If I have the following ...
a OrElse b
... and a is True then clearly b is never evaluated. But if I add an Or, then what?
a OrElse b Or c
Does/should c get evaluated? And what if I put in some brackets?
Apologies if this is basic. Of course I can test for the answer myself but I can't find this question answered here or elsewhere. Lots of questions dealing with Or versus OrElse but nothing dealing with Or with OrElse
This is an operator precedence problem. The relevant documentation is here:
http://msdn.microsoft.com/en-us/library/fw84t893.aspx?ppud=4
The important excerpts:
- Operators with equal precedence are evaluated left to right in the order in which they appear in the expression.
and
Inclusive disjunction (Or, OrElse)
So what we learn here is that Or and OrElse have the same precedence and that operators with the same precedence are evaluated from left to right.
Therefore, I would expect that in cases where a is true, b is not evaluated. However, c still will be. In cases where a is false, b is evaluated and regardless of the b's value the Or operator will evaluate c. So, yes, c is always evaluated.
As a practical matter, you should generally prefer OrElse in your code unless you have a good reason to use Or. Or exists now mainly for backwards compatibility.
And has higher precedence that Or/Orelse. Still, in my program when I used test(1) OrElse test(2) And test(3) I get 1 and true as output. Any idea? –
Finisterre 2,3,1,true from that. I suspect that it's jitter optimization of some kind. –
Hecate resultofAnd first because of And's higher precedence, and that should cause it to output 2 and 3 prior to the 1. –
Hecate test(x) function outputs x and returns True then test(1) OrElse test(2) And test(3) will always output just 1 and evaluate to true. From the OrElse documentation "If the first expression is True, the second is not evaluated". Are you supposing that using an expression as the second operand to OrElse must cause the second operand to be evaluated before the first operand? That would be rather bizarre. It seems to me it would make OrElse rather useless! –
Seritaserjeant And has higher precedence, it should be as if the full expression were written like this: test(1) OrElse (test(2) And test(3)). –
Hecate test(1) OrElse (test(2) And test(3)). That doesn't necessarily determine the evaluation sequence. To evaluate test(2) And test(3) before test(1) would contradict the specification for OrElse. The compiler can easily parse the expressions for the operands before it finally generates any IL. Then it could generate IL to evaluate test(1) first, and then, only if test(1) evaluated to False, to evaluate test(2) And test(3). Surely this must be how it works? –
Seritaserjeant OrElse short circuits between the left and right side parameters (only 2 parameters). So I would say that C will always be evaluated as you could treat this as (A OrElse B) Or C.
In the case presented, c is evaluated. A small test will tell you:
Debug.WriteLine(test(1) OrElse test(2) Or test(3))
Function test(ByVal a As Integer) As Boolean
Debug.WriteLine(a)
Return True
End Function
The above example outputs:
1 3 True
This is an operator precedence problem. The relevant documentation is here:
http://msdn.microsoft.com/en-us/library/fw84t893.aspx?ppud=4
The important excerpts:
- Operators with equal precedence are evaluated left to right in the order in which they appear in the expression.
and
Inclusive disjunction (Or, OrElse)
So what we learn here is that Or and OrElse have the same precedence and that operators with the same precedence are evaluated from left to right.
Therefore, I would expect that in cases where a is true, b is not evaluated. However, c still will be. In cases where a is false, b is evaluated and regardless of the b's value the Or operator will evaluate c. So, yes, c is always evaluated.
As a practical matter, you should generally prefer OrElse in your code unless you have a good reason to use Or. Or exists now mainly for backwards compatibility.
And has higher precedence that Or/Orelse. Still, in my program when I used test(1) OrElse test(2) And test(3) I get 1 and true as output. Any idea? –
Finisterre 2,3,1,true from that. I suspect that it's jitter optimization of some kind. –
Hecate resultofAnd first because of And's higher precedence, and that should cause it to output 2 and 3 prior to the 1. –
Hecate test(x) function outputs x and returns True then test(1) OrElse test(2) And test(3) will always output just 1 and evaluate to true. From the OrElse documentation "If the first expression is True, the second is not evaluated". Are you supposing that using an expression as the second operand to OrElse must cause the second operand to be evaluated before the first operand? That would be rather bizarre. It seems to me it would make OrElse rather useless! –
Seritaserjeant And has higher precedence, it should be as if the full expression were written like this: test(1) OrElse (test(2) And test(3)). –
Hecate test(1) OrElse (test(2) And test(3)). That doesn't necessarily determine the evaluation sequence. To evaluate test(2) And test(3) before test(1) would contradict the specification for OrElse. The compiler can easily parse the expressions for the operands before it finally generates any IL. Then it could generate IL to evaluate test(1) first, and then, only if test(1) evaluated to False, to evaluate test(2) And test(3). Surely this must be how it works? –
Seritaserjeant In my personal experience VB tends to obtain the value for all of them regardless of whether they be actually evaulated or not.
This is only helpfull when order matters for items existance and the like. Just wanted to note it.
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