C using strtok to parse an input file

Question

Currently trying to parse an input file in C to extract variables.

The input file looks like:

% this is a comment
x 3
% another comment
y 5.0
% one last comment
z 4

x, y and z are predefined variables in my C class. My goal is to parse this file so that int x has the value 3, y has the value 5.0, and z has the value 4. Any line starting with % is ultimately ignored

I've managed to do this using fgets and sscanf - here is the attempt:

while (!feof(inputFile)) {
    fgets(ch,500,inputFile);
    if (sscanf(ch, "x %d", &x)) {
        printf("x: %d\n", x);
    } else if (sscanf(ch, "y %lf", &y)) {
        printf("y: %lf\n", y);
    } else if (sscanf(ch, "z %d", &z)) {
        printf("z: %d\n", z);
    }

and this prints out the desired results. However now I'm trying to use fgets and strtok because I don't think I can get the above code to work with a matrix (i.e. if I had in the input file (note in this case a will also be predefined in my c file):

a
1 -1 3
2 1 6
9 3 0

I would like to store those values in a 3x3 matrix which I don't think is possible using sscanf (especially if the matrix dimensonality is changeable - however it will always be an n * n matrix). My attempt with using fgets and strtok is:

while (!feof(inputFile)) {
    fgets(ch,500,inputFile);
    token = strtok(ch, " ");
    while (token) {
        if (strcmp(token, "x")) {
            printf("%s", "x found");
            // Here is my problem - how do I look at the next token where the value for x is stored

        }
        token = strtok(NULL, " ");
        break;
}
    break;
}

The problem in my code is stated in a comment. I've been thinking about this for a while, trying various things. I think the difficulty comes from understanding how strtok works - initially I was trying to store every token into an array.

Any help is appreciated in helping me work out how I can replicate my existing code to use strtok instead of sscanf so I can then work on parsing matrices.

I know there's a few parsing questions out there but I've seen none that tackle how to parse a matrix as such.

Thanks

Answer 1

Start by breaking the line into an array of tokens. Then, based on the first token, you can use either strtol to strtod to convert the remaining tokens. The following code demonstrates how to break the input lines into an array of tokens

char line[500];
char *token[200];
int tokenCount = 0;
while ( fgets( line, sizeof line, fp ) != NULL )
{
    token[0] = strtok( line, " " );
    if ( strcmp( token[0], "%" ) == 0 )   // skip comments
        continue;

    // break the line into tokens
    int i;
    for ( i = 1; i < 200; i++ )
        if ( (token[i] = strtok( NULL, " " )) == NULL )
            break;
    tokenCount = i;

    // output the token array
    for ( i = 1; i < tokenCount; i++ )
        printf( "%s: %s\n", token[0], token[i] );
}

The for loop generates an array index from 1 to 200. strtok( NULL, " " ) extracts the next token from the line, and returns a pointer to that token (or returns NULL if there are no more tokens). The returned value is stored in token[i]. If the return value was NULL the loop breaks.