努力编写一个简单的计算器

Question

I am trying to program a simple calculator.
Here is my code first:

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

int main(void) { 

char operator = 0;
float num1 = 0.0;
float num2 = 0.0;
float sol = 0.0;


while (operator != 'q') {

    printf("Operator: ");
    scanf("%c", &operator);
    printf("First Number: ");
    scanf("%f", &num1);
    printf("Second Number: ");
    scanf("%f", &num2);

    switch (operator)
    {
    case '+': sol = num1 + num2; break;
    case '-': sol = num1 - num2; break;
    case '*': sol = num1 * num2; break;
    case '/': sol = num1 / num2; break;
    case 'q': printf("Finished!"); exit(0);
    default: printf("Error!"); exit(0);
    }

    printf("The solution is: %.2f\n\n", sol); 
}

return 0;
}

So for me the code is fine. As you can see I did this with a while loop that lets you calculate until you type in 'q' as operator. The first run of the loop works fine but then it gets creepy (my console):

Operator: +
First Number: 5
Second Number: 4
The solution is: 9.00

Operator: First Number:

Why does the program not let me enter an operator in the second loop run?

Answer 1

Most format specifiers with scanf will skip leading whitespace. %c does not.
scanf("%f", &num2); at the end of the first iteration leaves a newline in the input buffer.
scanf("%c", &operator); at the start of the second iteration, reads the newline and proceeds.
using a space before %c in scanf(" %c", &operator); will allow %c to skip the leading whitespace and capture the operator.

Answer 2

You should check scanf for errors:

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

int main(void) {

    char operator = 0;
    float num1 = 0.0;
    float num2 = 0.0;
    float sol = 0.0;

    while (operator != 'q') {

        printf("Operator: ");
        if((scanf(" %c", &operator)) != 1){
            printf("Error, Fix it!\n");
            exit(1);
        }

        printf("First Number: ");
        if((scanf("%f", &num1)) != 1){
            printf("Error, Fix it!\n");
            exit(1);
        }

        printf("Second Number: ");
        if((scanf("%f", &num2)) != 1){
            printf("Error, Fix it!\n");
            exit(1);
        }

        switch (operator){
            case '+': sol = num1 + num2; break;
            case '-': sol = num1 - num2; break;
            case '*': sol = num1 * num2; break;
            case '/': sol = num1 / num2; break;
            case 'q': printf("Finished!"); exit(0);
            default: printf("Error!"); exit(0);
        }

        printf("The solution is: %.2f\n\n", sol);
    }

    return 0;
}

and as you can see I changed scanf("%c", &operator); to this scanf(" %c", &operator); to make scanf to ignore (skip) Whitespace .