simple extensible calculator

// TODO: Check return val of scanf()
#include <stdio.h>
#include <math.h>
#include <stdlib.h>

#define ARRAY_SIZE(x) (sizeof(x)/sizeof (x)[0])
#define CLEAR_STDIN do {} while ( (c = getchar()) != '\n' && c != EOF )

double _sum(double x, double y) { return x + y;}
double _sub(double x, double y) { return x - y;}
double _mul(double x, double y) { return x * y;}
double _div(double x, double y) { return x / y;}
double _curt(double x) { return pow(x, 1.f / 3);}
double _quit(void) { exit(0); return 0;}

struct
{
    const char *description;
    const char *symbol;
    int operands;
    double (*function)();
} ops[] = {{"sum", "+", 2, _sum}    ,{"sub", "-", 2, _sub}
      ,{"mul", "*", 2, _mul}    ,{"div", "/", 2, _div}
      ,{"pow", "^", 2, pow} ,{"abs", " ", 1, fabs}
      ,{"sqrt", "√", 1, sqrt},{"curt", "∛", 1, _curt}
       {"exit", "⎋", 0, _quit}
};


int main (void)
{
    int i, c;
    printf("****\n");
    for (i = 0; i < ARRAY_SIZE(ops); i++){
        printf("%d- %s (%s)", i + 1, ops[i].description, ops[i].symbol);

        if (i % 2)  printf("\n");
        else        printf("\t\t\t\t\t");
    }

    unsigned choice;
    double x,y;
    for (;;){
        printf("\n****\nSelect:  ");
        scanf("%u", &choice); choice--;
        CLEAR_STDIN;
        if (choice > i){
            printf ("Invalid; try again.");
            continue;
        }
        switch (ops[choice].operands){
        case 0: // constant or x(void) function
            printf("=%lg", ops[choice].function());
        break;
        case 1: // function with 1 parameter
            printf("Enter Number: ");
            scanf("%lf", &x);
            printf("%s(%lg) = %lg", ops[choice].description, x, ops[choice].function(x));
        break;
        case 2: // function with 2 parameters
            printf("Enter Number 1: ");
            scanf("%lf", &x);
            printf("Enter Number 2: ");
            scanf("%lf", &y);
            printf("%lg %s %lg = %lg", x,ops[choice].symbol, y,ops[choice].function(x,y));
        break;
        }
    }

}