Reverse the ordering of words in a string

Question

I have this string s1 = \"My name is X Y Z\" and I want to reverse the order of the words so that s1 = \"Z Y X is name My\".

I can do it u

Answer 1

This is not perfect but it works for me right now. I don't know if it has O(n) running time btw (still studying it ^^) but it uses one additional array to fulfill the task.

It is probably not the best answer to your problem because i use a dest string to save the reversed version instead of replacing each words in the source string. The problem is that i use a local stack variable named buf to copy all the words in and i can not copy but into the source string as this would lead to a crash if the source string is const char * type.

But it was my first attempt to write s.th. like this :) Ok enough blablub. here is code:

#include 
using namespace std;

void reverse(char *des, char * const s);
int main (int argc, const char * argv[])
{    
    char* s = (char*)"reservered. rights All Saints. The 2011 (c) Copyright 11/10/11 on Pfundstein Markus by Created";
    char *x = (char*)"Dogfish! White-spotted Shark, Bullhead";

    printf("Before: |%s|\n", x);
    printf("Before: |%s|\n", s);

    char *d = (char*)malloc((strlen(s)+1)*sizeof(char));  
    char *i = (char*)malloc((strlen(x)+1)*sizeof(char));

    reverse(d,s);
    reverse(i,x);

    printf("After: |%s|\n", i);
    printf("After: |%s|\n", d);

    free (i);
    free (d);

    return 0;
}

void reverse(char *dest, char *const s) {
    // create a temporary pointer
    if (strlen(s)==0) return;
    unsigned long offset = strlen(s)+1;

    char *buf = (char*)malloc((offset)*sizeof(char));
    memset(buf, 0, offset);

    char *p;
    // iterate from end to begin and count how much words we have
    for (unsigned long i = offset; i != 0; i--) {
        p = s+i;
        // if we discover a whitespace we know that we have a whole word
        if (*p == ' ' || *p == '\0') {
            // we increment the counter
            if (*p != '\0') {
                // we write the word into the buffer
                ++p;
                int d = (int)(strlen(p)-strlen(buf));
                strncat(buf, p, d);
                strcat(buf, " ");
            }
        }
    }

    // copy the last word
    p -= 1;
    int d = (int)(strlen(p)-strlen(buf));
    strncat(buf, p, d);
    strcat(buf, "\0");

    // copy stuff to destination string
    for (int i = 0; i < offset; ++i) {
        *(dest+i)=*(buf+i);
    }

    free(buf);
}