Operators not inserting in the stack in C while trying to convert an infix expression to a postfix one

Question

I am trying to implement an infix-to-postfix conversion program. On executing the code I get to see only alpha-numeric characters. Arithmetic operators are not printing. Upon debugging, I found the operators are not inserting in the stack. I couldn't find out the reason behind this.

Any help is appreciated.

#include<stdio.h>
#include <ctype.h>

#define MAX 50

char st[MAX];
int top = -1;
void push(char);
char pop();
int priority(char);

int main()
{
  int i=0;
  char x,s[50];
  printf("Enter infix expression: ");
  gets(s);
  while(s[i]!='\0')
  {
    if(isalnum(s[i]))
      printf("%c",s[i]);
    else if(s[i] == '(')
      push(s[i]);
    else if(s[i] == ')')
    {
      while((x=pop())!='(')
              printf("%c",x);
    }
    else
    {
      while(priority(st[top])>=priority(s[i]))
        printf("%c",pop());
      push(st[i]);
    }
    i++;
  }
  while(top!=-1)
    printf("%c",pop());
}

void push(char x)
{
  st[++top] = x;
}

char pop()
{
  if(top == -1)
    return -1;
  else
    return (st[top--]);
}

int priority(char x)
{
  if(x == '(')
      return 0;
  if(x == '+' || x == '-')
    return 1;
  if(x == '*' || x == '/' || x == '%')
    return 2;
}

Answer 1

As you correctly detected on your debugging session, you don't see operators in your postfix expression because you never push() them to your stack.

In fact

• in the first if you check for alphanumeric characters
• in the following else if you check for opening parenthesis
• in the second else if you check for closing parenthesis
• in the final else you manage pops from stack... but you didn't push anything! ⁽¹⁾

---

What you need to fix is the last else, where you have at least two distinct problems:

1. You access st[top] without checking top value. You need to manage the case in which top = -1, that would lead to out of bounds access of the stack array and to undefined behavior. I think that in that scenario you just need to push the operator
2. You push in the stack st[i]. Probably you meant s[i]

In this way the while analysing the expression becomes

  while(s[i]!='\0')
  {
    if(isalnum(s[i]))
      printf("%c ",s[i]);
    else if(s[i] == '(' )
      push(s[i]);
    else if(s[i] == ')')
    {
      while((x=pop())!='(')
              printf("%c ",x);
    }
    else
    {
      if( top != -1 )
      {
        while(priority(st[top])>=priority(s[i]))
          printf("%c ",pop());
      }
      push(s[i]);
    }
    i++;
  }

Input:

4*6+3

Output:

4 6 * 3 +

(I added a further space after each %c in printfs in order to improve output readability).

---

Note: You still have to fix some problems in operators priority management.