swap_strings_v3.c

/*

    swap_strings_v3.c

    https://web.facebook.com/groups/c.programing/permalink/1558823557609822/

    A string consists of two strings separated by ';' .
    The task is to change the order of the two stings.

    Version with pointers and without string.h .


    You can find all my C programs at Dragan Milicev's pastebin:

    https://pastebin.com/u/dmilicev

*/

#include <stdio.h>
#include <stdlib.h>

char *swap(char *s)
{
    int i, j, length;
    char *tmp, *first, *second;
    char delimiter = ';';

    i=0;                                        // calculate length of s, implementation of strlen()
    length=0;
    while( *(s+i) != '\0' )
    {
        length++;
        i++;
    }

    tmp = (char *)malloc( length + 1 );         // reserve memory for tmp, + 1 for '\0'
    if(tmp == NULL)                             // checking that it is successful
    {
        printf("\n malloc error for tmp ! \n");
        exit(1);
    }

    first = (char *)malloc( length + 1 );       // reserve memory for first, + 1 for '\0'
    if(first == NULL)                           // checking that it is successful
    {
        printf("\n malloc error for first ! \n");
        exit(1);
    }

    second = (char *)malloc( length + 1 );      // reserve memory for second, + 1 for '\0'
    if(second == NULL)                          // checking that it is successful
    {
        printf("\n malloc error for second ! \n");
        exit(1);
    }

    i=0;                                        // get first string from string s
    j=0;
    while( *(s+i) != delimiter )
    {
        *(first+j) = *(s+i);
        i++;
        j++;
    }
    *(first+j) = '\0';                          // finish string first

    i++;                                        // get second string from string s
    j=0;
    while( *(s+i) != '\0')
    {
        *(second+j) = *(s+i);
        i++;
        j++;
    }
    *(second+j) = '\0';                         // finish string second

    i=0;                                        // put second string in string s
    j=0;
    while( *(second+i) != '\0')
    {
        *(s+j) = *(second+i);
        i++;
        j++;
    }

    *(s+j) = delimiter;                         // put delimiter in string s

    i=0;                                        // put first string in string s
    j++;
    while( *(first+i) != '\0')
    {
        *(s+j) = *(first+i);
        i++;
        j++;
    }
    *(s+j) = '\0';                              // finish string s

    return s;
}


int main(void)
{
    char str[] = "first;second";

    printf("\n before: |%s| \n", str );

    swap(str);

    printf("\n after:  |%s| \n", str );

    printf("\n again:  |%s| \n", swap(str) );


    return 0;
}