ds-lab-codes/InfixToPosfix.c

#include<stdio.h>
#define MAX 20

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

precedence stack[30];
int top = -1;
char EXPR[MAX];

int isp[] = { 0, 19, 12, 12, 13, 13, 13, 0 };   // in-stack precedence
int icp[] = { 20, 19, 12, 12, 13, 13, 13, 0 };  // incoming precedence

void push(precedence token)
{
    stack[++top] = token;
}

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

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 print_token(precedence token)
{
    switch (token)
    {
        case plus:   printf("+");  break;
        case minus:  printf("-");  break;
        case times:  printf("*");  break;
        case divide: printf("/");  break;
        case mod:    printf("%%"); break;
        default:     break;
    }
}

void postfix()
{
    char symbol;
    precedence token;
    int n = 0;

    top = 0;
    stack[0] = eos;

    for (token = get_token(&symbol, &n); token != eos; token = get_token(&symbol, &n))
    {
        if (token == operand)
            printf("%c", symbol);
        else if (token == rparen)
        {
            while (stack[top] != lparen)
                print_token(pop());
            pop(); // pop '('
        }
        else
        {
            while (isp[stack[top]] >= icp[token])
                print_token(pop());
            push(token);
        }
    }

    while ((token = pop()) != eos)
        print_token(token);

    printf("\n");
}

int main()
{
    printf("\nEnter the infix expression\n");
    scanf("%s", EXPR);
    postfix();
    return 0;
}