I wrote a program which is used to evaluate postfix expression, I am not getting any compiler error/warnings but I am not getting the correct output, which probably means the issue is with the calculation but I don't know where.

My Code:

#include <ctype.h>
#include <conio.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define MAX 20
char s[MAX], top = 0;

void main() {
    char postfix[MAX], ch;
    int i, op1, op2, res;
    clrscr();
    printf("\n\t\t program to evaluate postfix expression");
    printf("\n\t\t.......");
    printf("\n enter the postfix expression:\n");
    scanf("%s", &postfix);
    for (i = 0; i < strlen(postfix); i  ) {
        ch = postfix[i];
        if (isdigit(ch))
            push(ch = '0');
        else {
            op2 = pop();
            op1 = pop();
            switch (ch) {
              case ' ':
                res = op1   op2;
                break;
              case '-':
                res = op1 - op2;
                break;
              case '*':
                res = op1 * op2;
                break;
              case '/':
                res = op1 / op2;
                break;
              case '^':
                res = pow(op1, op2);
                break;
              default:
                printf("invalid choice");
            }
            push(res);
        }
    }
    printf("result of above expression is:%d\n", pop());
    getch();
}
push(int element) {
      top;
    s[top] = element;
}
int pop() {
    int element;
    element = s[top];
    --top;
    return (element);
}

CodePudding user response：

You should fix the typo and change push(ch = '0'); to

    push(ch - '0');

ch is a character, isdigit(ch), or better isdigit((unsigned char)ch) tells you it is a digit, ch - '0'is the digit value, a number in the range0to9`.

Your code ch = '0' stores the digit '0' into ch and pushes this value, which is the character code or 0 on your system, 48 in ASCII.