Question

Ok. Let me start again. I am trying to write a program that would evaluate reverse Polish notation arguments to it. (i.e. ./program 2 4 +, which evaluates to 6).

int main(int argc, char *argv[]) 
{
   char oper[MAXOP];
   char *ptr;
   unsigned char c;
   double op2;
   while (--argc > 0 && ++argv != NULL) {
     ptr =  oper;
     if (isdigit(c = **argv)) 
     {
        while (isdigit(c = **argv++)) 
        {
            *ptr++ = c;
        }
        *ptr = '\0';
        push(atof(oper));
     } 
     else if (isalpha(c)) 
     {  
        while (isalpha(c = **argv++)) 
        {
            *ptr++ = c;
        }
                   ...
         ...
    }
}

The program should check the first character of each arguments if it is alpha or digit or an operator but it always turns out to be an alpha.

+++++++++ UPDATE +++++++++++++

Well, that’s odd. It is now working. Before the condition,
isdigit(c = **argv) wasn’t evaluating to true for numeric arguments (didn’t matter if the rest of the code was wrong). Now it does, but the condition isdigit(c = **argv++) is wrong.

It should be isdigit(c = *argv[0]++).

I had two files of the same name. They were in different directories but contain the same code. While working with this one, I must have been in the other directory and compiling the other file, and thus getting the same output despite edits to this one. My bad.

Answer 1

Have you tried using a debugger and figuring out what happens?

My first guess would be that **argv++ is wrong (are you increasing argv? *argv? **argv? Even if it’s not wrong, it’s just confusing code). Switch to:

char *cur_arg = *argv;

And go over cur_arg (using cur_arg++ to get to the next character, and not c=*cur_arg++ – again, confusing code).