Check if the input is an integer

Question

I need a function that reads a keyboard input and validates whether it is an integer (negative or positive). If it is, it should return the value. Otherwise, it should report that the entry is invalid for the user and ask again.

The function I wrote validates this, however, it fails if the user reports characters mixed with numbers. Besides, there is also a problem with printing. The printf runs the number of times the user has typed an invalid character. From now on, thank you for your patience.

Examples of Input and Output:

Input 15 Output Return 15;

Input -5 Exit Return -5;

Entry 0 Exit Return 0;

Input asefga*/+. Output Printf("Invalid Number") scanf Again;

Entry sdasddddas55546 Output Printf("Invalid Number") scanf Again;

Basically, the scanf must return any int, and continue while while some char is typed;

int InserirValido(){

    int valor;
    int x=1;

    do
    {
        x =scanf("%d", &valor);
        getchar();

        if(x==0)
            printf("Numero Inválido");

    }while(x==0);

    return valor;
}

Answer 1

The problem here is that you’re using scanf, who’s interpreting the entry as an indepentende number of what’s really there. What you can do is read your input as text and then use the function strtol in your input. This function already does both: converts to number and reports errors. A code that does this would look like this:

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

long ler_apenas_se_for_inteiro();

int main() {
    printf("%ld", ler_apenas_se_for_inteiro());
    return 0;
}

long ler_apenas_se_for_inteiro(){
    char buffer_de_entrada[64];
    char* onde_a_funcao_de_conversao_parou;

    while (1) {
        fgets(buffer_de_entrada, 64, stdin);
        long numero_lido = strtol(buffer_de_entrada, &onde_a_funcao_de_conversao_parou, 0);

        /* Se o número resultante é zero e o ponteiro que aponta para o caractere da string
         * no final da conversão não andou é porque não ocorreu conversão, ou seja, falhou. Além disso, o caractere onde a conversão parou
         * tem que ser uma quebra de linha, caso contrário há caracteres não numéricos
         * que foram ignorados pelo strtol, o que significa que a entrada 
         * não possui somente números.
         */
        if ((numero_lido == 0 && onde_a_funcao_de_conversao_parou == buffer_de_entrada)
            || *onde_a_funcao_de_conversao_parou != '\n') {
            printf("Número inválido. Digite novamente.\n");
            continue;
        } else {
            return numero_lido;
        }
    }

}

The function is returning long. You can adapt convert to int if necessary. You may also want to place more validations if the person type a number larger than the size of the long. Behold Documentation of the function for more details.