Postfix Expression - Checking for Spaces - c

Question

So I am trying to add an if statement in my main function to check if there is some whitespace. If so it would just go on to the next number/operator in line. So for example, if I would type 2 3 4 * +, I should get 14. However, when I run the code I would get a random number. When I do it with no spaces such as 234*+, then I would get 14. Could anyone tell me what I'm doing wrong and how I would fix this?
#include <ctype.h>
#include <stdio.h>
#define MAX 20
typedef struct stack {
int data[MAX];
int top;
} stack;
int evaluate(char x, int op1, int op2) {
if (x == '+')
return (op1 + op2);
if (x == '-')
return (op1 - op2);
if (x == '*')
return (op1 * op2);
if (x == '/')
return (op1 / op2);
if (x == '%')
return (op1 % op2);
}
void init(stack *s) {
s->top = -1;
}
void push(stack *s, int x) {
s->top = s->top + 1;
s->data[s->top] = x;
}
int pop(stack *s) {
int x;
x = s->data[s->top];
s->top = s->top - 1;
return (x);
}
int main() {
stack s;
char x;
int op1, op2, val;
init(&s);
printf("Enter a Postfix Expression: ");
while ((x = getchar()) != '\n') {
if (isdigit(x))
push(&s, x - 48); //x-48 for removing the effect of ASCII
if (isspace(x))
continue;
else {
op2 = pop(&s);
op1 = pop(&s);
val = evaluate(x, op1, op2);
push(&s, val);
}
}
val = pop(&s);
printf("\nValue of expression=%d",val);
return 0;
}

Answer 1

First of all, char x needs to be replaced with int x. The value returned by getchar won't always fit in a char. But that's not the problem you asked about.
If the character is a digit, you treat it as a digit and a operator. This
if(isdigit(x))
push(&s,x-48); //x-48 for removing the effect of ASCII
if (isspace(x))
continue;
else
{
op2=pop(&s);
op1=pop(&s);
val=evaluate(x,op1,op2);
push(&s,val);
}
should be
if (isdigit(x)) {
push(&s, x-48);
}
else if (isspace(x)) {
continue;
}
else {
int op2 = pop(&s);
int op1 = pop(&s);
int val = evaluate(x, op1, op2);
push(&s, val);
}
or just
if (isdigit(x)) {
push(&s, x-48);
}
else if (!isspace(x)) {
int op2 = pop(&s);
int op1 = pop(&s);
int val = evaluate(x, op1, op2);
push(&s, val);
}
You shouldn't count on a trailing line feed. So you should really have the following:
while ( ( x = getchar() ) != EOF ) {
if (isspace(x))
continue;
if (isdigit(x)) {
push(&s, x-48);
} else {
int op2 = pop(&s);
int op1 = pop(&s);
int val = evaluate(x, op1, op2);
push(&s, val);
}
}
Finally, you should handle someone entering 23++, 234+ or 23!.