C Programming - File io parsing strings using sscanff

You need to get the filename from argv if they use the -f option. And you need to get the number of lines from a different argument depending on whether this option was supplied.

Use strcmp() to compare strings, rather than testing each character separately. And use atoi() to convert the lines argument to an integer, since your method only works for single-digit numbers.

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

#define MAXCHAR 1000

function usage() {
    fprintf(stderr, "Usage: reader [-f filename] lines\n");
    exit(1);
}

int main(int argc, char *argv[]) {
    FILE *file;
    char string[MAXCHAR];
    char* filename = "c:\\cprogram\\fileio-activity\\bar.txt";
    int integer;
    int i; //for loops

    if (argc < 2) {
        usage();
    }

    # Process arguments
    if (strcmp(argv[1], "-f") == 0)
    {
        if (argc < 4) {
            usage();
        }
        filename = argv[2];
        integer = atoi(argv[3]);
    } else {
        integer = atoi(argc[1]);
    }

    file = fopen(filename, "r");
    if (file == NULL){
        fprintf(stderr, "Could not open file %s\n",filename);
        return 1;
    }
    while (fgets(string, MAXCHAR, file) != NULL)
        printf("%s", string);
    fclose(file);
    return 0;
}

To add to what Barmar already answered, for the further steps in completing the assignment:

Splitting a string into separate words is usually called tokenization, and we normally use strtok() for this. There are several ways how one can use sscanf() to do it. For example:

1. Use sscanf(string, "%s %s %s", word1, word2, word3) with however many word buffers you might need. (If you use e.g. char word1[100], then use %99s, to avoid buffer overrun bugs. One character must be reserved for the end-of-string character \0.)

   The return value of sscanf() tells you how many words it copied to the word buffers. However, if string contains more than the number of words you specified, the extra ones are lost.

   If the exercise specifies the maximum length of strings, say N, then you know there can be at most N/2+1 words, each of maximum length N, because each consecutive pair of words must be separated by at least one space or other whitespace character.  
    

2. Use sscanf(string + off, " %s%n", word, &len) to obtain each word in a loop. It will return 1 (with int len set to a positive number) for each new word, and 0 or EOF when string starting at off does not contain any more words.

   The idea is that for each new word, you increment off by len, thus examining the rest of string in each iteration.  
    

3. Use sscanf(string + off, " %n%*s%n", &start, &end) with int start, end to obtain the range of positions containing the next word. Set start = -1 and end = -1 before the call, and repeat as long as end > start after the call. Advance to next word by adding end to off.

   The beginning of the next word (when start >= 0) is then string + start, and it has end - start characters.

   To "emulate" strtok() behaviour, one can temporarily save the terminating character (which can be whitespace or the end of string character) by using e.g. char saved = string[off + end];, then replace it with an end-of-string character, string[off + end] = '\0';, so that (string + start) is a pointer to the word, just like strtok() returns. Before the next scan, one does string[off + end] = saved; to restore the saved character, and off += end; to advance to the next word.
    

The first one is the easiest, but is the least useful in practical programs. (It works fine, but we do not usually know beforehand the string length and word count limitations.)

The second one is very useful when you have alternate patterns you can try for the next "word" or item; for example, when reading 2D or 3D vectors (points in a plane, or in three-dimensional space), you can support multiple different formats from <1 2 3> to [1,2,3] to 1 2 3, by trying to parse the most complicated/longest first, and trying the next one, until one of them works. (If none of them work, then the input is in error, of course.)

The third one is most useful in that it describes essentially how strtok() works, and what its side effects are. (It's saved character is hidden internally as a static variable.)