Operators - Logical and Bitwise Operators Continued (Page 3 of 4 )
And
The And operator performs a logical or bitwise conjunction on the two source operands. In logical operations, it returns True if and only if both operands evaluate to True. If either operand is False, then the result is False. The syntax is:
result = expression1 And expression2
For example, consider the following statement:
If (x = 5) And (y < 7) Then
In this case, the code within the Then clause will be executed only if the value of x is 5 and the value of y is less than 7.
As a bitwise operator, And returns 1 in a bit position if the compared bits in the same position in both expressions are 1, and it returns 0 in all other cases, as shown in the following table:
Bit in expression1 0 | Bit in expression2 0 | Result 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
For example, the bitwise result of 15 And 179 is 3, as the following binary representation shows:
00001111 And 10110011 -> 00000011
AndAlso
The AndAlso operator works exactly like the logical And operator, but short-circuiting is enabled. If the first operand evaluates to False, the second operand is not evaluated at all, even if that expression includes function calls. Operands are evaluated from left to right. AndAlso does not perform bitwise operations.
Or
The Or operator performs a logical or bitwise disjunction on the two source operands. In logical operations, it returns True if either of the operands evaluates to True. If both operands are False, then the result is False. The syntax is:
result = expression1 Or expression2
For example, consider the following statement:
If (x = 5) Or (y < 7) Then
In this case, the code within the Then clause will be executed if either the value of x is 5 or the value of y is less than 7.
As a bitwise operator, Or returns 1 in a bit position if either of the compared bits in the same position in the source expressions are 1, and it returns 0 in all other cases, as shown in the following table:
Bit in expression1 0 | Bit in expression2 0 | Result 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
For example, the bitwise result of 15Or179 is 191, as the following binary representation shows:
00001111 Or 10110011 -> 10111111
OrElse
The OrElse operator works exactly like the logical Or operator, but short-circuiting is enabled. If the first operand evaluates to True, the second operand is not evaluated at all, even if that expression includes function calls. Operands are evaluated from left to right. OrElse does not perform bitwise operations.
Not
The Not operator performs a logical or bitwise negation on a single expression. In logical operations, it returns True if the operand is False, and False if the operand is True. The syntax is:
result = Not expression1
For example, consider the following statement:
If Not IsNumeric(x) Then
In this example, the code within the Then clause will be executed if IsNumeric returns False, indicating that x is not a value capable of being represented by a number.
As a bitwise operator, Not simply toggles the value of each bit in the source expression between 0 and 1, as shown in the following table:
Bit in expression1 | Result |
0 | 1 |
1 | 0 |
For example, the bitwise result of Not 16 is 239, as the following binary representation shows:
Not 00010000 -> 11101111
Xor
The Xor (an abbreviation for "eXclusive OR") operator performs a logical or bitwise exclusion on the two source operands. In logical operations, it returns True if and only if the two expressions have different truth values. If both expressions are True, or both are False, this operator returns False. If one of the operands is True but the other False, then Xor returns True. The syntax is:
result = expression1 Xor expression2
As a bitwise operator, Xor returns 1 in a bit position if the compared bits are different from each other, and it returns 0 if they are the same, as shown in the following table:
Bit in expression1 0 | Bit in expression2 0 | Result 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
For example, the result of 15 Xor 179 is 188, as the following binary representation shows:
00001111 Xor 10110011 -> 10111100
Eqv and Imp
Eqv and Imp, two logical and bitwise operators present in VB 6, have been removed from .NET implementations of Visual Basic. Eqv can be replaced with the = (equal to) comparison operator. The expression:
expression1 Eqv expression2
is the same as the logical comparison:
expression1 = expression2
Imp can be replaced with a logical expression using the Not and Or operators. For example:
expression1 Imp expression2
can also be expressed as:
(Not expression1) Or expression2
If you need more precise replacements using bitwise calculations, see the "Logical and Bitwise Operators" section in Appendix D.
<< (Shift Left)
New in 2003. The << (shift left) operator performs a left shift of the bits in the first operand by the number of bits specified in the second operand. All bits shifted off the left are lost. All bits newly vacated on the right are filled with zeros.
The number of bits you can shift is limited by the number of possible bits in the first operand. Any excess number of shift positions will be ignored. This operator never throws an overflow exception. The syntax is:
result = source << bits
For example, the bitwise result of 15 << 5 is 224, as the following binary representation shows:
00001111 << 5 -> 11100000
>> (Shift Right)
New in 2003. The >> (shift right) operator performs a right shift of the bits in the first operand by the number of bits specified in the second operand. All bits shifted off the right are lost. All bits newly vacated on the left are filled with the bit value of the leftmost bit position before shifting. When shifting unsigned data values (Byte , UShort, UInteger, ULong), the newly vacated bits on the left are filled with zero (0).
The number of bits you can shift is limited by the number of possible bits in the first operand. Any excess number of shift positions will be ignored. This operator never throws an overflow exception. The syntax is:
result = source >> bits
For example, the bitwise result of 12 >> 1 is 6, as the following binary representation shows:
00001100 >> 1 -> 00000110
Next: Assignment Operators >>
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 This article is excerpted from chapter five of Visual Basic 2005 in a Nutshell, Third Edition, written by Tim Patrick, Steven Roman, Ph.D., Ron Petrusha and Paul Lomax (O'Reilly; ISBN: 059610152X). Check it out today at your favorite bookstore. Buy this book now.
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