How do I parse out the fields in a comma separated string using sscanf while supporting empty fields?

Question

I have a comma separated string which might contain empty fields. For example:

1,2,,4

Using a basic

sscanf(string,"%[^,],%[^,],%[^,],%[^,],%[^,]", &val1, &val2, &val3, &val4);

I get all the values prior to the empty field, and unexpected results from the empty field onwards.

When I remove the expression for the empty field from the sscanf(),

sscanf(string,"%[^,],%[^,],,%[^,],%[^,]", &val1, &val2, &val3, &val4);

everything works out fine.

Since I don't know when I'm going to get an empty field, is there a way to rewrite the expression to handle empty fields elegantly?

Answer 1

If you use strtok with the comma as your separator character you'll get a list of strings one or more of which will be null/zero length.

Have a look at my answer here for more information.

Answer 2

man sscanf:

[ Matches a nonempty sequence of characters from the specified set of accepted characters;

(emphasis added).

Answer 3

This looks like you are currently dealing with CSV values. If you need to extend it to handle quoted strings (so that fields can contain commas, for example), you will find that the scanf-family can't handle all the complexities of the format. Thus, you will need to use code specifically designed to handle (your variant of) CSV-format.

You will find a discussion of a set CSV library implementations in 'The Practice of Programming' - in C and C++. No doubt there are many others available.

Answer 4

scanf() returns the number of items assigned. Maybe you can use that info ...

char *data = "1, 2,,, 5, 6";
int a[6];
int assigned = sscanf(data, "%d,%d,%d,%d,%d,%d", a, a+1, a+2, a+3, a+4, a+5);
if (assigned < 6) {
    char fmt[18];
    switch (assigned) {
        default: assert(0 && "this did not happen"); break;
        case 0: fmt = ",%d,%d,%d,%d,%d"; break;
        case 1: fmt = "%d,,%d,%d,%d,%d"; break;
        case 2: fmt = "%d,%d,,%d,%d,%d"; break;
        case 3: fmt = "%d,%d,%d,,%d,%d"; break;
        case 4: fmt = "%d,%d,%d,%d,,%d"; break;
        case 5: fmt = "%d,%d,%d,%d,%d,"; break;
    }
    sscanf(data, fmt, a+(assigned<=0), a+1+(assigned<=1), a+2+(assigned<=2),
                      a+3+(assigned<=3), a+4+(assigned<=4));
}

Ugh! And that's only for 1 missing value
As has been pointed out by other answers, you're much better off parsing the string in the 'usual' way: fgets() and strtok().