請問這C語言程式碼傳前序式的字串進入eval函式為什麼計算出來的答案都是0.000000，是switch內出問題嗎?

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include <cmath>
#define MAX 256
void inToPrefix(char*, char*);
int priority(char);
double eval(char*); 
double cal(char, double, double);
main()
{ 
    char infix[MAX] = {'\0'}; 
    char prefix[MAX]= {'\0'};
    printf("中序運算式：");
    gets(infix);
    fflush(stdin);
   	inToPrefix(infix, prefix);
    int i;
    printf("前序運算式：");
    for(i = strlen(prefix) -1; i >= 0; i--)
	{
      	 printf("%c", prefix[i]); 
   	}
   	printf("\n");
   	printf("前序運算結果：");
   	printf("%f\n\n", eval(infix));
} 
void inToPrefix(char* infix, char* prefix)
{
	char stack[MAX] = {'\0'};
	int i,j,top;
	for(i = strlen(infix) -1 ,j = 0,top = 0 ; i >= 0 ; i--)
	{
		switch(infix[i])
		{
			case ')':
				stack[++top] = infix[i];
				break;
			case '+': case '-': case '*': case '/':case '%':
				while(priority(stack[top]) > priority(infix[i]))
				{ 
                	prefix[j++] = stack[top--];
            	}
            	stack[++top] = infix[i];
				break;
			case '(':
				while(stack[top] != ')')
				{
					prefix[j++] = stack[top--];
				}
				top--;
				break;
			default:
                prefix[j++] = infix[i];
                break;
		}
	}
	while(top > 0)
	{ 
       	prefix[j++] = stack[top--];
	}
}
int priority(char op)
{
	switch(op)
	{
	case '+': case '-':
		return 1;
		break;
	case '*': case '/':case '%':
		return 2;
		break;
	default:
		return 0;
	}
}
double eval(char *prefix)
{
	char opne[2],stack[MAX];
	int top,i;
	for(top = -1 , i = strlen(prefix) ; i >= 0 ; i--)
	{
		switch(prefix[i])
		{
			case '+':case '-':case '*':case '/':case '%':
				stack[top -1] = cal(prefix[i],stack[top - 1],stack[top]);
				top--;
				break;
			default:
				opne[0] = prefix[i];
				stack[++top] = atof(opne);
				break;
		}
	}
	return stack[top];
}
double cal(char op, double p1, double p2) { 
    switch(op){ 
        case '+': 
			return p1 + p2;
			break; 
        case '-': 
			return p1 - p2;
			break;  
        case '*': 
			return p1 * p2;
			break;  
        case '/': 
			return p1 / p2;
		case '%':
			return fmod(p1,p2);
			break; 
		default:
			break; 
    } 
}