Sunday, April 13, 2014

Restrict a java text field length

There are many different way to do this. You might write your own once you figure out what has to be done exactly. But I came up with this. Additionally I wanted the input to be numeric as well. So below is what I came up with.

    private class NumericAndLengthFilter extends DocumentFilter {

         * Number of characters allowed.
        private int length = 0;

         * Restricts the number of charcacters can be entered by given length.
         * @param length Number of characters allowed.
        public NumericAndLengthFilter(int length) {
            this.length = length;

        public void insertString(FilterBypass fb, int offset, String string,
                AttributeSet attr) throws
                BadLocationException {
            if (isNumeric(string)) {
                if (this.length > 0 && fb.getDocument().getLength() + string.
                        > this.length) {
                super.insertString(fb, offset, string, attr);

        public void replace(FilterBypass fb, int offset, int length, String text,
                AttributeSet attrs) throws
                BadLocationException {
            if (isNumeric(text)) {
                if (this.length > 0 && fb.getDocument().getLength() + text.
                        > this.length) {
                super.insertString(fb, offset, text, attrs);

         * This method tests whether given text can be represented as number.
         * This method can be enhanced further for specific needs.
         * @param text Input text.
         * @return {@code true} if given string can be converted to number; otherwise returns {@code false}.
        private boolean isNumeric(String text) {
            if (text == null || text.trim().equals("")) {
                return false;
            for (int iCount = 0; iCount < text.length(); iCount++) {
                if (!Character.isDigit(text.charAt(iCount))) {
                    return false;
            return true;

Friday, April 11, 2014

Restricting a Java Text field to just a range of integers

This time I came across a slightly different way than extending the documentfilter. That is I will write my own plain document here.
class IntegerRangeDocument extends PlainDocument {

  int minimum, maximum;

  int currentValue = 0;

  public IntegerRangeDocument(int minimum, int maximum) {
    this.minimum = minimum;
    this.maximum = maximum;

  public int getValue() {
    return currentValue;

  public void insertString(int offset, String string, AttributeSet attributes)
      throws BadLocationException {

    if (string == null) {
    } else {
      String newValue;
      int length = getLength();
      if (length == 0) {
        newValue = string;
      } else {
        String currentContent = getText(0, length);
        StringBuffer currentBuffer = new StringBuffer(currentContent);
        currentBuffer.insert(offset, string);
        newValue = currentBuffer.toString();
      try {
        currentValue = checkInput(newValue);
        super.insertString(offset, string, attributes);
      } catch (Exception exception) {

  public void remove(int offset, int length) throws BadLocationException {
    int currentLength = getLength();
    String currentContent = getText(0, currentLength);
    String before = currentContent.substring(0, offset);
    String after = currentContent.substring(length + offset, currentLength);
    String newValue = before + after;
    try {
      currentValue = checkInput(newValue);
      super.remove(offset, length);
    } catch (Exception exception) {

  public int checkInput(String proposedValue) throws NumberFormatException {
    int newValue = 0;
    if (proposedValue.length() > 0) {
      newValue = Integer.parseInt(proposedValue);
    if ((minimum <= newValue) && (newValue <= maximum)) {
      return newValue;
    } else {
      throw new NumberFormatException();
Now you can attach your text field with this plain document as below to achieve our requirement.

Document rangeOne = new IntegerRangeDocument(0, 255);
JTextField textFieldOne = new JTextField();

Thursday, April 10, 2014

Restricting a Java Text field to alphabet

This is slightly opposite to what we have seen  in one of the previous posts
Please use the below DocumentFilter for the same.

class MyDocFilter extends DocumentFilter {
   private static final String REMOVE_REGEX = "\\d";
   private boolean filter = true;

   public boolean isFilter() {
      return filter;

   public void setFilter(boolean filter) {
      this.filter = filter;

   public void insertString(FilterBypass fb, int offset, String text,
         AttributeSet attr) throws BadLocationException {
      if (filter) {
         text = text.replaceAll(REMOVE_REGEX, "");
      super.insertString(fb, offset, text, attr);


   public void replace(FilterBypass fb, int offset, int length, String text,
         AttributeSet attrs) throws BadLocationException {
      if (filter) {
         text = text.replaceAll(REMOVE_REGEX, "");
      super.replace(fb, offset, length, text, attrs);


Restricting a Java Text field to just Integers

I have read a lot for this and the best way I found to do this is by extending the DocumentFilter class and using this new DocumentFilter
class MyIntFilter extends DocumentFilter {
   public void insertString(FilterBypass fb, int offset, String string,
         AttributeSet attr) throws BadLocationException {

      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.insert(offset, string);

      if (test(sb.toString())) {
         super.insertString(fb, offset, string, attr);
      } else {
         // warn the user and don't allow the insert

   private boolean test(String text) {
      try {
         return true;
      } catch (NumberFormatException e) {
         return false;

   public void replace(FilterBypass fb, int offset, int length, String text,
         AttributeSet attrs) throws BadLocationException {

      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.replace(offset, offset + length, text);

      if (test(sb.toString())) {
         super.replace(fb, offset, length, text, attrs);
      } else {
         // warn the user and don't allow the insert


   public void remove(FilterBypass fb, int offset, int length)
         throws BadLocationException {
      Document doc = fb.getDocument();
      StringBuilder sb = new StringBuilder();
      sb.append(doc.getText(0, doc.getLength()));
      sb.delete(offset, offset + length);

      if (test(sb.toString())) {
         super.remove(fb, offset, length);
      } else {
         // warn the user and don't allow the insert

Now after this use this filter with your JtextField like below:
PlainDocument doc = (PlainDocument) textField.getDocument();
doc.setDocumentFilter(new MyIntFilter());

A new addition to this blog - Java

I recently started working on Java and so from now on I would like to share all the interesting things that I have come across about this beautiful programming language. So I am starting a new tag by name JAVA in this blog which will direct all the posts related to java. Happy coding to myself and all.

Tuesday, March 11, 2014

Searching multiple strings in multiple files in a directory

I have a list of strings in a file separated by a new line.
for example:

File: input.txt

Now I have a directory with multiple dat files like:
and many more dat like like above with random names.
Now I want to search for all the strings in input.txt in all the dat files present in  directory(let's say current working directory).

This is what I came up with:
create a perl script given below and name it as anything you wish(I named here as
place the file input.txt in the current working directory.

#!/usr/bin/perl -w

open (INP,"input.txt") or die $!;
my $cmd="find . -name \"*.dat\"|xargs grep -w -i $_";
my $output=`$cmd`;
 print $_."\n";
 print "------------------\n";
 print $output."\n";
 print "-------------------\n";
Run this script as :


This solved my need.I hope it solves yours too :).

Monday, February 17, 2014

Creating a graph using STL's in C++

Below is a implementation of Graph Data Structure in C++ as Adjacency List.

I have used STL vector for representation of vertices and STL pair for denoting edge and destination vertex.

struct vertex{
 typedef pair ve;
 vector adj; //cost of edge, destination vertex
 string name;
 vertex(string s)

class graph
  typedef map vmap;
  vmap work;
  void addvertex(const string&);
  void addedge(const string& from, const string& to, double cost);

void graph::addvertex(const string &name)
 vmap::iterator itr=work.begin();
  vertex *v;
  v= new vertex(name);
  cout<<"\nVertex already exists!";

void graph::addedge(const string& from, const string& to, double cost)
 vertex *f=(work.find(from)->second);
 vertex *t=(work.find(to)->second);
 pair edge = make_pair(cost,t);

Friday, November 29, 2013

Finding Memory leaks on solaris

There are lot of ways where one can identify memory leaks and lots of blogs and materieals
are available to help a designer to identify the same.

Some say using dbx and others say use mdb or gdb. I agree that all those tools will be useful, but it takes a lot of effort for understanding as well as putting it to work takes some time.

I have thought to break the legacy rules and do something new with the knowledge that
I have on various tools of solaris. So I thought of giving it a try to use tools like
dtrace, perl, ps command etc along with c++.

Putting this toll to action is an effort less job.

I am happy to have created a tool which I think would be very use for any solaris unix programmer.
This tool will give the list of all memory leaks of a process running in the background.

It uses dtrace to identify the memory allocations,
It uses the ps command to identify the memory used by a process.
It uses perl to filter and generate reports of meory leaks.

Main functionalities of this tool are:

  • Generate a report which contains actual physical memory used by the process, Virtual memory used, % of cpu used and %memory used.
  • Generate a report which contains all the memory leaks of the process due to inefficient coding techniques.
  • Generate a separate report for each and every process.

I have tested this on:
SunOS 5.10 Generic_147440-27 sun4u sparc SUNW,SPARC-Enterprise

Thanks to Frederic, I have taken part of his logic written in his blog:

You can download the zip file from the link here:

The file name is cpuleaks.gz.
Unzip this file using gzip -d cpuleaks.gz and you can place it on any server and give the execute permissions and run this process as a root user(dtrace can be run only using root access on my server. If dtrace can be used by a user other than root, well and fine).

Execution of the process should be as below:

cpuleaks <comma separated pid list> -st <time interval for mem usage>
ex: cpuleaks 1234,2341 -st 2

This above command will give 4 files as output after either the processes 1234,2341 complete or if you press CTRL +C(an interrupt signal).

  • .leaks report will have the list of all memory leaks
  • .cpumem report will have the memory usage of the process for every interval passed as an argument to the cpuleaks binary.

Thursday, September 5, 2013

Search Multiple strings and print them in the order of search strings

One of my colleagues asked this query to me. And I liked the query very much.
So I started thinking about that.
For example i have a below file:
1 name1
2 name2
3 name3
4 name1
5 name4
6 name2
7 name1
Now i want to find strings in the file , lets say in the order name1, name2, name3, name4.
And i also want the output to be in the same order in the way i searched as below:
1 name1
4 name1
7 name1
2 name2
6 name2
3 name3
5 name4
Obviously we cannot do this with a single grep command without any temporary files being created.
So I thought perl would be a better option for this. So I came up with a simple perl solution.
perl -lne '/name1/?push @a,$_:
          (/name2/?push @b,$_:
          (/name3/?push @c,$_:
           /name4/?push @d,$_:next));
          END{print join "\n",@a,@b,@c,@d}' your_file

And this worked like a a charm.

Monday, September 2, 2013

Some Useful Perl one liner's

We'll start with a brief refresher on the basics of perl one-liners before we begin. The core of any perl one-liner is the -e switch, which lets you pass a snippet of code on the command-line:perl -e 'print "hi\n"' prints "hi" to the console.
The second standard trick to perl one-liners are the -n and -p flags. Both of these make perl put an implicit loop around your program, running it once for each line of input, with the line in the $_ variable. -p also adds an implicit print at the end of each iteration.
Both of these use perl's special "ARGV" magic file handle internally. What this means is that if there are any files listed on the command-line after your -e, perl will loop over the contents of the files, one at a time. If there aren't any, it will fall back to looping over standard input.
perl -ne 'print if /foo/' acts a lot like grep foo, and perl -pe 's/foo/bar/' replaces foo with bar
Most of the rest of these tricks assume you're using either -n or -p, so I won't mention it every time.

The top 10 one-liner tricks

One Liner-1:   -l
Smart newline processing. Normally, perl hands you entire lines, including a trailing newline. With -l, it will strip the trailing newline off of any lines read, and automatically add a newline to anything you print (including via -p).
Suppose I wanted to strip trailing whitespace from a file. I might naïvely try something like
perl -pe 's/\s*$//'
The problem, however, is that the line ends with "\n", which is whitespace, and so that snippet will also remove all newlines from my file! -l solves the problem, by pulling off the newline before handing my script the line, and then tacking a new one on afterwards:
perl -lpe 's/\s*$//'

One Liner-2: -0
Occasionally, it's useful to run a script over an entire file, or over larger chunks at once. -0 makes -n and -p feed you chunks split on NULL bytes instead of newlines. This is often useful for, e.g. processing the output of find -print0. Furthermore, perl -0777 makes perl not do any splitting, and pass entire files to your script in $_.
find . -name '*~' -print0 | perl -0ne unlink
Could be used to delete all ~-files in a directory tree, without having to remember how xargs works.
One Liner-3: -i
-i tells perl to operate on files in-place. If you use -n or -p with -i, and you pass perl filenames on the command-line, perl will run your script on those files, and then replace their contents with the output. -i optionally accepts an backup suffix as argument; Perl will write backup copies of edited files to names with that suffix added.
perl -i.bak -ne 'print unless /^#/'
Would strip all whole-line commands from, but leave a copy of the original in
One Liner-4: The .. operator
Perl's .. operator is a stateful operator -- it remembers state between evaluations. As long as its left operand is false, it returns false; Once the left hand returns true, it starts evaluating the right-hand operand until that becomes true, at which point, on the next iteration it resets to false and starts testing the other operand again.
What does that mean in practice? It's a range operator: It can be easily used to act on a range of lines in a file. For instance, I can extract all GPG public keys from a file using:
perl -ne 'print if /-----BEGIN PGP PUBLIC KEY BLOCK-----/../-----END PGP PUBLIC KEY BLOCK-----/' FILE
One Liner-5: -a
-a turns on autosplit mode – perl will automatically split input lines on whitespace into the @F array. If you ever run into any advice that accidentally escaped from 1980 telling you to use awk because it automatically splits lines into fields, this is how you use perl to do the same thing without learning another, even worse, language.
As an example, you could print a list of files along with their link counts using
ls -l | perl -lane 'print "$F[7] $F[1]"'

One Liner-6: -F
-F is used in conjunction with -a, to choose the delimiter on which to split lines. To print every user in /etc/passwd (which is colon-separated with the user in the first column), we could do:
perl -F: -lane 'print $F[0]' /etc/passwd
One Liner-7: \K
\K is undoubtedly my favorite little-known-feature of Perl regular expressions. If \K appears in a regex, it causes the regex matcher to drop everything before that point from the internal record of "Which string did this regex match?". This is most useful in conjunction with s///, where it gives you a simple way to match a long expression, but only replace a suffix of it.
Suppose I want to replace the From: field in an email. We could write something like
perl -lape 's/(^From:).*/$1 Nelson Elhage <nelhage\>/'
But having to parenthesize the right bit and include the $1 is annoying and error-prone. We can simplify the regex by using \K to tell perl we won't want to replace the start of the match:
perl -lape 's/^From:\K.*/ Nelson Elhage <nelhage\>/'
One Liner-8: $ENV{}
When you're writing a one-liner using -e in the shell, you generally want to quote it with ', so that dollar signs inside the one-liner aren't expanded by the shell. But that makes it annoying to use a ' inside your one-liner, since you can't escape a single quote inside of single quotes, in the shell.
Let's suppose we wanted to print the username of anyone in /etc/passwd whose name included an apostrophe. One option would be to use a standard shell-quoting trick to include the ':
perl -F: -lane 'print $F[0] if $F[4] =~ /'"'"'/' /etc/passwd
But counting apostrophes and backslashes gets old fast. A better option, in my opinion, is to use the environment to pass the regex into perl, which lets you dodge a layer of parsing entirely:
env re="'" perl -F: -lane 'print $F[0] if $F[4] =~ /$ENV{re}/' /etc/passwd
We use the env command to place the regex in a variable called re, which we can then refer to from the perl script through the %ENV hash. This way is slightly longer, but I find the savings in counting backslashes or quotes to be worth it, especially if you need to end up embedding strings with more than a single metacharacter.
One Liner-9: BEGIN and END
BEGIN { ... } and END { ... } let you put code that gets run entirely before or after the loop over the lines.
For example, I could sum the values in the second column of a CSV file using:
perl -F, -lane '$t += $F[1]; END { print $t }'
One Liner-10: -MRegexp::Common
Using -M on the command line tells perl to load the given module before running your code. There are thousands of modules available on CPAN, numerous of them potentially useful in one-liners, but one of my favorite for one-liner use is Regexp::Common, which, as its name suggests, contains regular expressions to match numerous commonly-used pieces of data.
The full set of regexes available in Regexp::Common is available in its documentation, but here's an example of where I might use it:
Neither the ifconfig nor the ip tool that is supposed to replace it provide, as far as I know, an easy way of extracting information for use by scripts. The ifdata program provides such an interface, but isn't installed everywhere. Using perl and Regexp::Common, however, we can do a pretty decent job of extracing an IP from ip's output:
ip address list eth0 | \

  perl -MRegexp::Common -lne 'print $1 if /($RE{net}{IPv4})/'
So, those are my favorite tricks, but I always love learning more. What tricks have you found or invented for messing with perl on the command-line? What's the most egregious perl "one-liner" you've wielded, continuing to tack on statements well after the point where you should have dropped your code into a real script?

Tuesday, July 9, 2013

What is Undefined Behavior?

In computer programming, undefined behavior refers to computer code whose behavior is unpredictable. It is a feature of some programming languages—most famously C.[1] In these languages, to simplify the specification and allow some flexibility in implementation, the specification leaves the results of certain operations specifically undefined, meaning that the programmer can't predict what will happen.

For example, in C the use of any automatic variable before it has been initialized yields undefined behavior, as would division by zero or indexing an array outside of its defined bounds (see buffer overflow). This specifically frees the compiler to do whatever is easiest or most efficient, should such a program be submitted. In general, any behavior afterwards is also undefined. In particular, it is never required that the compiler diagnose undefined behavior — therefore, programs invoking undefined behavior may appear to compile and even run without errors at first, only to fail on another system, or even on another date. When an instance of undefined behavior occurs, so far as the language specification is concerned anything could happen, maybe nothing at all. In particular, anything may include apparently-impossible behavior because the compiler has made assumptions that lead to erroneous code generation that does not match the source code.

Under some circumstances there can be specific restrictions on undefined behavior. For example, the instruction set specifications of a CPU might leave the behavior of some forms of an instruction undefined, but if the CPU supports memory protection then the specification will probably include a blanket rule stating that no user-accessible instruction may cause a hole in the operating system's security; so an actual CPU would be permitted to corrupt any or all user registers in response to such an instruction but would not be allowed to, for example, switch into supervisor mode.
In C and C++, implementation-defined behavior is also defined which requires the implementation to document what it does, thus more restrictive than undefined behavior.

Examples in C and C++ :

Attempting to modify a string literal causes undefined behavior:[2]

char * p = "wikipedia"; // ill-formed C++11, deprecated C++98/C++03
p[0] = 'W'; // undefined behaviour
One way to prevent this is defining it as an array instead of a pointer.

char p[] = "wikipedia"; /* RIGHT */
p[0] = 'W';
In C++ one can use STL string as follows.

std::string s = "wikipedia"; /* RIGHT */
s[0] = 'W';

Division by zero results in undefined behavior:[3]
return x/0; // undefined behavior

Certain pointer operations may result in undefined behavior:[4]
int arr[4] = {0, 1, 2, 3};
int* p = arr + 5;  // undefined behavior

Reaching the end of a value-returning function (other than main()) without a return statement may result in undefined behavior:
int f()
}  /* undefined behavior */

The C Programming Language cites the following examples of code that have undefined behavior in Section 2.12:
The order in which function arguments are evaluated is not specified, so the statement
printf("%d %d\n", ++n, power(2, n));    /* WRONG */

results in undefined behavior.
The following statement is ambiguous as it is not clear whether the array index is the old value of i or the new.
a[i] = i++;

This results in undefined behavior.

Thursday, July 4, 2013

Fork() system call and a duplicating buffer

Did you try this program:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
int main(void)
int i;    
      for(i = 0; i < 2; i++)    
return 0;

How many dots will this program print.
If you analyse and count , you will come up with a total of 6 dots.
But in real after you execute you will see not 6 but 8 dots printed on the console.

The reason for this is :
So, at first, there is one process. That creates a second process, both of which print a dot and loop. On their second iteration, each creates another copy, so there are four processes print a dot, and then exit. So we can easily account for six dots, like you expect.

However, what printf() really does is buffer its output. So the first dot from when there were only two processes does not appear when written. Those dots remain in the buffer—which is duplicated at fork(). It is not until the process is about to exit that the buffered dot appears. Four processes printing a buffered dot, plus the new one gives 8 dots.

If you wanted to avoid that behavior, call fflush(stdout); after printf().

Thursday, June 20, 2013

Removal of unnecessary white spaces

I have a text file which looks something like this:
> cat temp
1, 2, 3, "Test, Hello"
4, 5, 6, "Well, Hi There!"

You can see that there is a space after comma(,) which i feel its not needed for me.
But i need that space in between the string in last field. So the output i am expecting is :
> cat temp
1,2,3,"Test, Hello"
4,5,6,"Well, Hi There!"

I used the below command for doing the same:
perl -lne 'if(/(.*?\")(.*)/){$b=$2;$a=$1;$a=~s/,[\s]/,/g;print "$a$b"}' temp

(.*?\")(.*)- capture thye string till " in $1 and and remaining in $2.
$2 should be obviously unchanged. so store it in $b
$1 should be changed . so store it in $a and replace all ", " with ",".

Same thing can be achieved in awk as below:
>nawk -F'\"' -v OFS='\"' '{gsub(/ /,"",$1)}1' temp

Monday, June 17, 2013

Replacing the instances of a string only in the last line of occurance

How do you replace  instances of a string only in the line where they last occured.
for example if you have a file like below:
The output should look like below:
Below is the perl command that will perform the same:

Friday, June 7, 2013

Keyword auto in C++ - Basics

The auto Keyword: The Basics
Consider the following code snippet:
std::vector<int> vect;
// ...

  std::vector<int>::const_iterator cit = vect.begin();
  cit != vect.end();
  std::cout << *cit;
Perfectly fine, we all write code like this or similar all the time. Now let us consider this instead:
std::vector<int> vect;
// ...

  std::vector<int>::iterator it = vect.begin();
  it != vect.end();
  std::cin >> *it;
When writing this code, you should be getting slightly annoyed. The variable it is of the exact same type as the expression vect.begin() with which it is being initialized, and the compiler knows what that type is. Therefore, for us to have to write std::vector<int>::iterator is redundant. The compiler should use this as the default for the type of the variable it. And that is exactly the issue that the auto keyword addresses. In C++11, we can write:
std::vector<int> vect;
// ...

for(auto it = vect.begin(); it != vect.end(); ++it)
  std::cin >> *it;
The compiler will use the type of the initializing expression vect.begin(), which is std::vector<int>::iterator, as the type of the variable it. This sounds easy enough, but it's not quite the full story on auto. Read on.
The auto Keyword: The Rest of the Story
Consider this example of using auto:
int x = int(); // x is an int, initialized to 0
assert(x == 0);

const int& crx = x; // crx is a const int& that refers to x
x = 42;
assert(crx == 42 && x == 42);

auto something = crx;
The crucial question is, what is the type of something? Since the declared type of crx is const int&, and the initializing expression for something is crx, you might think that the type of something is const int&. Not so. It turns out that the type of something is int:
assert(something == 42 && crx == 42 && x == 42);
// something is not const:
something = 43;
// something is not a reference to x:
assert(something == 43 && crx == 42 && x == 42);
Before we discuss the rationale behind this behavior, let us state the exact rules by which autoinfers the type from an initializing expression:

When auto sets the type of a declared variable from its initializing expression, it proceeds as follows:
  1. If the initializing expression is a reference, the reference is ignored.
  2. If, after Step 1 has been performed, there is a top-level const and/or volatile qualifier, it is ignored.

As you have probably noticed, the rules above look like the ones that function templates use to deduce the type of a template argument from the corresponding function argument. There is actually a small difference:auto can deduce the type std::initializer_list from a C++11-style braced list of values, whereas function template argument deduction cannot. Therefore, you may use the rule "auto works like function template argument deduction" as a first intuition and a mnemonic device, but you need to remember that it is not quite accurate.Continuing with the example above, suppose we pass the const int& variable crx to a function template:
template<class T>
void foo(T arg);
Then the template argument T resolves to int, not to const int&. So for this instantiation of foo, the argument arg is of type int, not const int&. If you want the argument arg of foo to be a const int&, you can achieve that either by specifying the template argument at the call site, like this:
foo<const int&>(crx);
or by declaring the function like this:
template<class T>
void foo(const T& arg);
The latter option works analogously with auto:
const auto& some_other_thing = crx;
Now some_other_thing is a const int&, and that is of course true regardless of whether the initializing expression is an int, an int&, a const int, or a const int&.
assert(some_other_thing == 42 && crx == 42 && x == 42);
some_other_thing = 43;  // error, some_other_thing is const
x = 43;
assert(some_other_thing == 43 && crx == 43 && x == 43);
The possibility of adding a reference to the auto keyword as shown in the example above requires a small amendment to auto's const-stripping behavior. Consider this example:
  const int c = 0;
  auto& rc = c;
  rc = 44; // error: const qualifier was not removed
If you went strictly by the rules stated earlier, auto would first strip the const qualifier off the type of c, and then the reference would be added. But that would give us a non-const reference to the const variable c, enabling us to modify c. Therefore, auto refrains from stripping the const qualifier in this situation. This is of course no different from what function template argument deduction does.
Let's do one more example to demonstrate that auto drops const and volatilequalifiers only if they're at the top or right below an outermost reference:
int x = 42;
const int* p1 = &x;
auto p2 = p1;
*p2 = 43; // error: p2 is const int*
Now that we know how auto works, let's discuss the rationale behind the design. There is probably more than one way to argue. Here's my way to see why the behavior is plausible: being a reference is not so much a type characteristic as it is a behavioral characteristic of a variable. The fact that the expression from which I initialize a new variable behaves like a reference does not imply that I want my new variable to behave like a reference as well. Similar reasoning can be applied to constness and volatility1. Therefore,auto does not automatically transfer these characteristics from the initializing expression to my new variable. I have the option to give these characteristics to my new variable, by using syntax like const auto&, but it does not happen by default.

Wednesday, June 5, 2013

Print range of lines in perl based upon line numbers

Below is a simple way to extract certain lines in perl based on line numbers.
for eg: If I need to extract lines 5 -10 then i need to do:
perl -lne 'print if($.>=5&&$.<=10) your_file

Also this can done in awk  in a much simpler way:
awk 'NR>=5&&NR<=10 your_file

Classic way of doing this is :

head -10 your_file | tail -6

Thursday, May 30, 2013

Copy multiple files with same name in different directories into one target directory

I regularly  do this.So I automated it.
Below is a small simple script for copying files which are present in two different directories but without same names into a single directory with having name collision.


for i in `find <source_dir_path> -type f`
new=`echo $i|nawk -F"/" '{split($NF,a,".");print "<target_dir_path>"a[1]"_"$(NF-1)"."a[2]}'`
cp $i $new

Wednesday, May 29, 2013

Pattern Match Using Perl-Append file contents

If i want to append all the lines of one file  to a line in second file only if there is a pattern match:

For example:



pattern to match:



Below is the perl command to achieve this:

perl -lne 'BEGIN{open(A,"first_file.txt");@f=<A>}
           print;if(/2222/){print @f}' 

Friday, May 17, 2013

Retrieving a List of files from Remote Server

Below is the perl script for retrieving the list of files from Remote server:

    use strict;
    use warnings;
    use Net::FTP;
    my $ftp_site     = 'localhost';
    my $ftp_dir      = '/home/simon/software';
    my $ftp_user     = 'a_login_name';
    my $ftp_password = 'a_password';
    my $glob         = 'ex*';
    my @remote_files;
    my $ftp = Net::FTP-&gt;new($ftp_site)
        or die "Could not connect to $ftp_site: $!";
    $ftp-&gt;login($ftp_user, $ftp_password)
        or die "Could not login to $ftp_site with user $ftp_user: $!";
        or die "Could not change remote working " .
                 "directory to $ftp_dir on $ftp_site";
    @remote_files = $ftp-&gt;ls($glob);
    foreach my $file (@remote_files) {
        print "Saw: $file\n";

Wednesday, May 15, 2013

List of Bash Conditional statements

At times you need to specify different courses of action to be taken in a shell script, depending on the success or failure of a command. The if construction allows you to specify such conditions.

The most compact syntax of the if command is:

The TEST-COMMAND list is executed, and if its return status is zero, the CONSEQUENT-COMMANDS list is executed. The return status is the exit status of the last command executed, or zero if no condition tested true.
The TEST-COMMAND often involves numerical or string comparison tests, but it can also be any command that returns a status of zero when it succeeds and some other status when it fails. Unary expressions are often used to examine the status of a file. If the FILE argument to one of the primaries is of the form /dev/fd/N, then file descriptor "N" is checked. stdin, stdout and stderr and their respective file descriptors may also be used for tests.

Expressions used with if
The table below contains an overview of the so-called "primaries" that make up the TEST-COMMAND command or list of commands. These primaries are put between square brackets to indicate the test of a conditional expression.

Primary expressions
[ -a FILE ] True if FILE exists.
[ -b FILE ] True if FILE exists and is a block-special file.
[ -c FILE ] True if FILE exists and is a character-special file.
[ -d FILE ] True if FILE exists and is a directory.
[ -e FILE ] True if FILE exists.
[ -f FILE ] True if FILE exists and is a regular file.
[ -g FILE ] True if FILE exists and its SGID bit is set.
[ -h FILE ] True if FILE exists and is a symbolic link.
[ -k FILE ] True if FILE exists and its sticky bit is set.
[ -p FILE ] True if FILE exists and is a named pipe (FIFO).
[ -r FILE ] True if FILE exists and is readable.
[ -s FILE ] True if FILE exists and has a size greater than zero.
[ -t FD ]   True if file descriptor FD is open and refers to a terminal.
[ -u FILE ] True if FILE exists and its SUID (set user ID) bit is set.
[ -w FILE ] True if FILE exists and is writable.
[ -x FILE ] True if FILE exists and is executable.
[ -O FILE ] True if FILE exists and is owned by the effective user ID.
[ -G FILE ] True if FILE exists and is owned by the effective group ID.
[ -L FILE ] True if FILE exists and is a symbolic link.
[ -N FILE ] True if FILE exists and has been modified since it was last read.
[ -S FILE ] True if FILE exists and is a socket.
[ FILE1 -nt FILE2 ] True if FILE1 has been changed more recently than FILE2, or if FILE1 exists and FILE2 does not.
[ FILE1 -ot FILE2 ] True if FILE1 is older than FILE2, or is FILE2 exists and FILE1 does not.
[ FILE1 -ef FILE2 ] True if FILE1 and FILE2 refer to the same device and inode numbers.
[ -o OPTIONNAME ] True if shell option "OPTIONNAME" is enabled.
[ -z STRING ] True of the length if "STRING" is zero.
[ -n STRING ] or [ STRING ] True if the length of "STRING" is non-zero.
[ STRING1 == STRING2 ]  True if the strings are equal. "=" may be used instead of "==" for strict POSIX compliance.
[ STRING1 != STRING2 ]  True if the strings are not equal.
[ STRING1 < STRING2 ]  True if "STRING1" sorts before "STRING2" lexicographically in the current locale.
[ STRING1 > STRING2 ]  True if "STRING1" sorts after "STRING2" lexicographically in the current locale.
[ ARG1 OP ARG2 ] "OP" is one of -eq, -ne, -lt, -le, -gt or -ge. These arithmetic binary operators return true if "ARG1" is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to "ARG2", respectively. "ARG1" and "ARG2" are integers.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
Combining expressions
[ ! EXPR ]          True if EXPR is false.
[ ( EXPR ) ]        Returns the value of EXPR. This may be used to override the normal precedence of operators.
[ EXPR1 -a EXPR2 ]  True if both EXPR1 and EXPR2 are true.
[ EXPR1 -o EXPR2 ]  True if either EXPR1 or EXPR2 is true.

The [ (or test) built-in evaluates conditional expressions using a set of rules based on the number of arguments. More information about this subject can be found in the Bash documentation. Just like the if is closed with fi, the opening square bracket should be closed after the conditions have been listed.

Monday, May 13, 2013

Semicolon is optional in perl

I guess most of the people have noticed about this and have just ignored it.
Or they must not have noticed it at all.
Do you know this fact about perl:
If you write this :
print $_
Its not an error in perl.
DO remember this taken from perldoc:
Every simple statement must be terminated with a semicolon, unless it is the final statement in a block, in which case the semicolon is optional.

Thursday, May 9, 2013

Row width and Column separator in Sybase isql

Many of you either donot know or perhaps you are not aware of these since there is very less documentation available in google search for these

In general I use:
isql -U<user> -P<passsword>  -D<dbname>

There are two ioptions in isql.
which are
-s     Column separator.This wil separate columns with what ever character is followed by this option
       another option is (and probably very important):
-w     This will set the row width.By default is set to 80 characters and if the row length exceeds 80          
       characters, a new line is added in the console output.
for eg:
isql -U<user> -P<passsword>  -D<dbname> -s ',' -w 65535

Tuesday, May 7, 2013

Consequetive pattern match in Perl

Make a pattern that will match three consecutive copies of whatever is currently contained in $what. That is, if
$what="fred" , your pattern should match "fredfredfred".
$what is "fred|barney", your pattern should match
"fredfredbarney" and
"barneyfredfred" and
"barneybarneybarney" and  many other variations.
Well, This is quite tricky. But the magic match to do over here is :

 Below is the example code:
use warnings;
use strict;
# notice the example has the `|`. Meaning
# match "fred" or "barney" 3 times.
my $str = 'fred|barney';
my @tests = qw(fred fredfredfred barney barneybarneybarny barneyfredbarney);
for my $test (@tests) {
if( $test =~ /^($str){3}$/ ) {
print "$test matched!\n";
} else {
print "$test did not match!\n";

Below is the execution:
> ./
fred did not match!
fredfredfred matched!
barney did not match!
barneybarneybarny did not match!
barneyfredbarney matched!

Thursday, May 2, 2013

How does free/delete in C/C++ know how much memory to free/delete

This is probably very important for every C/C++ programmer to know.
And most of the interviewers too ask this question.I myself had asked in many interviews, but hardly I can get a good response from the people.

So I thought it would be better that post it here so that people will get to know.
char *ptr =malloc(5*sizeof(char));
The above statement will allocate 5 bytes of memory for ptr on heap.
I had written the word heap in bold, since most people will think that memory is allocated on stack.

So after you use ptr in your code and get the task done you have to delete or free the memory.
so you write this for it.
Remember, you are not passing 5 bytes to free as an argument, but how come free function know that it has to free 5 bytes?

When you call malloc(), you specify the amount of memory to allocate. The amount of memory actually used is slightly more than this, and includes extra information that records (at least) how big the block is. You can't (reliably) access that other information - and nor should you :-).
When you call free(), it simply looks at the extra information to find out how big the block is.

This is the logic behind this magic(atleast for a programmer this is a magic provided by the standard).

also see the link in c-faqs:
How does free know how many bytes to free?

Same principle is followed for delete and new.

Tuesday, April 30, 2013

Replace a string in a file with the value in environment variable.

Many times its required inside a shell script that you need to replace a string in a file with a value which is present in a environment variable.

Most of the time people will face this in thier daily work life if they work on unix specially.

Well below is trich to do it.
echo $LINE | sed -e "s/12345678/${replace}/g"

Small test below:
> export replace=987654321
> echo X123456789X | sed "s/123456789/${replace}/"

Thursday, April 25, 2013

C++ overcome the definition inthe standard libraries

I have a function abs in one of the libraries and i wanted to use that function by linking to that library.
But i face this error below:
function abs used is #define abs(x) (x &gt; 0) ? x : -(x).

This is obviously from the stadard includes that i used.
So i found a very nice way to overcome this like below:

Wrapping the function name abs in paranthesis has resolved the problem right away.
Well, I also found one more way to achieve the same thing.
#undef abs

Before you call the function abc in the library.

Tuesday, April 23, 2013

Print Lines which have some unique fields in awk

I have a table in which most of the values in a given row are the same. What I want to pull out are any rows where at least one of the values is different.

For eg i have a file:

$ cat file
one one one one two
two two two two two
three four four five
I found a nice trick for doing this:

$ awk '{f=$0;gsub($1,"")}NF{print f}' file
one one one one two
three four four five

First we store the line in original state f=$0 then we do a global substitution on everything matching the first field, if all fields are the same then nothing will be left therefor NF will be 0 and nothing will be printed else we print the original line.

Wednesday, April 17, 2013

Sorting a file with related lines

I have a file in which first row contains the number and second row contains a statement associated with it and so on like the below example

I need to print the output like sorting reversely as per numbers and so the statement related to it
Output needed is :
Time = 18
Stat = stat2
Time = 15
Stat = stat3
Time = 12
Stat = stat1
Below is the perl command that i have written which does the needed thing.
> perl -lne 'if(/^\d+/){$k=$_}else{$x{$k}=$_}END{for(sort {$b<=>$a} keys %x){print "time=$_\nStat=$x{$_}";}}' file


precondition is time will come first and then the stetement associated with it.
1. so store in a variable $k if the line start with a decimal point number.
2. Else case it is nothing but the statement. So stroe this line as a value to the previous stored $k.
3. After parsing complete file.sort the keys of the hash in descending order by
   sort  $b<=>$a and then print each and ever key and value in needed format.
1. sort $a<=>$b will sort the numbers in ascending order.
2. sort $b<=>$a will sort the numbers in descending order

Friday, March 29, 2013

Column wise comaprision using awk

There was a requirement once where i need to compare two files not with thier rows instead  i needed to do the comparision with columns.
I wanted only those rows where any of the columns in the lines differ.

for eg:
1 A B C D
2 E F G H
1 A Z C D
2 E F Y H
3 M N O P
Below is the Output I need:
file1 1 col2 B
file2 1 col2 Z
file1 2 col3 G
file2 2 col3 Y

Below is the solution in awk that i have written.
awk 'FNR==NR{a[FNR]=$0;next} {
printf "file1 "b[1]" col"b[i-1]" "b[i]"\n";
printf "file2 "$1" col"b[i-1]" "$i"\n";
Below is the test i made on my solaris server:

> nawk 'FNR==NR{a[FNR]=$0;next}{if(a[FNR]){split(a[FNR],b);for(i=1;i<=NF;i++){if($i!=b[i]){printf "file1 "b[1]" col"i-1" "b[i]"\n";printf "file2 "$1" col"i-1" "$i"\n";}}}}' file1 file2
file1 1 col2 B
file2 1 col2 Z
file1 2 col3 G
file2 2 col3 Y

Tuesday, March 26, 2013

Increment a number for each repeat

I have a file where column one has a list of family identifiers
Is there a way that I can create a new column where each repeat is numbered creating a new label for each repeat i.e.
Below is a pretty simple solution for this:
awk '{print $1"_"++a[$1]}' file