How to extract various integers from a string in C?

Question

I was wondering how to extract various numbers from a string. I understand that strtol works, however it appears to only work for the first digit.

Here is my code

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

int main(){
    long v1, v2, v3;
    char str[20] = "16,23";
    char *d;
   v1 = strtol(str, &d, 10);
   v2 = strtol(str, &d, 10);
   printf("string is %s\nv1 is:%i\nv2 is:%d\n",str , v1,v2);
   return 0;
}

In this example I would like to output v1 = 16 and v2 = 23.

Another example, if the str was "12,23,34", I would like v3= 34

Thanks in advance :)

This is called ***tokenizing*** a string. If you search for `tokenize c` you'll get many useful results. Here is a related question here on SO. http://stackoverflow.com/questions/53849/how-do-i-tokenize-a-string-in-c — Dan, Oct 01 '15 at 15:16
Then you can tokenize string using strtok_r function and then convert tokens to integers using strtol or atoi — Gangadhar, Oct 01 '15 at 15:18

score 2 · Answer 1 · answered Oct 01 '15 at 15:15

You can have many approaches. One of them is to make use of the endptr, populated by the previous strtol() call as the source of the next strtol().

Otherwise, for a better and flexible approach, you also have an option of using strtok() with a predefined delimiter (the , here) to get the tokens one by one and convert them to int or long (as you wish) until strtok() returns NULL.

chux - Reinstate Monica · Answer 2 · 2015-10-01T15:59:10.833

Use long strtol(const char * nptr, char ** endptr, int base). The endptr allows for easy subsequent parsing as that is where parsing stopped.

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

int string_to_longs(const char *s) {
  #define N 3
  long v[N];
  int i;
  for (i=0; i<N; i++) {
    errno = 0;
    char *end;
    v[i] = strtol(s, &end, 10);

    if (errno) return -1; // overflow
    if (s == end) return -1; // no conversion
    printf("v[%d] = %ld\n", i, v[i]);
    if (*end == 0) break; // we are done
    if (*end != ',') return -1; // missing comma
    s = (const char *) (end + 1);
  }
  return i;
}

int main(void) {
  string_to_longs("16,23");
  string_to_longs("12,23,34");
  return 0;
}

vdaras · Answer 3 · 2015-10-01T15:21:55.683

1

strtol just converts a character array to a long int. It stops when it finds the first character that wouldn't make sense into interpreting an integer from.

There is a function in string.h named strtok which helps you tokenize a string.

Beware that strtok mutates the original character array contents.

edited Oct 01 '15 at 15:21

answered Oct 01 '15 at 15:18

vdaras

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How to extract various integers from a string in C?

3 Answers3