Infix_postfix_eval.c

/*
* C Program to convert a given infix expression to postfix expression and evaluate it.
* (c) www.c-program-example.com
*/

#define SIZE 50            /* Size of Stack */
#include <ctype.h>
#include <stdio.h>

char s[SIZE];
int top = -1; /* Global declarations */

/* Function to remove spaces from given string */
void RemoveSpaces(char* source) {
	char* i = source;
	char* j = source;
	while(*j != 0) {
		*i = *j++;
		if(*i != ' ')
			i++;
	}
	*i = 0;
}

/* Function for PUSH operation */
void push(char elem) { 
	s[++top] = elem;
}

/* Function for POP operation */
char pop() { 
	return (s[top--]);
}

/* Function for precedence */
int pr(char elem) { 
	switch (elem) {
		case '#':
			return 0;
	case '(':
		return 1;
	case '+':
	case '-':
		return 2;
	case '*':
	case '/':
		return 3;
	}
}

/*
* Function to convert from infix to postfix expression
*/
void infix_to_postfix(char *infix, char *postfix) {
	char ch, elem;
	int i = 0, k = 0;
	
	RemoveSpaces(infix);
	push('#');
	
	while ((ch = infix[i++]) != '\n') {
		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 */
}

/*
* Function to evaluate a postfix expression
*/
int eval_postfix(char *postfix) {
	char ch;
	int i = 0, op1, op2;
	while((ch = postfix[i++]) != 0) {
		if(isdigit(ch)) 
			push(ch-'0'); /* Push the operand */
		else {        /* Operator,pop two  operands */
			op2 = pop();
			op1 = pop();
			switch(ch) {
				case '+' : push(op1+op2); 
				break;
				case '-' : push(op1-op2); 
				break;
				case '*' : push(op1*op2);
				break;
				case '/' : push(op1/op2);
				break;
			}
		}
	}
	return s[top];
}

void main() { /* Main Program */
	
	char infx[50], pofx[50];
	printf("\nInput the infix expression: ");
	fgets(infx, 50, stdin);
	
	infix_to_postfix(infx, pofx);

	printf("\nGiven Infix Expression: %sPostfix Expression: %s", infx, pofx);
	top = -1;
	printf("\nResult of evaluation of postfix expression : %d", eval_postfix(pofx));
}