postfix <=> infix

pfix.c

#include <stdio.h>
#include <ctype.h>   // For isdigit() and isalpha()
#include <string.h>  // For strlen()

#define MAX 100      // Maximum size of the stack

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

// Function to push onto the stack
void push(char item) {
    if (top >= MAX - 1) {
        printf("Stack Overflow\n");
    } else {
        stack[++top] = item;
    }
}

// Function to pop from the stack
char pop() {
    if (top < 0) {
        printf("Stack Underflow\n");
        return -1;
    } else {
        return stack[top--];
    }
}

// Function to return precedence of operators
int precedence(char operator) {
    if (operator == '+' || operator == '-') {
        return 1;
    } else if (operator == '*' || operator == '/') {
        return 2;
    } else if (operator == '^') {
        return 3;
    }
    return 0;
}

// Function to check if the character is an operator
int isOperator(char symbol) {
    return (symbol == '+' || symbol == '-' || symbol == '*' || symbol == '/' || symbol == '^');
}

// Infix to Postfix conversion function
void infixToPostfix(char infix[], char postfix[]) {
    int i, j = 0;
    char symbol, temp;
    
    for (i = 0; i < strlen(infix); i++) {
        symbol = infix[i];

        // If the character is an operand (letter or digit), add to the postfix expression
        if (isalnum(symbol)) {
            postfix[j++] = symbol;
        }
        // If the character is an opening parenthesis, push it onto the stack
        else if (symbol == '(') {
            push(symbol);
        }
        // If the character is a closing parenthesis, pop from the stack until '(' is found
        else if (symbol == ')') {
            while ((temp = pop()) != '(') {
                postfix[j++] = temp;
            }
        }
        // If the character is an operator
        else if (isOperator(symbol)) {
            while (top != -1 && precedence(stack[top]) >= precedence(symbol)) {
                postfix[j++] = pop();
            }
            push(symbol);
        }
    }

    // Pop all remaining operators from the stack
    while (top != -1) {
        postfix[j++] = pop();
    }

    postfix[j] = '\0';  // Null terminate the postfix expression
}

int main() {
    char infix[MAX], postfix[MAX];

    printf("Enter an infix expression: ");
    gets(infix);  // Using gets() for simplicity, but it's unsafe in production code

    infixToPostfix(infix, postfix);

    printf("Postfix Expression: %s\n", postfix);

    return 0;
}