Parsing input with scanf in C

Question

I've been having a lot of problems trying to figure out how to use scanf(). It seems to work fine with integers, being fairly straight forward scanf("%d", &i).

Where I am running into issues is using scanf() in loops trying to read input. For example:

do {
  printf("counter: %d: ", counter);
  scanf("%c %c%d", &command, &prefix, &input);
} while (command != 'q');

1. When I enter in a validly structured input like c P101, it seems to loop again before prompting me. This seems to happen even with a single:

   scanf("%c", &c) 
   

   in a while loop. It'll do the loop twice before prompting me again. What is making it loop twice, and how do I stop it?

2. When I enter in less amount of input that programmatically wouldn't have another character or number such as q, pressing enter seems to prompt me to enter more. How do I get scanf() to process both single and double character entries?

Answer 1

When you enter "c P101" the program actually receives "c P101\n". Most of the conversion specifiers skip leading whitespace including newlines but %c does not. The first time around everything up til the "\n" is read, the second time around the "\n" is read into command, "c" is read into prefix, and "P" is left which is not a number so the conversion fails and "P101\n" is left on the stream. The next time "P" is stored into command, "1" is stored into prefix, and 1 (from the remaining "01") is stored into input with the "\n" still on the stream for next time. You can fix this issue by putting a space at the beginning of the format string which will skip any leading whitespace including newlines.

A similiar thing is happening for the second case, when you enter "q", "q\n" is entered into the stream, the first time around the "q" is read, the second time the "\n" is read, only on the third call is the second "q" read, you can avoid the problem again by adding a space character at the beginning of the format string.

A better way to do this would be to use something like fgets() to process a line at a time and then use sscanf() to do the parsing.

Answer 2

It's really broken! I didn't know it

#include <stdio.h>

int main(void)
{
    int counter = 1;
    char command, prefix;
    int input;

    do 
    {
        printf("counter: %d: ", counter);
        scanf("%c %c%d", &command, &prefix, &input);
        printf("---%c %c%d---\n", command, prefix, input);
        counter++;
    } while (command != 'q');
}

counter: 1: a b1
---a b1---
counter: 2: c d2
---
 c1---
counter: 3: e f3
---d 21---
counter: 4: ---e f3---
counter: 5: g h4
---
 g3---

The output seems to fit with Robert's answer.

Answer 3

Once you have the string that contains the line. i.e. "C P101", you can use the parsing abilities of sscanf.

See: http://www.cplusplus.com/reference/clibrary/cstdio/sscanf.html