How to scan enter key as input

Question

I am trying to learn c Language, and I have to create a calculator, the thing is, if I don't type anything and press the enter key it should print out an error, I have tried to do it with scanf but it is not working.

#include <stdio.h>
#include <stdlib.h>
int main()
{
    float a,b,c;
    char op;
    int q=1;


    while(q=1){
   scanf("%f%c%f",&a,&op,&b);



    if (scanf("%f%c%f",&a,&op,&b)=='\n')
    {
        printf("error");

    }


switch (op)
    {
        case '+':c=a+b;
        break;
        case '-':c=a-b;
        break;
        case'*':c=a*b;
        break;
        case'/':c=a/b;
        break;
        default:printf("error");
        q=2;
        break;
    }
    {printf("%f\n",c);}

}}

Answer 1

Try to scan for a newline using a scanset %1[\n].
Another scanset %*[^\n] will scan and discard everything that is not a newline.
Another approach would be to use fgets to read a line and then parse the line with sscanf.

#include <stdio.h>
#include <stdlib.h>
int main ( void) {
    char newline[2] = "";
    char op = 0;
    int scanned = 0;
    double a = 0.0;
    double b = 0.0;
    double c = 0.0;

    while ( 1) {
        printf ( "input a number, an operator and a number\n");
        printf ( "        or just enter to quit\n");
        if ( 1 == scanf ( "%1[\n]", newline)) { // try to scan a newline
            printf ( "done\n");
            break;
        }
        if ( 3 != ( scanned = scanf("%lf %c%lf", &a, &op, &b))) {
            printf ( "bad input error\n");
            printf ( "try again\n");
        }

        if ( EOF == scanned) {
            printf ( "EOF error\n");
            break;
        }

        scanf ( "%*[^\n]"); // scan and discard up to newline
        scanf ( "%1[\n]", newline); // scan a newline

        if ( 3 == scanned) {
            switch (op) {
                case '+':
                    c = a + b;
                    break;
                case '-':
                    c = a - b;
                    break;
                case '*':
                    c = a * b;
                    break;
                case '/':
                    c = a / b;
                    break;
                default:
                    printf ( "bad op error\n");
                    printf ( "try again\n");
                    op = 0;
                    break;
            }
            if ( op) {
                printf ( "%f\n", c);
            }
        }
    }
    return 0;
}