Error while reading of loop variable

Question

0.00/5 (No votes)

See more:

Hello!
I'm working with this code:

C++

int counter = 0;
int dest_pos = 20;

for (int z = 0; z < 1000; z++)
{
/***2***/

   if(counter++ == dest_pos)
   {
   /***1***/
   //if reach dest position - than would set new value for dest_pos
   dest_pos = 56;
   
   }
}

At the line (marked as /***1***/) I tried to read z-value:

C++

Memo1->Lines->Add(IntToStr(z));

And I've got 0;

But if I try to assign value of z to another variable at str line marked as /***2***/, something like this:

C++

int y = z;

than I've got right z value at the line marked as /***1***/
Why is this happening?
And in what cases the value of z is outputed as 0 inside "for" loop?

Posted 31-Jul-14 9:13am

Igor-84

Add a Solution

Comments

CPallini 31-Jul-14 15:23pm

It smells like a bug of IntToStr function.

Sergey Alexandrovich Kryukov 31-Jul-14 15:43pm

I think this is not just a smell, but can be proven — please see my answer.
Your guess is appreciated and credited.
—SA

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Answer 1 · 2014-07-31T09:42:00

Solution 1

CPallini is right: this is a bug in IntToStr. I can tell you more: it is certainly the bug there. Here is why: there is no a way for z to get any wrong value, because its values are fully determined by the line with for statement head. (Well, people faced some extreme perverts who tried to modify the loop variable, and a C++ compiler — what a shame! — can permit that, but there is no a sign of such a bad thing in your code sample.)

At the same time, we cannot see the definition of IntToStr in your code sample. Therefore, the problem is in that function. ∎

:-)

—SA

Posted 31-Jul-14 9:42am

Sergey Alexandrovich Kryukov

Updated 31-Jul-14 9:44am

v2

Comments

CPallini 1-Aug-14 3:21am

:)5.

Sergey Alexandrovich Kryukov 1-Aug-14 11:44am

Thank you, Carlo.
—SA

mbue 1-Aug-14 17:22pm

sorry to intercept. but nothing is proven!
what do you think the function IntToStr should look alike?


#pragma once
#include <stdio.h>
#include <tchar.h>
#include <malloc.h>

static TCHAR					__intBuff[64];
#define	IntToStr1(i)		(const TCHAR*)__IntToStr1(i,__intBuff,sizeof(__intBuff)/sizeof(__intBuff[0]))
#define	IntToStr2(i)		(const TCHAR*)__IntToStr2(i,__intBuff,sizeof(__intBuff)/sizeof(__intBuff[0]))

const TCHAR* __IntToStr1(int& i,TCHAR* buff,const unsigned int blen)
{
	_itot_s(i,buff,blen,10);
	i = 0;// <--- parameter 'i' may be lost or nulled by malfunction
	return buff;
}

const TCHAR* __IntToStr2(int& i,TCHAR* buff,const unsigned int blen)
{
	i = 0;// <--- parameter 'i' may be lost or nulled by malfunction
	_itot_s(i,buff,blen,10);
	return buff;
}

int _tmain(int argc, _TCHAR* argv[])
{
	int counter = 0;
	int dest_pos = 20;

	_tprintf(__T("loop IntToStr1\r\n"));
	for (int z = 0; z < 1000; z++)
	{
		/***2***/
	 int	y = z;
		/***3***/

		 if(counter++ == dest_pos)
		 {
			 /***1***/
			 _tprintf(__T("y = %s (OP says here comes z unchanged)\r\n"),IntToStr1(y));
			 _tprintf(__T("z = %s (OP says here comes 0)\r\n"),IntToStr1(z));
			 //if reach dest position - than would set new value for dest_pos
			 dest_pos = 56;
	   
		 }
	}
	
	_tprintf(__T("loop IntToStr2\r\n"));

	counter = 0;
	dest_pos = 20;
	for (int z = 0; z < 1000; z++)
	{
		/***2***/
		 int	y = z;
		/***3***/

		 if(counter++ == dest_pos)
		 {
			 /***1***/
			 _tprintf(__T("y = %s (OP says here comes z unchanged)\r\n"),IntToStr2(y));
			 _tprintf(__T("z = %s (OP says here comes 0)\r\n"),IntToStr2(z));
			 //if reach dest position - than would set new value for dest_pos
			 dest_pos = 56;
	   
		 }
	}
	_tprintf(__T("<key> ")); _gettch();
	return 0;
}

the result:


loop IntToStr1
y = 20 (OP says here comes z unchanged)
z = 20 (OP says here comes 0)
y = 36 (OP says here comes z unchanged)
z = 36 (OP says here comes 0)
loop IntToStr2
y = 0 (OP says here comes z unchanged)
z = 0 (OP says here comes 0)
y = 0 (OP says here comes z unchanged)
z = 0 (OP says here comes 0)

conclusion:
the only place the value of z can be changed to 0 is between '/***3***/' and '/***1***/'.
how is this possible with the code we've seen?

regards.

Sergey Alexandrovich Kryukov 1-Aug-14 20:57pm

This is irrelevant. The proof is based on the fact that there cannot be other source of this problem. (I don't consider such cases when one use one code but shows different code in the issue report; it happens all the time.) Please use the debugger and you will see what exactly is going on. If you prove me wrong I'll be much grateful.
Good luck.
—SA

mbue 2-Aug-14 5:46am

Yes indeed your statement IntToStr is the reason is irrelevant, cause its impossible. There must be other code the OP did not posted here. Use your debugger and see that any kind of that function cannot change parameters in a different way.
regards

Sergey Alexandrovich Kryukov 4-Aug-14 3:01am

By the way, you comment above shows code lines
#include
#include
#include
...

Do you call it source code? I understand that this is the artifact of HTML not properly formed, but who is going to fix it?
Anyway, you already got an answer...
—SA

barneyman 4-Aug-14 2:48am

your intTostr is passing the int by ref, then nulling it ...