infixToPostfix.c

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

char stack[100];
int top = -1;

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

int pop()
{
    if (top == -1)
        return -1;
    else
        return stack[top--];
}

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

int main()
{
    char exp[100];
    char *e, x;
    printf("Enter an expression in infix form\n");
    scanf("%s", exp);

    printf("The postfix expression is: ");
    e = exp;
    while (*e != '\0')
    {
        if (isalnum(*e))
        {
            printf("%c", *e);
        }
        else if (*e == '(')
            push(*e);
        else if (*e == ')')
        {
            while ((x == pop()) != '(')
                printf("%c", x);
        }
        else
        {
            while (priority(stack[top] > priority(*e)))
            {
                printf("%c", pop());
            }
            push(*e);
        }
        e++;
    }
    while (top != -1)
        printf("%c", pop());

    return 0;
}