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BITCLR.PRG
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1991-08-16
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3KB
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90 lines
/*
* File......: BITCLR.PRG
* Author....: Forest Belt, Computer Diagnostic Services, Inc.
* Date......: $Date: 15 Aug 1991 23:02:50 $
* Revision..: $Revision: 1.2 $
* Log file..: $Logfile: E:/nanfor/src/bitclr.prv $
*
* This is an original work by Forest Belt and is placed in the
* public domain.
*
* Modification history:
* ---------------------
*
* $Log: E:/nanfor/src/bitclr.prv $
*
* Rev 1.2 15 Aug 1991 23:02:50 GLENN
* Forest Belt proofread/edited/cleaned up doc
*
* Rev 1.1 14 Jun 1991 19:50:58 GLENN
* Minor edit to file header
*
* Rev 1.0 01 Apr 1991 01:00:40 GLENN
* Nanforum Toolkit
*
*/
/* $DOC$
* $FUNCNAME$
* FT_BITCLR()
* $CATEGORY$
* String
* $ONELINER$
* Clear (reset) selected bit in a byte
* $SYNTAX$
* FT_BITCLR( <cByte>, <nBitPos> ) -> cByte
* $ARGUMENTS$
* <cByte> is a character from CHR(0) to CHR(255).
*
* <nBitPos> is a number from 0 to 7 conforming to standard
* right-to-left bit numbering convention and representing the
* position of the bit within the byte.
* $RETURNS$
* Returns new byte, with designated bit cleared (reset).
* If parameters are faulty, returns NIL.
* $DESCRIPTION$
* In effect, ANDs argument byte with a byte that has all bits set except
* the target bit. If bit is already clear (0), it remains clear.
* Note: Calls FT_ISBIT() which is also in this Library.
*
* This function is presented to illustrate that bit-wise operations
* are possible with Clipper code. For greater speed, write .C or
* .ASM versions and use the Clipper Extend system.
* $EXAMPLES$
* This code would clear bit 4 in a byte represented by CHR(115):
*
* cNewByte := FT_BITCLR( CHR(115), 4 )
* ? ASC( cNewbyte ) // result: 99
* ? cNewByte // result: 'c'
*
* This code would clear bit 5 in the byte represented by letter 'A':
*
* FT_BITCLR( 'A', 5 ) // result: 'A', since
* // bit 5 already clear
*
* For a demonstration of Clipper bit manipulations, compile and
* link the program BITTEST.PRG in the Nanforum Toolkit source code.
* $SEEALSO$
* FT_BITSET() FT_ISBIT()
* $END$
*/
FUNCTION FT_BITCLR(cInbyte, nBitpos)
LOCAL cByte
IF valtype(cInbyte) != "C" .or. valtype(nBitpos) != "N" // parameter check
cByte := NIL
ELSE
IF (nBitPos > 7) .or. (nBitPos < 0) .or. (nBitPos != int(nBitPos))
cByte := NIL
ELSE
cByte := iif( .not. FT_ISBIT(cInByte, nBitpos), cInByte, ;
chr(asc(cInByte) - (2 ^ nBitpos)))
ENDIF
ENDIF
RETURN cByte