Using Push and Pop.

dark314 has asked for the wisdom of the Perl Monks concerning the following question:

When I run this as:

dark314@somebox:~/Perl$ ./tac.pl test
zyxwvutsrqpon
[download]

I only get 13 characters... the test file is simply a file with letters a - z, it should print all characters backwards
I'm sure there is something wrong here!

#!/usr/bin/perl

use strict;
#perl excercise, reverse cat...


my @file;


while (<>) {
    chomp;
    push @file, $_;

}

foreach (@file) {   my $test = pop (@file); print $test;  }
print "\n";
[download]

------------------------------------UPDATE-------------------------------------

Thanks. I am aware of reverse() lol. The idea was to use push() and pop(). I guess I understand now my problem, using a foreach there evaluates the size of @file each time I think, and by the time it gets halfway thru it believes that it has gone thru the entire set? ?? how confusing. I will use while from now on

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Re: Using Push and Pop. by ikegami (Patriarch) on Aug 23, 2006 at 19:31 UTC
`foreach (@file) { my $test = pop (@file); print $test; }` [download] should be `while (@file) { my $test = pop (@file); print $test; }` [download] In reply to your updated question, `while (@file)` works because it evaluates the size of `@file` before each iteration. `foreach (@file)` processes element 0 on the first pass, element 1 on the second pass, etc. On the 14th pass, it should process element 13. However, the loop ends because `@file` only has elements 0 through 12 left.	[reply] [d/l] [select]
Re: Using Push and Pop. by Joost (Canon) on Aug 23, 2006 at 19:33 UTC
`foreach (@file) { my $test = pop (@file); print $test; }` [download] That line modifies @file (with pop) while looping through it with foreach. You should drop one of the statements for something better. I.e. change the `foreach` to `while`. ps: `print reverse <>;` :-) "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Using Push and Pop. - response to update by imp (Priest) on Aug 23, 2006 at 22:07 UTC
This will demonstrate why popping items off of the list you are iterating over is a bad idea: `use strict; use warnings; my @list = 'a'..'z'; for my $letter (@list) { my $popped = pop @list; printf "letter: %s popped: %s list: %s\n",$letter,$popped,join('', +@list); }` [download] Output: letter: a popped: z list: abcdefghijklmnopqrstuvwxy letter: b popped: y list: abcdefghijklmnopqrstuvwx letter: c popped: x list: abcdefghijklmnopqrstuvw letter: d popped: w list: abcdefghijklmnopqrstuv letter: e popped: v list: abcdefghijklmnopqrstu letter: f popped: u list: abcdefghijklmnopqrst letter: g popped: t list: abcdefghijklmnopqrs letter: h popped: s list: abcdefghijklmnopqr letter: i popped: r list: abcdefghijklmnopq letter: j popped: q list: abcdefghijklmnop letter: k popped: p list: abcdefghijklmno letter: l popped: o list: abcdefghijklmn letter: m popped: n list: abcdefghijklm [download] By the time you reach letter 'm' you have removed enough items from the end of the list that you are now at the new end. Update - added quote from perlsyn If any part of LIST is an array, "foreach" will get very confused if you add or remove elements within the loop body, for example with "splice". So don't do that	[reply] [d/l] [select]
Re^2: Using Push and Pop. - response to update by ikegami (Patriarch) on Aug 23, 2006 at 22:28 UTC
Based on that perlsyn quote, another (inefficient) solution would be to convert the array into a list: `foreach (map $_, @file) { my $test = pop (@file); print $test; }` [download]	[reply] [d/l]
Re: Using Push and Pop. by imp (Priest) on Aug 23, 2006 at 19:32 UTC
Why are you popping entries off the end of the @file list inside a loop where you are iterating over the entries in @file? `for my $f (reverse @file) { print $f; }` [download] Or `while (@file) { print pop @file; }` [download]	[reply] [d/l] [select]
Re: Using Push and Pop. by chromatic (Archbishop) on Aug 24, 2006 at 05:20 UTC
A: It's confusing and difficult to read otherwise. Q: Why do updates go after the original text?	[reply]