reverse(char *s, int dim); Recursive reverse a string in place without using strlen

Question

Is it possible to reverse a string in place without using strlen, using recursion and with this definition?

void reverse(char *s, int dim);

The only thing I could do was:

void reverse(char *s, int dim)
{
    int a = dim;
    int b = strlen(s) - 1 - dim;

    if ((a - b) <= 0)
        return;

    swap(s[a], s[b]);

    reverse(s, dim - 1);
}

But I would like to do that without using strlen and without defining a similar function. Is it possible?

You cannot reverse a range whose length you don't know. (You can perhaps discover the length by ways other than, but equivalent to, strlen, but you'd only do a worse job of it that strlen would.) — Kerrek SB
– Kerrek SB, Commented Jul 19, 2015 at 12:52

Vlad from Moscow · Accepted Answer · 2015-07-19 13:00:07Z

3

Ciao. Come va?

As for me I would declare the function the following way

char * reverse( char *s, size_t n );

Here is a demonstrative program

#include <stdio.h>

char * reverse( char *s, size_t n )
{
    if ( !( n < 2 ) )
    {
        char c = s[0];
        s[0] = s[n-1];
        s[n-1] = c;
        reverse( s + 1, n - 2 );
    }

    return s;
}    


int main( void )
{
    char s[] = "Hello misiMe";

    puts( s );
    puts( reverse( s , sizeof( s ) - 1 ) );
}

The program output is

Hello misiMe
eMisim olleH

answered Jul 19, 2015 at 13:00

Vlad from Moscow

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1 Comment

edmz Over a year ago

Io bene, tu? Sorry, couldn't resist.

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