ds-lab-codes/PostfixEval.c

#include<stdio.h>
#define MAX 40

typedef enum { lparen, rparen, plus, minus, times, divide, mod, eos, operand } precedence;

char EXPR[MAX];
int stack[20];
int top = -1;

precedence get_token(char *symbol, int *n)
{
    *symbol = EXPR[(*n)++];
    switch (*symbol)
    {
        case '(': return lparen;
        case ')': return rparen;
        case '+': return plus;
        case '-': return minus;
        case '*': return times;
        case '/': return divide;
        case '%': return mod;
        case '\0': return eos;
        default:  return operand;
    }
}

void push(int num)
{
    stack[++top] = num;
}

int pop()
{
    return stack[top--];
}

int eval()
{
    precedence token;
    char symbol;
    int op1, op2, n = 0;

    token = get_token(&symbol, &n);

    while (token != eos)
    {
        if (token == operand)
            push(symbol - '0');   // assumes single-digit operands
        else
        {
            op2 = pop();
            op1 = pop();
            switch (token)
            {
                case plus:   push(op1 + op2); break;
                case minus:  push(op1 - op2); break;
                case times:  push(op1 * op2); break;
                case divide: push(op1 / op2); break;
                case mod:    push(op1 % op2); break;
                default:     break;
            }
        }
        token = get_token(&symbol, &n);
    }
    return pop();
}

int main()
{
    int res;
    printf("\nEnter the postfix expression\n");
    scanf("%s", EXPR);
    res = eval();
    printf("\nAfter evaluation:\t%d\n", res);
    return 0;
}