Infix to Postfix using stack

Question

My lecturer gave me an assignment to create a program to convert an infix expression to postfix using Stack. I\'ve made the stack classes and some functions to read the infix ex

Answer 1

[Program to convert infix to postfix using stack][1]

C program to convert infix to postfix using stackC

#define SIZE 50            
#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;
    }
}

main()
{                         /* Main Program */
    char infix[50],postfix[50],ch,elem;
    int i=0,k=0;
    printf("\n\nEnter Infix Expression : ");
    scanf("%s",infix);
    push('#');
    while( (ch=infix[i++]) != '\0')
    {
        if( ch == '(') push(ch);
        else
            if(isalnum(ch)) postfix[k++]=ch;
            else
                if( ch == ')')
                {
                    while( s[top] != '(')
                        postfix[k++]=pop();
                    elem=pop(); /* Remove ( */
                }
                else
                {       /* Operator */
                    while( pr(s[top]) >= pr(ch) )
                        postfix[k++]=pop();
                    push(ch);
                }
    }
    while( s[top] != '#')     /* Pop from stack till empty */
        postfix[k++]=pop();
    postfix[k]='\0';          /* Make postfix as valid string */
    printf("\nPostfix Expression =  %s\n",postfix);
}

Answer 2

You really should learn how to use a debugger, it's a great tool for figuring out problems like these. However, I ran it and figured out your problem:

On this line:

while(precedence(list->top->data)<=precedence(string[i])){

When you're iterating through the list, you need to check whether the stack is empty each time, not only before you go into the loop. So, do something like this:

while(!isEmpty(list) && precedence(list->top->data)<=precedence(string[i])){

instead.