postfix evaluation

#include <iostream>
#include <stack>
#include <string>

using namespace std;

float evaluate(string);
int main(){
    char exp[1000];
    cout<<"Enter a postfix expression separated by','(e.g. 54,6,+):"<<endl;
    cin.getline(exp,1000);
    float x;

    x=evaluate(exp);

    cout<<"Answer is "<<x<<endl;
    return 0;
}

float evaluate(string s){
    int i=0,id=0;
    stack <float> post;

    float op1,op2,result,num=0;
    while(s[i]!='\0'){
        if(isdigit(s[i])){
            num=num*10+(int)(s[i]-'0');
            id=1;
        }
        else if(s[i]==','&&id){
            post.push(num);
            id=0;
            num=0;
        }
        else{
            op2=post.top();
            post.pop();
            op1=post.top();
            post.pop();
            switch(s[i]){
                case '+':result=op1+op2;break;
                case '-':result=op1-op2;break;
                case '*':result=op1*op2;break;
                case '/':result=op1/op2;break;
                case '%':result=(int)op1%(int)op2;break;
            }
            post.push(result);
        }
        i++;
    }
    return result;
}