Thread: RPN calculator, identify different terms in a string.

If I were going to write an RPN calculator I would not actually write my own parser for it unless I needed to convert from infix notation.

It would be beneficial to break up the string into pieces before you try to calculate:

Code:

char **
rpn_break(char *eq, char **stack, int *n)
{
    int size = 6; // stack size guess.
 
    stack = malloc(size * sizeof(*stack));
    if (stack == NULL)
        return NULL;
 
    char *token;
    int i = 0;
 
    for (token = strtok(eq, "\n\t "); token != NULL; token = strtok(NULL, "\n\t "))
    {
        if (i == size) {
            char *temp;
 
            size *= 2;
            temp = realloc(stack, size);
            if (temp == NULL)
            {
                free(stack);
                return NULL;
            }
            stack = temp;
        }
 
        stack[i++] = token;
    }
 
    *n = i;
    return stack;
}
 
int 
main(void) {
    int size = 0;
    char **stack = NULL;
    char eq[BUFSIZ];
 
    puts("enter RPN equation here:");
    fgets(eq, sizeof eq, stdin);
    stack = rpn_break(eq, stack, &size);
 
    for (int i = 0; i < size; i++) {
        printf("stack[%d] = \"%s\"\n", i, stack[i]);
    }
 
    free(stack);
    stack = NULL;
 
    return 0;
}

We can reasonably expect the result on your example to be
stack[0] = "3"
stack[1] = "2"
stack[2] = "+"
stack[3] = "pi"
stack[4] = "-"
The code that you currently have is exceptionally brittle, because the stack is just a string, so every operand has to fit in a character. This just isn't a reasonable expectation. The constant pi can't be stored as a single character, and neither can multiple digit numbers.

The other thing I want to ask you is how to make the program realise the different types of ispunct and act differently according to that type (+, -, *, and /).

Well, if the tokens are strings, then strcmp(stack[i], foo) would return 0 for any string foo, like "-", "+", etc.

I think push is coded incorrectly, as well. Variable top is initially 0, instead of -1 like you apparently expect in your code, so I would actually write something to the effect of:

Code:

int top = -1;

void push(stack, x)
{
   if (top == MAX - 1) 
   {
      status = 0;
   }
   else 
   {
      stack[++top] = x;
      status = 1;
   }
}

Now top always points to the first element in the stack instead of something else without wasting any stack space.