表达式从左向右求值（所有运算符的优先级都一样，仅包括+ - * /）

#include <stdio.h>

int main(void){
    float a, b;
    char ch;
    
    printf("Enter an expression:");
    scanf("%f", &a);
    printf("%.3f", a);
    ch = getchar();
    putchar(ch);
    while (ch != ‘\n‘) {
        scanf("%f", &b);
        printf("%.3f", b);
        switch(ch){
            case ‘+‘:
                a += b;
                break;
            case ‘-‘:
                a -= b;
                break;
            case ‘*‘:
                a *= b;
                break;
            case ‘/‘:
                a /= b;
                break;
        }
        ch = getchar();
        putchar(ch);
    }
    printf("%.3f\n", a);
    return 0;
}