Array rearrangement hangup.

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

Dear Monks,

I'm attempting to write an encryption (and matching decryption) program, but it just was not working. No warnings or anything, but it wasn't giving me the correct output. As for the encryption half, I found the section that wasn't working, but I can't figure out why. Anybody think they'll be able to help? I isolated the task and gave an example input.

my @array=(1,2,3,4,5,6,7,8);
my @array2;
my $length = @array;
my $length2 = $length-1;
my $i;


    for $i(0..$length2){
        push @array2, $array[$i];
        $i+=2;
    }
    for $i(1..$length){
        push @array2, $array[$i];
        $i+=2;
    }
    undef @array;
    @array=reverse(@array2);
    
        
    undef @array2;
    for $i(1..$length){
        push @array2, $array[$i];
        $i+=2;
    }
    for $i(0..$length2){
        push @array2, $array[$i];
        $i+=2;
    }
    undef @array;
    @array=reverse(@array2);

print @array;
[download]

The desired output is "51627384" if that helps.

Comment on Array rearrangement hangup. Download Code

Replies are listed 'Best First'.
Re: Array rearrangement hangup. by ikegami (Patriarch) on Mar 15, 2006 at 23:13 UTC
The `$i+=2` in your loop does nothing. The `for` will put the next value from the list into `$i` without checking its current value. You want `for ($i=0; $i<=$length2; $i+=2){ push @array2, $array[$i]; } for ($i=1; $i<=$length; $i+=2){ push @array2, $array[$i]; }` [download] or `for $i(0..$length/2){ push @array2, $array[$i2]; } for $i(0..$length/2){ push @array2, $array[$i2+1]; }` [download]	[reply] [d/l] [select]
Re^2: Array rearrangement hangup. by Andrew_Levenson (Hermit) on Mar 15, 2006 at 23:15 UTC
...Looking at it, that makes sense. It just reads a new value for i, doesn't it? Thanks.	[reply]
Re^3: Array rearrangement hangup. by ikegami (Patriarch) on Mar 15, 2006 at 23:17 UTC
You also have an off-by-one error. $length contains the number of elements in the array, which is one beyond the end of the array. The `$i+=2` in your loop does nothing. The `for` will put the next value from the list into `$i` without checking its current value. You want `for ($i=0; $i<$length2; $i+=2){ push @array2, $array[$i]; } for ($i=1; $i<$length; $i+=2){ push @array2, $array[$i]; }` [download] or `for $i(0..($length-1)/2){ push @array2, $array[$i2]; } for $i(0..($length-1)/2){ push @array2, $array[$i2+1]; }` [download] or `for $i(0..$#array/2){ push @array2, $array[$i2]; } for $i(0..$#array/2){ push @array2, $array[$i2+1]; }` [download] That doesn't fix your decrpytion (which is totally wrong), but it puts you at a point where you can start having a look at it.	[reply] [d/l] [select]
Re^4: Array rearrangement hangup. by Andrew_Levenson (Hermit) on Mar 15, 2006 at 23:54 UTC
Re: Array rearrangement hangup. by QM (Parson) on Mar 15, 2006 at 23:20 UTC
`for $i(0..$length2){ push @array2, $array[$i]; $i+=2; }` [download] Don't modify the loop variable in the loop. This is bad programming practice. If you want to do something like this, make the loop do it for you: `for ($i=0;$i<=@array-2;$i+=2) { push @array2, $array[$i]; }` [download] For the next section: `for $i(1..$length){ push @array2, $array[$i]; $i+=2; }` [download] Careful you don't step off the array. While this works, it's fragile and easy to break. You should `use strict;` and `use warnings;`, and then use the debugger to find out where it goes horribly wrong ;) I'd do it for you now, but I have to run for the carpool! -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]
Re: Array rearrangement hangup. by Anonymous Monk on Mar 15, 2006 at 23:34 UTC
surprisingly, List::MoreUtils doesn't seem to have `unzip` `sub unzip { my ($a, $b) = ([], []); while (@_) { push @$a, shift; push @$b, shift } [$a, $b] }` [download]	[reply] [d/l] [select]
Re: Array rearrangement hangup. by Anonymous Monk on Mar 16, 2006 at 07:09 UTC
`$ perl -le' my @array = 1 .. 8; print "@array"; @array = map { $array[ $_ + @array / 2 ], $array[ $_ ] } 0 .. $#array +/ 2; print "@array"; ' 1 2 3 4 5 6 7 8 5 1 6 2 7 3 8 4` [download]	[reply] [d/l]