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Re: reverse a string in place

by Athanasius (Abbot)
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,

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Re^2: reverse a string in place
by j0se (Pilgrim) 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

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