Infix To Postfix Conversion in C

This post is all about the conversion of an infix expression to a postfix expression.
This program is basically implemented with a stack. This program is done using the c language. Here the main element is the precedence of the operator and the parentheses !

Screenshots[Demo]

The Code

#define SIZE 50            /* Size of Stack */
#include <ctype.h>
char s[SIZE];
int top=-1;       /* Global declarations */
push(char elem)
{                       /* Function for PUSH operation */
    s[++top]=elem;
}
char pop()
{                      /* Function for POP operation */
    return(s[top--]);
}
int pr(char elem)
{                 /* Function for precedence */
    switch(elem)
    {
    case '#': return 0;
    case '(': return 1;
    case '+':
    case '-': return 2;
    case '*':
    case '/': return 3;
    }
}
int main()
{                         /* Main Program */
    char infx[50],pofx[50],ch,elem;
    int i=0,k=0;
    printf("\n\nRead the Infix Expression ? ");
    scanf("%s",infx);
    push('#');
    while( (ch=infx[i++]) != '\0')
    {
        if( ch == '(') push(ch);
        else
            if(isalnum(ch)) pofx[k++]=ch;
            else
                if( ch == ')')
                {
                    while( s[top] != '(')
                        pofx[k++]=pop();
                    elem=pop(); /* Remove ( */
                }
                else
                {       /* Operator */
                    while( pr(s[top]) >= pr(ch) )
                        pofx[k++]=pop();
                    push(ch);
                }
    }
    while( s[top] != '#')     /* Pop from stack till empty */
        pofx[k++]=pop();
    pofx[k]='\0';          /* Make pofx as valid string */
    printf("\n\nPostfix: %s \n",pofx);
}

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