How to read only whole numbers in the scanf?

Question

For example, in the code:

int main() {
    int x;

    scanf("%d", &x);          

 printf("%d", x);
}

If I type "A", it automatically converts to whole or just closes the program (both happened to me). then, in this case, it prints 39. how do I make the program accept only whole numbers?

Answer 1

You need to put the #include <stdio.h> for the scanf and the printf.
Also, you need to initialize the "x" variable, otherwise it will have "trash".
If you type "A" and enter, the "A" you entered is not placed in the "x", what the printf shows is the "trash" content of the "x" that you didn’t initialize.

In the example below, if you type "A" the "printf" will always show "-1".

#include <stdio.h>

int main()
{
  int x = -1;
  scanf("%d", &x);
  printf("%d", x);
}

Answer 2

There are basically two ways:

• or creates its own data-reading mechanism and then can control what can be typed
• or checks that the entered data is invalid and asks to be typed again. This is done through a loop by checking if the return of the function indicates that something valid has been entered. This was explained in the question We must disregard the return of functions in C that already receive the desired value by the parameter by reference?.

Answer 3

By putting what you already got in the other code answers, you can test the return of scanf until 1 which signals that it has been able to read 1 entire successfully:

int x;
while (scanf("%d", &x) != 1){ //enquanto não ler 1 inteiro
    static char temp[256];
    fgets(temp, sizeof(temp), stdin); //limpa tudo o que ficou na entrada lendo como string
    printf("Digite um numero ");
}

Although it may seem a bit boring to have to write all this to make sure you read an integer, it’s something you can easily abstract in an auxiliary function:

int ler_inteiro(){
    int num;
    while (scanf("%d", &num) != 1){
        static char temp[256];
        fgets(temp, sizeof(temp), stdin);
        printf("Digite um numero ");
    }
    return num;
}

int main() {
    int x = ler_inteiro();
    int y = ler_inteiro();

    printf("%d %d", x, y);
}

See this example in Ideone

Another possibility a little more laborious would be to always read everything as a string and interpret the data you want the string to read. In case a whole could try to interpret a whole at the expense of strtol and check if you were unsuccessful to re-order another.