Program to evaluate postfix expression

postfix_eval.c

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

#define SIZE 40

int pop();
void push(int);

char postfix[SIZE];
int stack[SIZE], top = -1;

int main()
{
	int i, a, b, result, pEval;
	char ch;
	
	for(i=0; i<SIZE; i++)
	{
		stack[i] = -1;
	}
	printf("\nEnter a postfix expression: ");
	scanf("%s",postfix);

	for(i=0; postfix[i] != '\0'; i++)
	{
		ch = postfix[i];

		if(isdigit(ch))
		{
			push(ch-'0');
		}
		else if(ch == '+' || ch == '-' || ch == '*' || ch == '/')
		{
			b = pop();
			a = pop();
			
			switch(ch)
			{
				case '+': result = a+b;
					  break;
				case '-': result = a-b;
					  break;
				case '*': result = a*b;
					  break;
				case '/': result = a/b;
					  break;
			}
			
			push(result);
		}		
	
	}

	pEval = pop();
	
	printf("\nThe postfix evaluation is: %d\n",pEval);
	
	return 0;
}

void push(int n)
{
	if (top < SIZE -1)
	{
		stack[++top] = n;
	}
	else
	{
		printf("Stack is full!\n");
		exit(-1);
	}
}
int pop()
{	
	int n;
	if (top > -1)
	{
		n = stack[top];
		stack[top--] = -1;
		return n;
	}
	else
	{
		printf("Stack is empty!\n");
		exit(-1);
	}
}	

    if(isdigit(ch))
    {
        push(ch-'0');
    }

i cant understand push(ch-'0');

if we just write push(ch) then we will be able to insert the ASCII value of that character 'ch' but not the actual value of 'ch'. So to get the actual value of 'ch' we must subtract ASCII value of 0 from the ASCII value of character.
Example: we need to push 1. ASCII value of 1 is 49 and the ASCII value of 0 is 48. So 49-48=1.

stack[i] = -1; what does it mean?

    if(isdigit(ch))
    {
        push(ch-'0');
    }

i cant understand push(ch-'0');

if we just write push(ch) then we will be able to insert the ASCII value of that character 'ch' but not the actual value of 'ch'. So to get the actual value of 'ch' we must subtract ASCII value of 0 from the ASCII value of character. Example: we need to push 1. ASCII value of 1 is 49 and the ASCII value of 0 is 48. So 49-48=1.

thank you

stack[i] = -1; what does it mean?

stack is empty