Thread: calculating postfix problem

I don't know what's going wrong with this. Here's the code that I think is messing it up: the main evaluation process, of course.
postfix: 8 4 + = 8 + 4

Code:

int evaluatePostfixExpression(char postfix[])
{
    int i, length;
    int x,y;
    STACK stack;

    initStack(&stack); // initializze stack
	get_postfix(postfix); // get postfix expression from user
    length = strlen(postfix);  

    // if strlen if postfix is more than zero
    if ( length )
    {     
		
        for ( i=0; i<length; i++ )
        {
			/* if current operator in postfix is a digit */
            if ( isdigit(postfix[i]) )
            {
				push(&stack, postfix[i]);
            }
            
            /* if current operator in postfix is an operator */
            else if ( isOperator(postfix[i]) )
            {
				x = pop(&stack);
				y = pop(&stack);

				push(&stack, calculate(y, x, postfix[i]) );
			}
		}

		return(pop(&stack));
	}

	else 
		return -1;
}

subtraction and mod seem to be working. Addition, multiplication and division are returning answers where 0=96, 1=97 etc. The exponent might as well be a random number generator :P

Calculate and isOperator functions just in case:

Code:

int calculate(int op1, int op2, char the_operator)
{
	if(the_operator == '+')
		return(op1 + op2);
	else if(the_operator == '-')
		return(op1 - op2);
	else if(the_operator == '*')
		return(op1 * op2);
	else if(the_operator == '/')
		return(op1 / op2);
	else if(the_operator == '^')
		return(op1 ^ op2);
	else if(the_operator == '%')
		return(op1 % op2);
}

int isOperator(char c) // is c and operator
{
     if ( c == '+' || c == '-' || c == '*' ||
           c == '/' || c == '%' || c == '^' )
     {
          return TRUE;
     }
     else
          return FALSE;
}

edit: http://k5-k.element80.net/files/saa1213.cpp for the entire file.

more edit: It was taking the ascii values. So if I subtract 48 from each op1 and op2 it fixes the strange output problems with simple input. Of course, then I had to add 48 when calling the calculate function so I can switch between working with ascii values and integers. So it's almost working. One more thing to fix. Don't worry about it. Might be useful later on for people who use the search, as there was lots on infix to postfix, but not much on postfix.

Originally Posted by Axolotl

Don't worry about it. Might be useful later on for people who use the search, as there was lots on infix to postfix, but not much on postfix.

What were the expressions that gave the bad output?

At the moment you are only handling single digit numbers so numbers of more than 1 digit may result in some unexpected output.

Ok first have you used the conversion functions on the input (i.e. from string or char to int) and the such. I can't remember what they are right now as I have to run. Also can you please say what type of errors exactly you get along with example answers. I don't understand what you explained.