Infix to post fix code

[lost110]

The last else will not run if the first if condition has worked. So how will we check the priority? Also, I couldn't understand the last two else conditions in main.

#include<iostream>
using namespace std;
char stack[20];
int top = -1;
void push(char x)
{
	stack[++top] = x;
}

char pop()
{
	if (top == -1)
		return -1;
	else
		return stack[top--];
}

int priority(char x)
{
	if (x == '(')
		return 0;
	if (x == '+' || x == '-')
		return 1;
	if (x == '*' || x == '/')
		return 2;
}

int main()
{
	char exp[20];
	char *e, x;
	cout << "Enter the expression : " << endl;
	cin >> exp;
	e = exp;
	while (*e != '\0')
	{
		if (isalnum(*e))
		{
			cout << *e;
		}
		else if (*e == '(')
		{
			push(*e);
		}
		else if (*e == ')')
		{
			while ((x = pop()) != '(')
				printf("%c", x);
		}
		else
		{
			while (priority(stack[top]) >= priority(*e))
				cout << pop();
			push(*e);
		}
		e++;
	}
	while (top != -1)
	{
		cout << pop();
	}
	system("pause");
}