scanf not reading properly because of gets function

Question

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

int main(){
    int n=1,i,cont;
    char string[50];

    scanf("%d",&n);
    while(n!=0){
        gets(string);
        cont=0;
        for(i=0;i<strlen(string);i++){
            if(string[i]=='.'){
                cont++;
            }
        }
        if(cont%2==0){
            printf("S\n");
        }else{
            printf("N\n");
        }
        scanf("%d",&n);
    }
    return 0;
}

My problem is quite simple but troublesome, I want to read an integer value n, and then read a string, after that read n again, but whenever I run the program, it only reads the string value... but if I digit 0 the program ends... it's like my scanf is within the gets function.

Answer 1

Mixing scanf with gets or fgets is troublesome because they each handle newlines differently.

Get rid of the gets call (which is unsafe anyway) and replace it with the following scanf call:

scanf("%49s", string);

This will read at most 49 characters into string (i.e. one less that its size).

Answer 2

From OP's comments, it sounds like the goal is to be able to read strings containing spaces. While there are ways to accomplish this using scanf(), it would be better to use fgets(), which is at the least less error-prone.

The fgets() function can be used to read input for the number into a buffer, and this buffer can then be processed by sscanf() to extract the number. Since fgets() keeps the newline character, it is not left behind to interfere with the next I/O operation.

But, when fgets() is used to get the string, since the newline is retained, it may be desirable to remove it. This can be accomplished in a number of ways, but here strcspn() is used to provide the index of the first \r or \n character encountered; a \0 character is then written to this location, removing the terminating newline from the string.

The code below illustrates these suggestions. Note that both buffer[] and string[] are generously allocated to accommodate reasonably large inputs. If a user enters a large number of characters (more than 999 in this case), the extra characters are left behind in the input stream for the next I/O function call. Also note that the main loop has been streamlined a bit; now there is a for(;;) loop that never terminates, broken out of when the user enters 0 for the number. And, there is a nested loop within the main loop that prompts the user to enter a number until a valid number is entered. Since the #include <stdlib.h> was unnecessary, it was removed. Better code would check the values returned from the calls to fgets() for possible errors.

#include<stdio.h>
#include<string.h>

int main(void)
{
    int n = 1, cont;
    char buffer[1000];
    char string[1000];

    for (;;) {

        /* Loop until user enters a number */
        do {
            printf("Please enter a number: ");
            fgets(buffer, sizeof buffer, stdin);
        } while (sscanf(buffer, "%d", &n) != 1);

        /* Break on 0 */
        if (n == 0) break;

        /* Get a string, and remove trailing newline */
        printf("Please enter a string\n");
        fgets(string, sizeof string, stdin);
        string[strcspn(string, "\r\n")] = '\0';

        cont = 0;
        for (size_t i = 0; i < strlen(string); i++) {
            if (string[i] == '.') {
                cont++;
            }
        }
        if (cont % 2 == 0){
            printf("S\n");
        } else {
            printf("N\n");
        }
    }

    return 0;
}

Answer 3

When you enter 5 for an example, you hit a new line character afterwards. So you are entering 2 characters: 5 and a new line character. That new line character is causing your headache.

The new line character is also considered an input.

In order to ignore this new line char, simply add a new line that acts as a garbage collection:

 char garbage[50];

 scanf( "%d", &n);
 fgets(garbage, sizeof(garbage), stdin);