CODEWITHSUNDEEP
cscanfansi-c
Orcinuss
Orcinuss

Reputation: 39

C - Using spaces in sscanf()

Im trying to parse an input string in the format x y z (spaces included) so that it will exit the loop when the entered string is valid. However, when I run this code, it outputs "incorrect format" 3 times and each time, valid = 1. Is there any other way to get this to work without having to parse it manually?

int valid = 0, x, y, z;
char str[20];
while (valid != 3) {
  scanf("%s", str);
  valid = sscanf(str, "%d %d %d", &x, &y, &z);
  if (valid != 3) {
    printf("Incorrect format\n");
  }
}

Upvotes: 0

Views: 771

Answers (2)

David C. Rankin
David C. Rankin

Reputation: 84579

In your code, the primary stumbling block you create is to attempt a read with scanf ("%s", str). The %s format specifier will only read characters up to the first whitespace encountered. If you enter 3-integers separated by a space, you will never read more than the first one into str. This makes it impossible to parse 3-integer values with sscanf from str that can contain no more than 1.

To correct the problem, either read 3-integer values directly with scanf, e.g.

#include <stdio.h>

int main (void) {
    
    int x, y, z;
    printf ("enter 3 integers: ");
    if (scanf ("%d %d %d", &x, &y, &z) != 3) {
        fprintf (stderr, "error: invalid input.\n");
        return 1;
    }
    printf ("x : %d\ny : %d\nz : %d\n", x, y, z);
    
    return 0;
}

Or, the preferred way to handle line-oriented input is by using the line-oriented input functions fgets or POSIX getline. This will avoid the many pitfalls with taking user input with scanf that new C programmers routinely fall into. Never, never, never use gets, it is so insecure it has been removed from the C11 library.

An equivalent example using fgets and sscanf would be:

#include <stdio.h>

#define MAXC 128

int main (void) {
    
    int x, y, z;
    char buf[MAXC] = "";
    printf ("enter 3 integers (on a single line): ");
    if (fgets (buf, MAXC, stdin) == NULL) {
        fprintf (stderr, "error: user canceled input.\n");
        return 1;
    }
    if (sscanf (buf, "%d %d %d", &x, &y, &z) != 3) {
        fprintf (stderr, "error: invalid input.\n");
        return 1;
    }
    printf ("x : %d\ny : %d\nz : %d\n", x, y, z);
    
    return 0;
}

Example Use/Output

(it is the same for both)

$ ./bin/rd3int
enter 3 integers: 10 12 14
x : 10
y : 12
z : 14

Look things over and let me know if you have further questions.

Upvotes: 4

delta
delta

Reputation: 3818

scanf("%s", str) scans text until space occurs. So for input 1 2 3, str is read 3 times, with content 1, 2, 3.

Try using gets(str) instead.

Upvotes: 0

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