C program strings - c

Question

For part of my program I would like to concatenate two strings together with an asterisk between each character. For example, if I had a first string of "abcde" and a second string of "1234567", I would like to have a result of "a*b*c*d*e*1*2*3*4*5*6*7*".
For this part of the program I have:
char *widen_stars(char *one, char *two)
{
int length_one = strlength(one); // length of first parameter
int length_two = strlength(two); // Length of second parameter
char *p = malloc((sizeof(char) * (length_one + length_two) * 2)+ 1), *p_start; //Allocate enough memory for both strings concatenated together with a * between each letter
p_start = p;
while(*one != '0')
{
if( (p - p_start) % 2 == 0) // Keeps track of where we are in the p string
{
*p = *one;
p++;
one++;
}
else
{
*p = '*';
p++;
}
}
while(*two != '0')
{
if( (p - p_start) % 2 == 0)
{
*p = *two;
p++;
two++;
}
else
{
*p = '*';
p++;
}
}
return p_start;
}
int main(int argc, char *argv[])
{
char first[31]= {0};
char second[31]= {0};
char *f = first, *s = second;
printf("Please enter a string of maximum 30 characters: ");
scanf("%s", f);
printf("Please enter a string of maximum 30 characters: ");
scanf("%s", s);
printf("The combined string is: %s\n", widen_stars(f, s));
}
return 0;
}
However, when I run the program with the above inputs, I get something like "a*b*c*d*e*", without any of the second string. If I block out the first while loop into comments to test the second loop, I get something like "1*2*3*4*5*5*7*", which leaves me scratching my head.

Answer 1

Your problem lies here:
while(*oneOrTwo != '0')
If you're looking for the end of the strings, it's '\0' that you should be looking for, not '0'. The former is the end-of-string marker, the latter is simply the character 0.
And, as an aside, there are much less ..., err, verbose ways to do this (assuming it's not class work - if it is, you should go with your current method). For example:
#include <stdio.h>
#include <string.h>
char *widen_stars(char *one, char *two) {
// Need to cater for memory exhaustion.
char *p = malloc((strlen(one) + strlen(two)) * 2) + 1);
if (p == NULL) return NULL;
// Init to empty string in case both inputs empty.
*p = '\0';
// Save string start for return.
char *p_start = p;
// Add character and asterisk for every character in both strings.
while (*one != '\0') {
sprintf(p, "%c*", *one++);
p += 2;
}
while (*two != '\0') {
sprintf(p, "%c*", *two++);
p += 2;
}
// Remove last asterisk (if needed).
// *(--p) = '\0';
// Return result.
return p_start;
}
That's based on your actual expected results, which place an asterisk after each character. However, your specifications call for an asterisk between each character. If you decide to got for the latter, it's a simple matter of un-commenting the penultimate statement in the function, to basically back up and replace the final asterisk with an end-of-string marker.

Answer 2

The problem in your code is in while conditions, you should increment pointers until '\0' not '0'. So instead of doing:
while(*one != '0')
...
while(*two != '0')
You do it like this:
while(*one != '\0')
...
while(*two != '\0')
And as you are returning a dynamically allocated memory, consider using a pointer to this memory, which you free after usage:
char *str = widen_stars(f, s);
printf("The combined string is: %s\n", str);
free(str);