how do I parse a ',' separated char string using c?

Question

What is the easiest way of parsing a comma separated list, where there can be zero elements between each token. The cstring could look like

1, 3, 4, 5, 6, 7, 8, ....

But could also look like

, , , , , , , , , ...

I've tried something like:

char *original = "1, 3, 4, 5, 6, 7, 8, ...."
char *tok = strtok(original," ,")
while(tok!=NULL){
    while(*tok!='\0'){
      //dostuff
      tok++;
    }
tok=strtok(NULL," ,");
}

This apparently only works, if there are elements between the comma's, for instance I've noticed that the first item list will be skipped if there are no elements.

I've tried other solutions like strchr(), but this gets very ugly, and I think there is an easier way.

Thanks

Update:

After some testing I noticed that tokenizing on "," seemed to work, on all cases except if the first item was missing. So I'm pulling that out as a special case.

char *original = "1, 3, 4, 5, 6, 7, 8, ...."
if(*original==',')
  //dostuff    
char *tok = strtok(original,",")
while(tok!=NULL){
    while(*tok!='\0'){
      //dostuff
      tok++;
    }
tok=strtok(NULL,",");
}

Thanks for your input and your help. (Maybe I should have given this a more careful thought before posting.)

Answer 1

strtok cannot cannot distinguish between `,` and `,,`.

Answer 2

You might want to look into the nonstandard strsep, which is designed to be a replacement for strtok which allows parsing of empty fields. See also the glibc manual chapter on Finding Tokens in a String. It's available on many systems (various BSDs, Linux, Mac OS X), but is not standardized, so I believe it may not be present on Windows or Solaris.

Answer 3

How about a simple for loop?

for (int begin = 0; original[begin]; ) {
  int end = begin;
  while (original[end] && original[end] != ',')
    ++end;

  // do something with original[begin] through original[end-1]

  begin = end;
}

Answer 4

If all you need to do is ignore empty "tokens", you can use the strspn function to detect whitespace-only strings. Here's an example:

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


/* Is the given string whitespace only?
*/
int iswhitespace(char* s)
{
    return (strspn(s, " \t") == strlen(s));
}


int main()
{
    char line[] = "1, , 3, 4, 5, 6";
    char sep[] = ",";
    char* tok;

    tok = strtok(line, sep);

    while (tok)
    {
        if (iswhitespace(tok))
            printf("empty token\n");
        else
            printf("new token: %s\n", tok);

        tok = strtok(0, sep);
    }

    return 0;
}

The key idea here is to tokenize on a comma only, and not " ," which skips the first element. Whitespace can then be handled separately.

Of course this still leaves the fact that strtok will skip spans of consecutive commas. If this isn't good for you, you can't use strtok and will have to employ another solution.