AND logical operator

[Mike Trader]

Can someone pls explain how this works?
IF (WFD.dwFileAttributes AND 16) = 0 THEN

in

    DO
        IF (WFD.dwFileAttributes AND 16) = 0 THEN  ' Exclude dirs 
            INCR Cnt
            REDIM PRESERVE Fi(Cnt)
            Fi(Cnt) = RTRIM$(wfd.cFileName, ANY CHR$(0,32))
        END IF
        IF ISFALSE FINDNEXTFILE(hFind, wfd) THEN CALL FINDCLOSE(hFind) : EXIT DO
    LOOP

How can you test "16"?
Obviously it is never equal to 0 so this should allways fail
yet it doesnt.
I dont get it.

------------------
Kind Regards
Mike

[Eric Pearson]

The parentheses tell PowerBASIC to perform a bitwise AND operation.

So that expression tests for whether or not "the 16 bit" (i.e. bit #4) is set in WFD.dwFileAttributes.

Bit #    Value
 0         1
 1         2
 2         4
 3         8
 4        16
 5        32
 6        64
 7       128
 8       256
etc.

If WFD.dwFileAttributes has a value that has the 16 bit set, that expression will return 1. Otherwise it will return zero.
-- Eric


------------------
Perfect Sync Development Tools
Perfect Sync Web Site
Contact Us: mailto:[email protected][email protected]</A>



[This message has been edited by Eric Pearson (edited August 01, 2001).]

[Mike Trader]

Eric,

thx, dont you mean NOT set, cos its testing = 0

and how do you translate: file to Bit #4

Here is what WinHlp says, but no reference to Bits ???

Specifies the file attributes of the file found. This member can be one or more of the following values:

Value Meaning
FILE_ATTRIBUTE_ARCHIVE
The file is an archive file. Applications use this value to mark files for backup or removal.
FILE_ATTRIBUTE_COMPRESSED
The file or directory is compressed. For a file, this means that all of the data in the file is compressed. For a directory, this means that compression is the default for newly created files and subdirectories.
FILE_ATTRIBUTE_DIRECTORY
The file is a directory.
FILE_ATTRIBUTE_HIDDEN
The file is hidden. It is not included in an ordinary directory listing.
FILE_ATTRIBUTE_NORMAL
The file has no other attributes set. This value is valid only if used alone.
FILE_ATTRIBUTE_OFFLINE
The data of the file is not immediately available. Indicates that the file data has been physically moved to offline storage.
FILE_ATTRIBUTE_READONLY
The file is read-only. Applications can read the file but cannot write to it or delete it.
FILE_ATTRIBUTE_SYSTEM
The file is part of the operating system or is used exclusively by it.
FILE_ATTRIBUTE_TEMPORARY
The file is being used for temporary storage.

What do these all translate to in Bits?

------------------
Kind Regards
Mike

[Borje Hagsten]

In binary world, everything is bits. We really only have 0 and 1 to use,
but today's compilers translate the bits to decimal system. This is why
it's faster to do bitwise compare - (almost) no need to translate anything.

Some people use the BIT() functions, but my own tests have show them to
be much slower than simple AND operator against mask, within paranthesis.

In this case, one must always test the bits. A folder can have several bits
(flags) set, like hidden, etc, so decimal value becomes 18 (whatever), but it
will always have at least bit 4 (=16) set. If bit 4 is zero, it is
a file (unless MS has come up with some new "invention" lately..)

To confuse things even more - values are usually given in hexadecimal form,
where H10 equals 16 (in HEX world, 0123456789ABCDEF). From latest win32api,inc:

%FILE_ATTRIBUTE_READONLY                     = &H1
%FILE_ATTRIBUTE_HIDDEN                       = &H2
%FILE_ATTRIBUTE_SYSTEM                       = &H4
%FILE_ATTRIBUTE_DIRECTORY                    = &H10
%FILE_ATTRIBUTE_ARCHIVE                      = &H20
%FILE_ATTRIBUTE_NORMAL                       = &H80
%FILE_ATTRIBUTE_TEMPORARY                    = &H100
%FILE_ATTRIBUTE_COMPRESSED                   = &H800

Now, if this wasn't clear as mud, I don't know what is..

(imagine the old days, when they had to use mechanical switches to set
each individual bit in order to create a running program. Now, that is
what I call programming..
------------------
PS: I like the way MS describes a directory: "then file is a directory"..
Then horse is an elephant? Car is a boat? ..?

[This message has been edited by Borje Hagsten (edited August 01, 2001).]

[George Bleck]

16 in "bits" (read as binary) is 000000000010000 (hex &h10)

                               Decimal       Hex       Binary
%FILE_ATTRIBUTE_READONLY       = 1           = &H1     = 0000000000000001
%FILE_ATTRIBUTE_HIDDEN         = 2           = &H2     = 0000000000000010
%FILE_ATTRIBUTE_SYSTEM         = 4           = &H4     = 0000000000000100
%FILE_ATTRIBUTE_DIRECTORY      = 16          = &H10    = 0000000000010000
%FILE_ATTRIBUTE_ARCHIVE        = 32          = &H20    = 0000000000100000
%FILE_ATTRIBUTE_NORMAL         = 128         = &H80    = 0000000010000000
%FILE_ATTRIBUTE_TEMPORARY      = 256         = &H100   = 0000000100000000
%FILE_ATTRIBUTE_COMPRESSED     = 2048        = &H800   = 0000100000000000

The AND operation checks bit for bit.  If BOTH bits are 1 then the final results is one all other cases are 0.

Example: if you AND the decimal numbers 186 and 16

186 in Binary is 0000000010111010
16  in binary is 0000000000010000
=================================   (perform AND operation)
                 0000000000010000   The answer is 16

                            ^
                            |____ see how the 1's line up for bit 4


Bits are numbered from right to left starting at 0.

see the numbering example below.  

1111110000000000    (bit numbering
5432109876543210     chart)

0000000010111010     Decimal 186 (hex &hba)
        ^  ^
        |  |___bit 4
        |
        |___bit 7

------------------
George W. Bleck
Senior System Engineer
KeySpan Corporation


[This message has been edited by George Bleck (edited August 01, 2001).]

[Mike Trader]

Thank you for such an excellent explanation guys!

------------------
Kind Regards
Mike

[Lance Edmonds]

Mike, you may also wish to read the section on Short-Circuit evaluation in the IF/THEN BLOCK STATEMENT section of the help file.


------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Daniel Corbier]

Lance, I've seen an explanation for short-circuiting in these
forums. However, I can't find the Short-Circuit section that you
are referring to in the help file. My PB/DLL help file is dated
7/2/99. Do I have the latest one?

By the way, in the help file, there's a combo-box, but it always
seems to be empty (other than the text bar). It would be nice
for it to list previous search words like MSDN does.

------------------
Daniel Corbier
UCalc Fast Math Parser
http://www.ucalc.com

[Lance Edmonds]

My mistake. The section on Short-Circuit evaluation is in the printed documentation only.

Combobox? Do you mean the edit box in the Index Tab or on the Search Tab?

------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]asic.com</A>

[Daniel Corbier]

Lance, I'm referring to the one in the search tab.

------------------
Daniel Corbier
UCalc Fast Math Parser
http://www.ucalc.com

[Lance Edmonds]

The dropdown list in the search tab is controlled by Robohelp. I don't believe that antries are preserved between launches of the help file, but I'll ask the documentation department to look into it.

------------------
Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>