C / C++ / MFC Discussion Boards

Re: pure C question about accessing bits

Justin Tay25-Mar-06 5:25

25-Mar-06 5:25

It looks like you may have either misrepresented your problem or I have misunderstood. You said that values are stored as a set of 2 bits (which can have 4 values from 0 to 3).. but it looks like it is just 1 bit (with a value of either 0 or 1)?

When doing bitwise operations though, always think of operating on the big-endian representation.

Since Windows is little endian... you can modify the function accordingly. While the values are held as little-endian, the bitwise operations are conceptually done in big-endian so you don't have to logically flip it yourself.

<br />
unsigned char GetAt(unsigned short *compressedArray, int i)<br />
{<br />
  int nArrayPos = i / 16;<br />
  int nBytePos = (15 - (i % 16));<br />
  return (compressedArray[nArrayPos] >> nBytePos) & 0x01;<br />
}<br />

edit: 1 0 0 0 is a short? does that mean it is using 4 bits to represent 1 value of either 0 or 1? Actually that snippet seems to make no sense since ordering is per byte.. so I don't know how 1 0 0 0 gets represented into 0 0 0 1.

-- modified at 12:01 Saturday 25th March, 2006

Re: pure C question about accessing bits

bouli25-Mar-06 6:16

bouli

25-Mar-06 6:16

Hi,

I was wrong by talking about short sorry.

by 1 0 0 0 I was meaning 01 00 00 00 = 8 bits = 1 byte.

The values are stored as set of 2 bits (values are between 0 - 3), but the sets are inverted in the file.

The values meaning are the following:
00 : nothing
01 : wall
10 : ladder
11 : tree

So, for example, when in the file there is the byte value:
0x68 : 01 10 10 00

the real values should be interpreted as:
0x29 : 00 10 10 01

1 byte means 4 cells of the array.
The series of 2 bits are inverted.
See what I mean?

So, the compressed array's size is (MapWidth/2)x(MapHeight/2).

for example if you have a tiny 4x8 map like this: (t1)
1 1 1 1 1 1 1 1
1 0 2 2 0 0 0 1
1 0 2 2 0 0 0 1
1 1 1 1 1 1 1 1

In the collision map coded like this in memory: (t2)
01 01 01 01 01 01 01 01
01 00 10 10 00 00 00 01
01 00 10 10 00 00 00 01
01 01 01 01 01 01 01 01

and in the file it is like this: (t3)
01 01 01 01 01 01 01 01
10 10 00 01 01 00 00 00
10 10 00 01 01 00 00 00
01 01 01 01 01 01 01 01

in hexa as bytes it looks like this:
0x55 0x55
0xa1 0x40
0xa1 0x40
0x55 0x55
the size of the compressed array is 2x4 like I wrote above (MapWidth/2)x(MapHeight/2)

In the unsigned char* it is coded like this (in the file)
0x55 0x55 0xa1 0x40 0xa1 0x40 0x55 0x55

so, in our example, GetAt(compressedArray, 3, 1) should returns 2 (0x10 - column 3, line 1 from t2)

while it is read from the compressedArray...

I hope it is clearer with a concrete example.

I'm sure there is a way to decode it on the fly without storing the decompressed array.

Thanks for the help

Best regards.

Fred.

There is no spoon.

-- modified at 12:20 Saturday 25th March, 2006

Re: pure C question about accessing bits

Justin Tay25-Mar-06 6:33

Justin Tay

25-Mar-06 6:33

Just a tweak to the function should get it to work.

You can just do GetAt(compressedArray, x + y * MapWidth) to get the cell you are interested in.

<br />
unsigned char GetAt(unsigned char *compressedArray, int i)<br />
{<br />
  int nArrayPos = i / 4;<br />
  int nBytePos = (i % 4) * 2;<br />
  return (compressedArray[nArrayPos] >> nBytePos) & 0x03;<br />
}<br />

Don't get what you mean by decode on the fly, isn't this decoding on the fly?

Re: pure C question about accessing bits

bouli25-Mar-06 6:50

bouli

25-Mar-06 6:50

it always return 1 Frown | :(

There is no spoon.

Re: pure C question about accessing bits

bouli25-Mar-06 7:05

bouli

25-Mar-06 7:05

ok got it!!!
It works!!!
many thanks Smile | :)

Fred.

There is no spoon.

Re: pure C question about accessing bits

bouli16-Apr-06 0:28

bouli

16-Apr-06 0:28

Hello hfry,

Do you remember me and my problem about bits?
You gave me the function that reads the values and it works perfectly. Now, I need the reciproque function: I need to write values...

I remind you the function for reading the data you kindly gave me:

int GetCollisionValue(unsigned char* collisionsMap, int x, int y, int nWidth)
{
int i=x+y*nWidth;
int nArrayPos = i / 4;
int nBytePos = (i % 4) * 2;
int value=(collisionsMap[nArrayPos] >> nBytePos) & 0x03;

return value;
}

Now, I need the "SetAt" function...
void SetAt(unsigned char* collisionsMap, int x, int y, int nWidth, int nValue);

Can you help me again please???

Best regards.

Fred.

There is no spoon.

OnFind()

fuschia24-Mar-06 21:35

fuschia

24-Mar-06 21:35

how to code for finding some related info based on keywords like "id", "name" etc

Re: OnFind()

toxcct25-Mar-06 4:53

toxcct

25-Mar-06 4:53

what's your question exactly ?
i doubt anybody can understand you.

please also indicate which data types you're handling...

sincerely, Rose | [Rose]

Re: OnFind()

fuschia25-Mar-06 16:07

fuschia

25-Mar-06 16:07

lets say i have a program, its a reservation system. i have a db connected to it, and when click a button "Find", it will find related info based on keywords provided like "ReserveNo", "Name", etc.

im not very good at telling wat i want, cause im very new with this... sry

Re: OnFind()

toxcct26-Mar-06 3:31

toxcct

26-Mar-06 3:31

well, what you want seems to be some SQL queries...

Runtime Compression

Waldermort24-Mar-06 21:19

Waldermort

24-Mar-06 21:19

I would like to distribute a font file with my app. The font I have is a little too large for my liking, 3.6mb. Using winrar I am able to compress it down to 1.2mb which is a little more realistic.

Obviously There is a chance that winrar will not be installed on the target machine so I need some sort of runtime decrompression algorithm. The idea is, I compress the file and add it to the resources/installer then simply deflate it onto the target machine.

I know of zlib, but I have never used it. Could anybody recomend a good algorithm, with not much overhead, will not increae the exe size too much, and fairly easy to implement? This is not an MFC project Smile | :)

Re: Runtime Compression

Saurabh.Garg25-Mar-06 0:48

Saurabh.Garg

25-Mar-06 0:48

you can try one of these...

http://www.codeproject.com/useritems/LiteZip.asp[^]

http://www.codeproject.com/cpp/xzipunzip.asp[^]

http://www.wischik.com/lu/programmer/index.html[^]

-Saurabh

Re: Runtime Compression

Waldermort25-Mar-06 1:28

Waldermort

25-Mar-06 1:28

This for the links, I will into them Smile | :)

Re: Runtime Compression

Hamid_RT25-Mar-06 1:34

Hamid_RT

25-Mar-06 1:34

maybe it is some helpful to you

http://www.codeguru.com/Cpp/Cpp/algorithms/compression/article.php/c5137[^]

Flat DateTimePicker Class

mikobi24-Mar-06 20:31

mikobi

24-Mar-06 20:31

I am using Visual c++ 6.0.

Where can I found a class that make DateTimePicker Flat like i found for ComboBox

Urgent

I learn my self

Re: Flat DateTimePicker Class

mikobi25-Mar-06 3:16

mikobi

25-Mar-06 3:16

No Body can reply me at this forum !

problem with m_strCursor

Kranti125198424-Mar-06 20:21

Kranti1251984

24-Mar-06 20:21

Hi,

I've an application which creates HTML files. For that i'm using some database operations, which includes classes such as, CDatabase and CRecordSet etc.

I'm getting an exception as "invalid cursor" since m_strCursor is getting set to some file name e.g. "test.html", which the application is creating. I'm not able to trace out, how the variable is getting set. Similarly, m_strFilter, m_strSort, m_strUpdateSQL, m_strRequeryFilter, m_strRequerySort are getting set to the same file name.

Thanks,
Kranti

VS2005 - ellipsis in a macro

Chintoo72324-Mar-06 18:42

Chintoo723

24-Mar-06 18:42

VS6 and VS2003 do not support varargs in a C macro. The link http://sourceforge.net/mailarchive/forum.php?forum_id=40270&max_rows=25&style=nested&viewmonth=200503[^] hints that this is possible in VS2005.

I dont have VS2005 installed, can someone who has VS2005 quickly check if the following compiles?

#define myprint(fmt, args_...) _snprintf(str, fmt, ##args_)<br />
char str[64];<br />
myprintf("hello %s", "world");

thanks!

Re: VS2005 - ellipsis in a macro

Michael Dunn25-Mar-06 0:23

Michael Dunn

25-Mar-06 0:23

VC8 does support it, you've got the syntax wrong though. It should be

#define foo(fmt,...) _snprintf(str,fmt,<font color=red>__VA_ARGS__</font>)

More info[^]

--Mike--
Visual C++ MVP Cool | :cool:

LINKS~! Ericahist | NEW!! PimpFish | CP SearchBar v3.0 | C++ Forum FAQ

Re: VS2005 - ellipsis in a macro

Chintoo72325-Mar-06 22:50

Chintoo723

25-Mar-06 22:50

Cool, that is what I was looking for. Thanks.

Now one more thing - is there any preprocessor variable that indicates what compiler is compiling the code? I want to write this macro seperately once for VC8 and once for VC6/VC7 so that the code can still be compiled with the older compiler, just in case someone wants to use the older compilers.

thanks!

Re: VS2005 - ellipsis in a macro

Michael Dunn26-Mar-06 12:22

Michael Dunn

26-Mar-06 12:22

Chintoo723 wrote:
is there any preprocessor variable that indicates what compiler is compiling the code?

Yep, look up _MSC_VER in the help

--Mike--
Visual C++ MVP Cool | :cool:

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Lan Connection

vivek.s.vivek24-Mar-06 17:55

vivek.s.vivek

24-Mar-06 17:55

Guys,

* How to find my system get connected to LAN alone
not internet.

* IsNetworkAlive() API results in compilation error. AS
It requires Sensapi.h and its not found in my sysem.
where could I get this file?.

You only have to do a very few things right in your life so long as you don't do too many things wrong.
Vivek.S

Re: Lan Connection

ThatsAlok24-Mar-06 18:52

ThatsAlok

24-Mar-06 18:52

vivek.s.vivek wrote:
It requires Sensapi.h and its not found in my sysem. where could I get this file?.

Download latest PSDK from MSDN site or seach cds comes with MSDN subscriptions.

Re: Lan Connection

Ganesh_T24-Mar-06 23:03

Ganesh_T

24-Mar-06 23:03

Try using this.

DWORD flags;

BOOL chk=InternetGetConnectedState(&flags, NULL);
if(chk)
AfxMessageBox("Connected");
else
AfxMessageBox("Not Connected");

Cheers
"Peace of mind through Technology"

Re: Lan Connection

ThatsAlok25-Mar-06 5:59

ThatsAlok

25-Mar-06 5:59

Ganesh_T wrote:
BOOL chk=InternetGetConnectedState(&flags, NULL);

This function doesnot gurrantee internet state

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