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Re: reverse a string in place

by Athanasius (Chancellor)
on Mar 24, 2013 at 08:04 UTC ( #1025128=note: print w/replies, xml ) Need Help??

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)])

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

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); }

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,

Replies are listed 'Best First'.
Re^2: reverse a string in place
by reisinge (Friar) 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

    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

Re^2: reverse a string in place
by perlynewby (Beadle) 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";

      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; }

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


        "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 ;-)

        is is a nice function

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


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