Reverse a string

Question posted in Computer Software on 03 2010

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one of the most common questions given to a developer is to take a string of characters and reverse it. Since Strings are immutable in almost all languages, it is supposed to be tricky.

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3 Answers

Shirly Leibovich

Any answer that shows the candidate thinking and figuring out the problem.
Examples : credits to Jim Burns

//------------------------------------------------------
// #1, While easy to understand suffers from a lot of memory reallocation.
// Each time the next letter is added to s2, it's added to the beginning of the
// string causing a reallocation of the entire string.
//
function ReverseString( s : string ) : string;
var
i : integer;
s2 : string;
begin
s2 := '';
for i := 1 to Length( s ) do
    s2 := s[ i ] + s2;
Result := s2;
end;

//------------------------------------------------------
// #2, Taking advantage of the fact that we can work at both ends of the string
// at once AND the fact that IF there is a middle character, ie. an odd number
// of characters in the string, it doesn't change position at all and we can
// eliminate all the memory allocations, work completely within the source
// string swapping from end to end working toward the middle and only having to
// make 1/2 of a loop through the string.
//
procedure ReverseStr(var Src : string);
var
i, j : integer;
C1   : char;
begin
j := Length(Src);
for i := 1 to (Length(Src) div 2) do
begin
    C1 := Src[i];
    Src[i] := Src[j];
    Src[j] := C1;
    Dec(j);
end;
end;

//------------------------------------------------------
// #3, One disadvantage of #2 can be seen when trying to fill one control with
// the contents of another. For example, two TEdits. Since TEdit.Text can't
// be sent as a var parameter you'll need to first make use of a temporary
// string and then set the second TEdit:
//
//   var
// tStr : string;
//   begin
// tStr := Edit1.Text;
// ReverseStr(tStr);
// Edit2.Text := tStr;
//
// However, using #3 this code turns into,
//
// Edit2.Text := ReverseStr(Edit1.Text);
//
// In addition, we lost 1 local var and the loop body was reduced since we
// could use Result directly swapping as we go!
//
function ReverseStr(const Src : string) : string;
var
i, j : integer;
begin
j := Length(Src);
SetLength(Result, j);
for i := 1 to (Length(Src) div 2) do
begin
    Result[i] := Src[j];
    Result[j] := Src[i];
    Dec(j);
end;
end;

03/24/2010

Shirly Leibovich

in PHP, you can simply use the strrev() function:

$string = "test string";
$string = strrev($string);

http://us2.php.net/manual/en/function.strrev.php

05/19/2010