Postfix to Evaluation

#include <stdio.h>
#include <ctype.h>
#include <math.h>
int stack[100];
int top=-1;

void push(int element)
{
    stack[++top]=element;
}

int pop()
{
    return(stack[top--]);
}

int main()
{
    char postfix[100],ch,j;
    int i=0,a,b;
    printf("Expression:\n");
    scanf("%s",postfix);
    while( (ch=postfix[i++]) != '\0')
    {
        if(isdigit(ch))
            push(ch-'0');
        else
        {
            b=pop();
            a=pop();
            if(ch=='+')
                push(a+b);
            if(ch=='-')
                push(a-b);
            if(ch=='*')
                push(a*b);
            if(ch=='/')
                push(a/b);
            if(ch=='^')
                push(pow((float)a,b));

        }
    }
    printf("\nPostfix: %s\n",postfix);
    printf("\nEvaluation: %d\n",stack[top]);
    getchar();
    getchar();
}