When should printf round a floating number up for display? - C / C++ / MFC Discussion Boards

Re: Empty String

David Crow31-Dec-12 5:25

David Crow

31-Dec-12 5:25

Something like:

BYTE buffer[512];
CFile file;
file.Read(buffer, sizeof(buffer));

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Re: Empty String

manem21k2-Jan-13 1:26

manem21k

2-Jan-13 1:26

You can make sure like this:

CFile file;
Char tempBuf[512];
file.Read(tempBuffer, 512);

Re: Empty String

jeron12-Jan-13 4:21

jeron1

2-Jan-13 4:21

manem21k wrote:

You can make sure like this:

CFile
file;
Char tempBuf[512];
file.Read(tempBuffer, 512);

Actually this will not work.

Char tempBuf[512]; <== wont compile there'e no Char data type
char tempBuf[512]; <== will compile

file.Read(tempBuffer, 512); <== wont compile, tempBuffer isn't declared
file.Read(tempBuf, 512); <== will compile

In other words see David's reply.

Re: Empty String

manem21k2-Jan-13 22:32

manem21k

2-Jan-13 22:32

Thanks for correcting the typos. Yeah I have't run this program but just sure the code will resolve the problem.

Re: Empty String

CPallini2-Jan-13 10:09

CPallini

2-Jan-13 10:09

Why do you want to initialize a CString with binary data?

Veni, vidi, vici.

MFC Grid control 2.27

BorisAnimal30-Dec-12 4:46

BorisAnimal

30-Dec-12 4:46

Hi I want to use this control.
I have VS2008. I create CMyDialog and create CGridCtrl m_Grid as a member. Then I fill grid without problem. I need to take focus cell text when I list grid by arrow down arrow up keys. What I have to do in message map of my class CMyDialog to react on these actions. Can You help me? Where I shoud look in documentation?

Re: MFC Grid control 2.27

Richard MacCutchan30-Dec-12 5:34

Richard MacCutchan

30-Dec-12 5:34

I already answered this question here[^]. Please post in one forum only.

One of these days I'm going to think of a really clever signature.

When should printf round a floating number up for display?

Member 419459328-Dec-12 5:11

Member 4194593

28-Dec-12 5:11

Specifically this statement:

C++

printf("Chi square distribution for %s samples is %1.2f (%1.4f), and randomly\n",
   String_Value, chisq, chisq);

I re-implemented this program in MASM to speed it up and I get the following difference (note that I added the (%1.4f)) to display the pre-rounded number to see why I was getting differences:

C++

------------------------------------------------------------------ 17
Chi square distribution for 2048 samples is 6.13 (6.1250), and randomly
-------------------------------------------------------------- 17
Chi square distribution for 2048 samples is 6.12 (6.1250), and randomly

The question is, which one is correct according to the C spec? Should 6.1250 round up to 6.13 or should it take 6.1250+ to round up?

There are other questions such as the following:

C++

printf("of this %s %s file by %d (%2.2f) percent.\n\n", String_Value, samp,
   (short) (((100 * ((binary ? 1 : 8) - ent) / (binary ? 1.0 : 8.0)) + 0.5)),
            ((100 * ((binary ? 1 : 8) - ent) / (binary ? 1.0 : 8.0))));

The cast to a (short) seems to truncate a 5.7 to 5, I had to add the (.... + 0.5) just before the cast in order to get the rounded number. Again, I added the (%2.2f) to display the rounding differences.

Which is correct according to the C spec, rounding or truncation?

The program being re implemented is John Walker's ENT.

Dave.

Re: When should printf round a floating number up for display?

Richard MacCutchan28-Dec-12 5:53

Richard MacCutchan

28-Dec-12 5:53

I think the rules for rounding when a floating point's decimal part ends in 5 is, round up if the previous digit is even, and down if it's odd. So 6.125 rounds up to 6.13 but 6.115 would round down to 6.11; this is easy to check. In the second instance if you cast a float to an integer it discards the decimal part.

One of these days I'm going to think of a really clever signature.

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 6:55

Member 4194593

28-Dec-12 6:55

Richard,

Very interesting. I pulled up my 8087 book "THE 8087 PRIMER", John F. Palmer and Steven P. Morse and looked up rounding. The nearest to what you described is the "round to nearest Even" mode, not the round to the nearest Odd as you described. Are you sure about this? Is it in some C spec?

If you are programming the FPU and want to cast a float to an integer, you just execute an FIST or FISTP, which will round to the specified rounding mode which is usually "round to nearest Even" unless you specifically set the mode to "round to 0" before you store the value.

Dave.

Re: When should printf round a floating number up for display?

Richard MacCutchan28-Dec-12 7:01

Richard MacCutchan

28-Dec-12 7:01

Well I did not have a reference to hand so it's just from what I recall during other development. However, you seem to have more up to date information than me.

One of these days I'm going to think of a really clever signature.

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 7:08

Member 4194593

28-Dec-12 7:08

Richard,

I'm not trying to argue, just trying to find out what the spec says.

"THE 8087 PRIMER" is just a spec, from the Senior Staff Engineers at Intel Corp. explaining why they did what they did. How C uses the hardware should be specified somewhere.

Dave.

Re: When should printf round a floating number up for display?

Richard MacCutchan28-Dec-12 7:18

Richard MacCutchan

28-Dec-12 7:18

Member 4194593 wrote:
I'm not trying to argue

I did not mean to imply that you were; I am more than happy to have my assumptions corrected. As to what the C/C++ compiler should do in these cases, you could look at http://www.stroustrup.com/[^], or the Microsoft reference pages[^].

One of these days I'm going to think of a really clever signature.

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 7:20

Member 4194593

28-Dec-12 7:20

Richard,

Thank you for the links. I'll report back with anything I find (if I can understand it).

Dave.

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 8:20

Member 4194593

28-Dec-12 8:20

Richard,

I did find out that a float cast as a (short) is truncated, but not yet found out about rounding in printf.

Dave.

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 8:58

Member 4194593

28-Dec-12 8:58

Richard,

I did find out only that the number will be rounded to the number of specified precision digits, but there is no mention of which rounding method is used! I did send feedback that I was dissatisfied with their answer.

Dave.

Re: When should printf round a floating number up for display?

jschell28-Dec-12 9:45

jschell

28-Dec-12 9:45

Member 4194593 wrote:
The question is, which one is correct according to the C spec?

Depends on what you mean by "C spec".

Prior to C99 it was left up to the implementation how rounding occurred.

According to the following (but not verified by me) C99 specifies how rounding is done (search for C99)

http://linuxgazette.net/144/misc/lg/a_question_of_rounding_in_issue_143.html[^]

Re: When should printf round a floating number up for display?

Member 419459328-Dec-12 12:51

Member 4194593

28-Dec-12 12:51

jschell,

Thank you for the excellent link, at least it references a spec, namely IEEE.

Personally, I do not think it really makes any difference what the least significant digit is rounded to, but when you are converting a program and want to insure that the conversion is accurate, and the program has many options, the easiest approach is a batch file calling the two programs, supplying the different input files and options to be processed, and then using a file compare routine to compare the outputs for each test instance.

In the case of ENT compiled under Visual Studio 2008, I can see truncation, rounding to an even, and common rounding, all in one program. Makes it a bit difficult to "emulate" the original program.

I have examined all of the test output (the diffs) for all of the test instances, and all of my results match the ENT results except possibly for that last digit. At this point I am calling this a valid conversion, and a huge learning experience. At least my version is about 5 times as fast for huge files as ENT (16 GB - all DWORD values for a full 2^32 period) 6 min vs 28 min.

Dave.

graphics.h multi-threading problem

yudhistira dewanata28-Dec-12 0:22

yudhistira dewanata

28-Dec-12 0:22

I'm not sure if i should put this question in this forum or the Graphics forum, as the description in graphics forum only says DirectX,OpenGL or GDI.

So I was trying this graphics.h library that can crate simple graphic for console app. Everything works just fine until I multithreaded? it.

Ok, here is the code:

#include<graphics.h>
void drawrect(int l,int t,int r,int d,int col){
  if(col==0)setcolor(RED);
  else setcolor(BLUE);
  rectangle(l,t,r,d);
}
void drawpiece(int l,int t,int r,int d,int col){
  for(int i=1;i<5;i++)
    drawrect(i*16+l,t,i*16+r,d,col);
}
DWORD WINAPI Thread2(void* lparam){
  int px2=161,py2=17;
  for(int i=0;i<10;i++,py2+=16,delay(100))
    drawpiece(px2,py2,px2+16,py2+14,1);
}
int main()
{   
    initwindow(600,600,"TEST");   
    CreateThread(0,0,&Thread2,NULL,0,0);
    int px1=17,py1=17;
    for(int i=0;i<10;i++,py1+=16,delay(100))
      drawpiece(px1,py1,px1+14,py1+14,0);    
    getch();
    return EXIT_SUCCESS;
}

What i'm trying to do is to make two set of blockline. where first set is red and second set is blue.
this code makes the color mixed up.
So that's it. Any help would be greatly appreciated.

Re: graphics.h multi-threading problem

Richard MacCutchan28-Dec-12 5:49

Richard MacCutchan

28-Dec-12 5:49

I have not checked your arithmetic, but it may be that the start points and limits of your rectangles are not correct.

One of these days I'm going to think of a really clever signature.

Re: graphics.h multi-threading problem

yudhistira dewanata28-Dec-12 16:23

yudhistira dewanata

28-Dec-12 16:23

I have checked the arithmetic and it works just fine if i don't multithread it.
This works just fine:

#include<graphics.h>
void drawrect(int l,int t,int r,int d,int col){
  if(col==0)setcolor(RED);
  else setcolor(BLUE);
  rectangle(l,t,r,d);
}
void drawpiece(int l,int t,int r,int d,int col){
  for(int i=1;i<5;i++)
    drawrect(i*16+l,t,i*16+r,d,col);
}
/*DWORD WINAPI Thread2(void* lparam){
  int px2=161,py2=17;
  for(int i=0;i<10;i++,py2+=16,delay(100))
    drawpiece(px2,py2,px2+16,py2+14,1);
}*/
int main()
{   
    initwindow(600,600,"TEST");   
 //   CreateThread(0,0,&Thread2,NULL,0,0);
    int px1=17,py1=17;
    for(int i=0;i<10;i++,py1+=16,delay(100))
      drawpiece(px1,py1,px1+14,py1+14,0);    

    int px2=161,py2=17;
    for(int i=0;i<10;i++,py2+=16,delay(100))
      drawpiece(px2,py2,px2+16,py2+14,1);

    getch();
    return EXIT_SUCCESS;
}

I'm really starting to think that this might be a problem from graphics.h

Re: graphics.h multi-threading problem

Richard MacCutchan28-Dec-12 23:07

Richard MacCutchan

28-Dec-12 23:07

It may well be a problem with the graphics library (not the header) which is rather old and was probably not designed with multi-threading in mind. I would suggest you switch to one of the more current graphics libraries available in the Windows API.

One of these days I'm going to think of a really clever signature.

Re: graphics.h multi-threading problem

yudhistira dewanata30-Dec-12 5:01

yudhistira dewanata

30-Dec-12 5:01

Yes that's what i thought. I've switched to SDL and it works.
Anyway, thanks for replying to my question. Really appreciate it.

Re: graphics.h multi-threading problem

Randor 29-Dec-12 8:25

Randor

29-Dec-12 8:25

Hi,

This is very basic computer science problem. Look at your code cafefully.

First read all about context switching[^] and once you completely understand look over your code again.

Now ask yourself what would happen if ThreadA context switches in the middle of drawrect() and ThreadB continues execution of drawrect().

Best Wishes,
-David Delaune

Re: graphics.h multi-threading problem

Richard MacCutchan29-Dec-12 21:28

Richard MacCutchan

29-Dec-12 21:28

David,
My reading of that is that it should, in theory, work. The variables used by each function are all stack variables and each thread has its own stack, so context switching should not affect it. I can only assume that there is some code in the actual library that is not thread safe.

One of these days I'm going to think of a really clever signature.

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