scanf Getting Skipped

Question

I am trying to make a simple C program for a class and one of the requirements is that I'm required to use scanf/printf for all input and output. My question is why is it that my scanf after the for loop in the main gets skipped and the program just terminates.

Here is my code

#include <stdio.h>

void main() {
    int userValue;
    int x;
    char c;

    printf("Enter a number : ");
    scanf("%d", &userValue);
    printf("The odd prime values are:\n");
    for (x = 3; x <= userValue; x = x + 2) {
        int a;
        a = isPrime(x);
        if (a = 1) { 
            printf("%d is an odd prime\n", x);
        }
    }   
    printf("hit anything to terminate...");
    scanf("%c", &c);    
}

int isPrime(int number) {
    int i;
    for (i = 2; i < number; i++) {
        if (number % i == 0 && i != number)
            return 0;
    }
    return 1;
}

I was able to "fix" it by adding another identical scanf after the first one, but I would prefer to just use the one.

Have you tried `system("PAUSE");`? – Cole Tobin Jan 23 '13 at 17:34 — Cole Tobin, Jan 23 '13 at 17:34
is that available in straight C or only C++? – Austin Davis Jan 23 '13 at 18:12 — Austin Davis, Jan 23 '13 at 18:12
Staight c. Notice the lack of a namespace? – Cole Tobin Jan 23 '13 at 21:24 — Cole Tobin, Jan 23 '13 at 21:24

hmjd · Accepted Answer · 2013-01-23T16:44:55.170

24

The new-line character present in stdin after the previous int was entered will not have been consumed by the last call to scanf(). So the call to scanf() after the for loop consumes the new-line character and continues without the user having to enter anything.

To correct without having to add another scanf() call you could use format specifier " %c" in the scanf() after the for loop. This will make scanf() skip any leading whitespace characters (including new-line). Note it means the user will have to enter something other than a new-line to end the program.

Additionally:

check the result of scanf() to ensure it actually assigns a value to the variables passed in:
```
/* scanf() returns number of assigments made. */
if (scanf("%d", &userValue) == 1)
```

this is an assignment (and will always be true):

if (a = 1){ /* Use == for equality check.
               Note 'a' could be removed entirely and
               replace with: if (isPrime(x)) */

edited Jan 23 '13 at 16:44

answered Jan 23 '13 at 16:29

hmjd

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Why would it not have been consumed by the last call to `scanf()`? Sorry newb question – SSH This Jan 23 '13 at 16:31
3

@SSHThis, `scanf("%d")` stops consuming when encounters something that is not a digit, and a new-line is not a digit so it will remain. – hmjd Jan 23 '13 at 16:32
thanks for catching that mistake – Austin Davis Jan 23 '13 at 16:34
1

Thank you, should it be `scanf("%d\n", &userValue);`? – SSH This Jan 23 '13 at 16:34
@SSHThis, yes. That would work also. – hmjd Jan 23 '13 at 16:39
Or use C++ with `std::cin` – Cole Tobin Jan 23 '13 at 17:21
`char temp; scanf("%c", &temp)` – Cole Tobin Jan 23 '13 at 17:28

scanf Getting Skipped

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