stack problem

Take the following code and look closely at the bold line:

Code:

---

if (function_stack.size() > 0)
                        {
                          string::size_type op_prec_2 = function_precedence[ function_operators.find(function_stack.top()) ];
                          string::size_type op_prec_1 = function_precedence[ function_operators.find(eq[counter]) ];
                         
                          if (op_prec_2 <= op_prec_1)
                          {
                              ostringstream oss;
                              oss << function_stack.top();
                              output_queue.push( oss.str() );
                              if (function_stack.size() > 0)
                              {
                                function_stack.pop();
                              }
                          }
                        }

---

It simply pops something off the top of the stack, but it causes a crash. I've tried using a try-catch clause and it still gives me the same thing without an error message. I don't see why this would cause a problem, since I'm checking the size first.

Also, if I change this line:

if (op_prec_2 <= op_prec_1)

to this:

if (op_prec_2 >= op_prec_1)


it works fine, but for some reason the top line causes the stack not to be able to pop();


Does anyone have a clue why this is happening?


PS: I've done cout statements all through out the program. Everything is good.

The size check is actually redundant. Have you stepped through the code and in particular into the pop() with a debugger?

Quote:

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Originally Posted by CornedBee

The size check is actually redundant. Have you stepped through the code and in particular into the pop() with a debugger?

---

That's what I thought. When I ran the debugger, it gave a segmentation fault error with the second size check I posted.... But not with the first. I don't see how calling pop() could illegally access memory... Any ideas on that?

No. That's why I suggested you step INTO the pop() call.

CB, I did all that yesterday. This morning, (after my computer restarted), everything is working fine.. I'm guessing the numeric comparision for the second statement caused a different number of pop() statements and some of which were illegal. The good thing is it's working. The bad thing is I can't step into the pop() statement like you told me to.


Thanks for the help. Hopefully the problem won't come back.

Ahh, it's doing it again. I don't get it. If stack is called twice, then it crashes, but if I turn the compare statement around, it runs fine with four calls.

Is there an alternative to the stack class that provides easy operations and LIFO?

What do you suggest I do CB?

Create a minimal testcase that reproduces the error and runs on Linux, zip it up and send it to me. The error intrigues me.

Thanks.

I'll attach it here. I really couldn't slim the code down any, but it's really only one method in a class, so it's not a big deal.

This isn't homework, so take your time and only look at it when you feel like :)


Thanks for any help you give. This problem has really annoying me and the only thing holding me back.

I guess it would help if i attached it.

Well, CB, I figured out the problem. First off, the code was fairly messy so I cleaned it up. Then I found out I was working with the operator precedence all wrong.

Great!