in reply to reverse a string in place

Just want to point out that the algorithm as given is incorrect. n is the number of elements in the array, so the first line should read:

function reverse_in_place(a[0 .. (n - 1)])

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Alternatively, you would need to change the arithmetic:

function reverse_in_place(a[0 .. n])
    for i from 0 to floor( ((n + 1) / 2) - 1 )
        tmp  := a[i]
        a[i] := a[n - i]
        a[n - i] := tmp

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Update 1: Implementing the algorithm in Perl is straightforward, provided you are allowed to use the built-in functions split and join (scalar isn’t really needed in the sub):

use strict;
use warnings;

my $string = 'abcdefghi';

print 'Original string: ', $string,                   "\n";
print 'Using function:  ', reverse_in_place($string), "\n";
print 'Using reverse:   ', scalar reverse  ($string), "\n";    # For c
+omparison

sub reverse_in_place
{
    my ($string) = @_;
    my  @array   = split //, $string;
    my  $n       = scalar @array;

    for (0 .. $n / 2 - 1)
    {
        my $tmp             = $array[$_];
        $array[$_]          = $array[$n - $_ - 1];
        $array[$n - $_ - 1] = $tmp;
    }

    return join('', @array);
}

[download]

Update 2: Fixed out-by-one error in the for loop. Thanks to j0se for pointing it out.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: reverse a string in place
by reisinge (Hermit) on Mar 24, 2013 at 10:27 UTC

    Hello, your Perl solution seems to have problems with strings that have an even number of characters: 

    Original string: ab
Using function:  ab
Using reverse:   ba

Original string: abcd
Using function:  dbca
Using reverse:   dcba

    [download]

    Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly. -- Will Durant

[reply]
[d/l]
Re^2: reverse a string in place
by perlynewby (Scribe) on Jun 28, 2015 at 00:26 UTC

    I don't know how old this thread is but I found it interesting to practice manipulation for strings,

    please keep in mind that I am but a few weeks into this perl code but did I meet the requirements? other than using length or split

    use strict;
use warnings;

my $string = 'abcdefghi';
#let's find the ways to count the elements in string;
my $l = length($string); #finding number of elemnts
my @ind=split //,$string; #finding the number of elements
#assigning the start of our last element for our looping
my $n = (scalar @ind )-1; #getting the last element to start count bac
+kwards optinal

for (my $i=(length ($string)-1) ; $i >= 0; $i-- ){
   print $ind[$i];
      }
      
print "\nfor comparison :", scalar reverse($string),"\n";

    [download]
[reply]
[d/l]

      Good job, especially the use strict; use warnings;. Here's another way based on your code that you may find interesting. (EDIT: My bad... I totally glanced over the fact no built-ins were allowed).

      while (my $letter = pop @ind){
    print $letter;
}

      [download]

      pop() removes the last element of an array. Also see shift(), unshift() and push()

      -stevieb

[reply]
[d/l]
[select]
        "without using any other array or built-in function"

        yeah, I had no idea how to get the number of elements without the use of at least one build-in option.

        Also, the no use of array is tricky for me.

        I think I will use these questions that other asks to practice my code but sometimes it is difficult to understand what a piece of code without a little help with comments.

        but then again, I am learning so everything is a little tricky...hmm, sometimes I think I should have picked to learn the piccolo over the summer ;-)

[reply]

        is is a nice function

        and now playing with shift and unshift to implement into next little prog

        thanks

[reply]

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