VBspeed: ShiftRight

VBspeed / Bits / ShiftRight

VBspeed © 2000-10, updated: 01-Oct-2001

ShiftRight OLD

NOTE --- 09-Oct-2001
All codes featured below go WRONG in most of the cases where Value is negative. For example, ShiftRight(-3,1) should return -2 (not -1): 11111111111111111111111111111101 -3 ShiftRight(-3, 1) -> 11111111111111111111111111111110 -2 The charts and codes below are presented only for internal reference (learning from mistakes, you know). Corrected versions here: ShiftRight.

The Definition

Function ShiftRight
Shifts the bits to the right the specified number of positions and returns the new value. Bits "falling off" the right edge do not wrap around. Fill bits coming in from left match bit 31 (the sign bit): if bit 31 is 1 the fill bits will be 1 (see ShiftRightZ for the alternative zero-fill-in version). A shift right is effectively a division by 2. Some common languages like C/C++ or Java have an operator for this job: ">>".
Declaration
Function ShiftRight(Value As Long, ShiftCount as Long) As Long
Arguments
Value	value to use for operation
ShiftCount	number of positions to shift, valid range is 0 thru 31.
You may use this function (VB5/6-compatible) to verify the correctness of your code.

The Charts

Calls
	lRet = ShiftRight(Value, ShiftCount)
Call 1	Value = 256 ShiftCount = 8
Call 2	Value = &H87654321 ShiftCount = 24

VB5

Code	Author	Doping	Notes
ShiftRight01	Donald
ShiftRight02	Donald
ShiftRight03	Jost
ShiftRight04	Jost

Call 1
4	4.25	0.196µs
3	4.25	0.196µs
2	3.13	0.144µs
1	1.00	0.046µs

Call 2
3	4.26	0.196µs
4	4.26	0.196µs
2	3.14	0.144µs
1	1.00	0.046µs

VB6

Code	Author	Doping	Notes
ShiftRight01	Donald
ShiftRight02	Donald
ShiftRight03	Jost
ShiftRight04	Jost

Call 1
3	3.90	0.179µs
4	3.90	0.179µs
2	3.13	0.144µs
1	1.00	0.046µs

Call 2
4	3.90	0.180µs
3	3.90	0.179µs
2	3.13	0.144µs
1	1.00	0.046µs

Conclusions
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The Code

ShiftRight01
submitted 18-Dec-2000 by Donald Lessau
Doping: none
Public Function ShiftRight01(Value As Long, ByVal ShiftCount As Long) As Long ' by Donald, donald@xbeat.net, 20001218 Select Case ShiftCount Case 0 To 31 ShiftRight01 = Value \ Pow2(ShiftCount) End Select End Function
Author's comments: See Pow2
Donald's comments:

ShiftRight02
submitted 18-Dec-2000 by Donald Lessau
Doping: none
Public Function ShiftRight02(Value As Long, ByVal ShiftCount As Long) As Long ' by Donald, donald@xbeat.net, 20001218 If ShiftCount < 0 Or ShiftCount > 31 Then Exit Function ShiftRight02 = Value \ Pow2(ShiftCount) End Function
Author's comments: See Pow2
Donald's comments:

ShiftRight03
submitted 28-Sep-2001 by Jost Schwider vb-tec.de
Doping: none
Public Static Function ShiftRight03(Value As Long, ByVal ShiftCount As Long) As Long ' by Jost Schwider, jost@schwider.de, 20010928 Dim Pow2(0 To 31) As Long Dim i As Long Select Case ShiftCount Case 0 To 31 'Ggf. Initialisieren: If i = 0 Then Pow2(0) = 1 For i = 1 To 30 Pow2(i) = 2 * Pow2(i - 1) Next i End If ShiftRight03 = Value \ Pow2(ShiftCount) End Select End Function
Author's comments:
Donald's comments:

ShiftRight04
submitted 01-Oct-2001 by Jost Schwider vb-tec.de
Doping: none
Public Function ShiftRight04(ByVal Value As Long, ByVal ShiftCount As Long) As Long ' by Jost Schwider, jost@schwider.de, 20010928 Select Case ShiftCount Case 0&: ShiftRight04 = Value Case 1&: ShiftRight04 = Value \ &H2& Case 2&: ShiftRight04 = Value \ &H4& Case 3&: ShiftRight04 = Value \ &H8& Case 4&: ShiftRight04 = Value \ &H10& Case 5&: ShiftRight04 = Value \ &H20& Case 6&: ShiftRight04 = Value \ &H40& Case 7&: ShiftRight04 = Value \ &H80& Case 8&: ShiftRight04 = Value \ &H100& Case 9&: ShiftRight04 = Value \ &H200& Case 10&: ShiftRight04 = Value \ &H400& Case 11&: ShiftRight04 = Value \ &H800& Case 12&: ShiftRight04 = Value \ &H1000& Case 13&: ShiftRight04 = Value \ &H2000& Case 14&: ShiftRight04 = Value \ &H4000& Case 15&: ShiftRight04 = Value \ &H8000& Case 16&: ShiftRight04 = Value \ &H10000 Case 17&: ShiftRight04 = Value \ &H20000 Case 18&: ShiftRight04 = Value \ &H40000 Case 19&: ShiftRight04 = Value \ &H80000 Case 20&: ShiftRight04 = Value \ &H100000 Case 21&: ShiftRight04 = Value \ &H200000 Case 22&: ShiftRight04 = Value \ &H400000 Case 23&: ShiftRight04 = Value \ &H800000 Case 24&: ShiftRight04 = Value \ &H1000000 Case 25&: ShiftRight04 = Value \ &H2000000 Case 26&: ShiftRight04 = Value \ &H4000000 Case 27&: ShiftRight04 = Value \ &H8000000 Case 28&: ShiftRight04 = Value \ &H10000000 Case 29&: ShiftRight04 = Value \ &H20000000 Case 30&: ShiftRight04 = Value \ &H40000000 Case 31&: ShiftRight04 = Value \ &H80000000 End Select End Function
Author's comments:
Donald's comments:

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