Prefix Evaluation

C Program For Prefix Evaluation

Source Code:

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

#define SIZE 20

typedef struct Stack
{
	int a[50];
	int top;
}Stack;

void push(Stack *s, int c)
{
	s->a[++s->top] = c;
}

int pop(Stack *s)
{
	return s->a[s->top--];
}

char* strrev(char str[])
{
	int i, j = 0;
	char o[50];
	for (i = 0; i < strlen(str) ; ++i)
	{
		o[j++] = str[strlen(str) - i - 1] ;
	}
	j = 0;
	for (i = 0; i < strlen(str); ++i)
	{
		str[i] = o[j++];
	}
	str[i] = '\0';
	return str;
}


void evalPrefix(char prefixExp[])
{
	Stack *s = (Stack*)malloc(sizeof(Stack));
	int i, l, r, res;
	s->top = -1;
	strrev(prefixExp);
	for (i = 0; prefixExp[i] != '\0'; i++) {
		if(isdigit(prefixExp[i])) {
			push(s, prefixExp[i] - '0');
		}
		else {
			l = pop(s);
			r = pop(s);
			switch(prefixExp[i]) {
				case '+': push(s, l + r); break;
	            case '-': push(s, l - r); break;
	            case '*': push(s, l * r); break;
	            case '/': push(s, l / r); break;
			}
		}
	}
	res = pop(s);
	printf("Ans: %d.\n",res);
}

int main()
{
	char prefixExp[40];
	
	printf("Enter The Prefix Expression: ");
	scanf("%s",prefixExp);

	evalPrefix(prefixExp);
	return 0;
}