Question

I’m new to C, and I’m trying to scan a line from stdin and extract the nth word from it.
Right now I’ve hard-coded it where you can store the first, second, or third entry in the sentence, and this is what it looks like:

int set_to_nth_word(char* word, char* input, int n)
{
    char word1[20];
    char word2[20];
    char word3[20];
    if(sscanf(input, "%s %s %s", word1, word2, word3) < n)
    {
        printf("You didn't enter enough values\n");
        return 0;
    }
    else
    {
        if(n == 1) strcpy(word, word1);
        else if(n == 2) strcpy(word, word2);
        else if(n == 3) strcpy(word, word3);
        return 1;
    }
}

The code that calls this method is:

char *input = (char *) malloc (1);
if(getline(&input, (size_t)0, stdin) != -1)
{
    char word[20];
    if(set_to_nth_word(word, input, 1))
    {
        printf("Success");
    }
}

Besides finding a solution to this problem, I’d be happy if anyone points out any bad style or bad coding practices!

Answer 1

You can make use of %n conversion specifier supported by sscanf(). It requires an int * parameter, and returns the number of characters consumed from the input into that int.

int set_to_nth_word(char *word, const char *input, int n)
{
    int chars_used;

    word[0] = '\0';    /* In case n < 1 */

    while (n > 0 && sscanf(input, "%s%n", word, &chars_used) > 0)
    {
        input += chars_used;
        n--;
    }

    if (n > 0)
    {
        printf("You didn't enter enough values\n");
        return 0;
    }

    return 1;
}

As far as style goes, you should make the input parameter const char *, since the characters pointed to are not being modified in the function.

In terms of safety, word should be allocated with a length of strlen(input) + 1, rather than declared as a fixed-size array, since the words may be up to that length.