How to create a "X" terminated string in C?-CodePudding

I'm trying to create a counter that counts the amount of characters in a string before "?". I have issues with using strcmp to terminate the while-loop and end up with a segmentation fault. Here's what I have:

void printAmount(const char *s)
{
    int i = 0;
    while ( strcmp(&s[i], "?") != 0 ) {
        i  ;
    }
    printf("%i", i);
}

CodePudding user response：

Don't use strcmp for this. Just use the subscript operator on s directly.

Example:

#include <stdio.h>

void printAmount(const char *s) {
    int i = 0;
    while (s[i] != '?' && s[i] != '\0') {
        i  ;
    }
    printf("%d", i);
}

int main() {
    printAmount("Hello?world"); // prints 5
}

Or use strchr

#include <string.h>

void printAmount(const char *s) {
    char *f = strchr(s, '?');
    if (f) {
        printf("%td", f - s);
    }
}

CodePudding user response：

strcmp() compares strings, not characters. So, if you input is something like "123?456", your logic does not work, because "?" != "?456". Thus, your while loop never terminates and you start using stuff outside the string.

void printAmount(const char * s) {
  int i = 0;
  for (; s[i] != '?' && s[i] != '\0'; i  ) {
     /* empty */
  }
  if (s[i] == '?') {
    printf("%d",i); // the correct formatting string for integer is %d not %i
  }  
}

CodePudding user response：

Unless you have very strange or specialized requirements, the correct solution is this:

#include <string.h>

char* result = strchr(str, '?');
if(result == NULL) { /* error handling */ }

int characters_before = (int)(result - str);