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#defined calculating returns wrong value

 
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Guest
#defined calculating returns wrong value
PostPosted: Sun Apr 03, 2005 9:34 am     Reply with quote
another stupid question...

i'm defining some constans and making some calculations...

with this code DeviceNo always 0.
DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;

but this one DeviceNo has correct result.
DeviceNo = RecordSaveNo;
DeviceNo /= (int16) EEPROM_RECORD_COUNT;

here is the test code i used and results from terminal screen.

Code:

#include <18F452.h>
#device adc=10
#use delay(clock=4000000)
#fuses XT, NOWDT,NOPROTECT, NOLVP, NOPROTECT, PUT
#use rs232(baud=19200, xmit=PIN_C6, rcv=PIN_C7, ERRORS)


//headers


#Define EEPROM_SIZE           32768
#Define RECORD_SIZE           64
#Define EEPROM_RECORD_COUNT   EEPROM_SIZE/ RECORD_SIZE

void GetDeviceAddress(int16 RecordSaveNo)
{
   int16 DeviceNo;

   //wrong result
   DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;
   printf("\n\rDevNo=%lu RecNo=%lu EEPROM_REC_CNT=%lu", DeviceNo,RecordSaveNo,EEPROM_RECORD_COUNT);

   //correct result
   DeviceNo = RecordSaveNo;
   DeviceNo /= (int16) EEPROM_RECORD_COUNT;
   printf("\n\rDevNo=%lu RecNo=%lu EEPROM_REC_CNT=%lu", DeviceNo,RecordSaveNo,EEPROM_RECORD_COUNT);
}




if i call GetDeviceAddress with parameter 3750

Code:

void main
{
     GetDeviceAddress(3750);
}


it returns
DeviceNo=0 RecordSaveNo=3750 EEPROM_RECORD_COUNT=512
DeviceNo=7 RecordSaveNo=3750 EEPROM_RECORD_COUNT=512

why this happens?
Guest
PostPosted: Sun Apr 03, 2005 9:39 am     Reply with quote
i forgot to say:

if i remove (int16) still same
DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;
DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT; //WRONG

but i replace EEPROM_RECORD_COUNT with 512 i get correct results.
DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;
DeviceNo = RecordSaveNo / 512; //CORRECT


this is why title is '#defined calculating returns wrong value'
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Sun Apr 03, 2005 1:22 pm     Reply with quote
I didn't try to analyze your code, but the very first step is to
put parenthesis around your defined values. That way, you
are certain of the order of evaluation.

Do it like this:
Code:
#Define EEPROM_RECORD_COUNT  (EEPROM_SIZE/ RECORD_SIZE)
Guest
PostPosted: Sun Apr 03, 2005 3:10 pm     Reply with quote
as seen in the post above:

printf("\n\rDevNo=%lu
RecNo=%lu
EEPROM_REC_CNT=%lu",

DeviceNo,
RecordSaveNo,
EEPROM_RECORD_COUNT);

creates these lines in the console.

DeviceNo=0 RecordSaveNo=3750 EEPROM_RECORD_COUNT=512
DeviceNo=7 RecordSaveNo=3750 EEPROM_RECORD_COUNT=512


these means EEPROM_RECORD_COUNT=512 and no problem with it.

it works with constants but not work with defined values. i'm using working code.

Nothing much to anaylze... most of them printf...

Code:

   //defines
   #Define EEPROM_SIZE           32768
   #Define RECORD_SIZE           64

   //EEPROM_RECORD_COUNT=512
   #Define EEPROM_RECORD_COUNT   EEPROM_SIZE/ RECORD_SIZE


Code:

   //wrong result - DeviceNo always =0
   DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;

   //wrong result - DeviceNo always =0
   DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT;


Code:

   //correct result - DeviceNo has value as expected
   DeviceNo = RecordSaveNo;
   DeviceNo /= (int16) EEPROM_RECORD_COUNT;



Code:

   //correct result - DeviceNo has value as expected
   DeviceNo = RecordSaveNo / 512;
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

View user's profile Send private message

PostPosted: Sun Apr 03, 2005 5:48 pm     Reply with quote
Now I'm at a computer where I can do some testing, and I did so,
and it works with the parens added. I tested it with PCM vs. 3.222.
The output of the program is:
0
1
2
3
....
61
62
63
Code:
#include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

#Define EEPROM_SIZE           32768
#Define RECORD_SIZE           64

#Define EEPROM_RECORD_COUNT   (EEPROM_SIZE/ RECORD_SIZE)

//============================
main()
{
int16 RecordSaveNo;
int16 DeviceNo;

for(RecordSaveNo = 0; RecordSaveNo < 32768; RecordSaveNo+= 512)
   {
     DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT;
     printf("%lu\n\r", DeviceNo);
   }   

while(1);
}
Ttelmah
Guest
PostPosted: Mon Apr 04, 2005 2:21 am     Reply with quote
Anonymous wrote:
as seen in the post above:

Code:

   //defines
   #Define EEPROM_SIZE           32768
   #Define RECORD_SIZE           64

   //EEPROM_RECORD_COUNT=512
   #Define EEPROM_RECORD_COUNT   EEPROM_SIZE/ RECORD_SIZE


Code:

   //wrong result - DeviceNo always =0
   DeviceNo = RecordSaveNo / (int16) EEPROM_RECORD_COUNT;

   //wrong result - DeviceNo always =0
   DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT;


Code:

   //correct result - DeviceNo has value as expected
   DeviceNo = RecordSaveNo;
   DeviceNo /= (int16) EEPROM_RECORD_COUNT;



Code:

   //correct result - DeviceNo has value as expected
   DeviceNo = RecordSaveNo / 512;

In fact the code works exactly as expected in each case...
This is a 'classic' macro problem with C. In the line 'DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT;
Think for a moment about what this expands to:

DeviceNo=RecordSaveNo/32768/64;

Now C evaluates division left to right, and you are running in int16, so you get the RecordSaveNo, divided by 32768, which unless RecordSaveNo, is greater than 32768, will allways result in '0'. Then this is divided by 64, to again give zero.
This is not a problem with CCS, but is covered in some detail in most C books, where it is pointed out, that you _must_ always think through the evaluation order when using macros, and for safety, when deaing with expressions like this, it is worth always bracketting, to avoid this problem (this forces the macros division to be executed first, to give the result expected...
You will see in code posted in the pastm that several of the posters do this with regularity, to avoid exactly this problem.

Best Wishes
Guest
PostPosted: Mon Apr 04, 2005 3:06 am     Reply with quote
ahhh. after your post i understand what is my error. #defines are text replaces.

1) #Define EEPROM_RECORD_COUNT EEPROM_SIZE/ RECORD_SIZE //=512
2) DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT;

when 1 and 2 merged:
DeviceNo = RecordSaveNo / EEPROM_RECORD_COUNT EEPROM_SIZE/ RECORD_SIZE;

not
DeviceNo = RecordSaveNo / 512

i always accept it 'DeviceNo = RecordSaveNo / 512'. it's wrong.
as i said my first post, i made very stupid error. and i don't understand PCM programmer's first post because i always accept DeviceNo equals to 512.

i never think this caused by compiler because / 512 works well.i anayzed asm code, and the code generated by compiler is very optimized asm code.

i must read more C - ANSI C book.

i'm sorry. but now i understand.
thank you very much. you are great people.
Ttelmah
Guest
PostPosted: Mon Apr 04, 2005 3:15 am     Reply with quote
A very good point. The fact that a #define, is a macro expansion, and not a 'numeric' defintion, is not intrinsically 'obvious', and leads to exactly your problem.
Glad I managed to 'trigger' the light on this. :-)

Best Wishes
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