You want to use a stack.

A stack is an abstract concept in programming. It is basically a list that you can only access from the tail end. You can push things onto the stack, and pop the item on the top off the stack, and you can detect when the stack is empty.

As I said, stacks are an abstract concept; you need a concrete realisation of them. In Perl, that's an array. Perl helpfully provides built-in push and pop functions, and makes it easy to check if an array is empty.

This is the basic technique you want...

my @if_stack;
while (<$input>)
{
  if (/$start/) {
    push @if_stack, $_;
    next;
  }
  if (/$stop/) {
    die unless @if_stack;
    pop @if_stack;
    next;
  }
  if (@if_stack) {
    # got a line which is inside a conditional
  }
  else {
    # got a line which is not inside a conditional
    print $output $_;
  }
}
[download]

In fact, in this case you could get away with using a data structure even simpler than a stack - a counter will do. You simply pretend you've got an imaginary stack, and use a counter that keeps track of how many items are on the stack. Instead of pushing onto the stack, increment the counter; instead of popping, decrement.

my $if_stack;
while (<$input>)
{
  if (/$start/) {
    $if_stack++;
    next;
  }
  if (/$stop/) {
    die unless $if_stack;
    $if_stack--;
    next;
  }
  if ($if_stack) {
    # got a line which is inside a conditional
  }
  else {
    # got a line which is not inside a conditional
    print $output $_;
  }
}
[download]

I maintain that the stack is the better solution because you can, for example, print a dump of the current stack when you detect an error, which can help in identifying where things have gone wrong. That said, if you're crunching though vast quantities of text, the counter will probably perform slightly faster.

Hi Tobyink,

your code fails if the code encounters #else

Regards,

-Prassi

No it doesn't. I just neglected to post my version of $start and $stop.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

#!/usr/bin/perl

use warnings;
use strict;

my $depth;
while (<DATA>) {
    /#ifdef|#if|#ifndef/ and $depth++;
    print unless $depth;
    /#endif/ and $depth--; # this comes after the print not to be prin
+ted
}


__DATA__
{

 #if 1
  #define done 9
  #define do 10
  #ifdef
    #define def1 11
    #define thread 12
  #endif
  #define mutex 13
  #define inter 14 
 #endif
 #define intel 15
 #define intel_64 16
}
[download]

choroba:

Actually, you *do* need a stack if you're going to properly handle the #else clauses. Otherwise, you won't remember the previous value(s) of the print vs. don't print states.

#if 0
  this shouldn't print
  #if 1
     nor should this
  #else
     nor this
  #end
#else
  but this should
  #if 1
     as should this
  #else
     but not this
  #end
  This should print, but where are you storing the information
  to know it without a stack?
#else
  If you got that one right, howzabout this one?
#end
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.

That was not part of the spec :-)

You probably don't want #else to increment $depth.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

Thanks, removed. I took it from the OP, and it is not present in the test data :-)

BTW, /#if/ matches both /#ifdef/ and /#ifndef/...

prassi:

Since 1 is true, I'd expect to get exactly what you're getting. If you change it to "#if 0" do you get what you were wanting?

Update: The quick hack I mentioned previously used the stack (as does tonyink's reply). It mostly wanted a way to handle #include files for getting the definitions of other #defines & such.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Hi Roboticus,

In my actual code of .h file I am interested to extract only those #define which are out of any directive statement, so it is either #if 1 or #if 0 it has to be ignored. Yes even "#if 0" the code works

Regards, -Prassi