In an article I was reading
(
http://www.ftponline.com/vsm/2005_06/magazine/columns/desktopdeveloper/),
I read the following:
"The ending condition of a VB.NET for loop is evaluated only once, while the
C# for loop ending condition is evaluated on every iteration."
Is this accurate? I don't understand how you could get away without
evaluating the ending condition at every iteration. Otherwise, how would you
know whether or not to exit?
This is arguably accurate, but misleading. (What follows assumes
knowledge of C#)
The correct VB.NET construction to compare a C# for loop to is not a
For ... Next loop. Let us review a C# for loop:
for ( <initial statement> ; <loop continuation condition> ; <step
statement> )
<statment block>
[no nitpicks please
]
Note that the initial statement and step statement can be ANY statement
you like. What this is equivalent to is a particular form of the VB.NET
Do ... Loop loop:
<initial statement>
Do While <loop continuation condition>
<statment block>
<step statement>
Loop
Whereas a VB.NET For ... Next loop:
For <index> = <start value> To <end value> Step <step value>
<statement block>
Next
is equivalent to this, *different*, Do ... Loop loop:
<index = start value>
<CONST end value = whatever>
Do While <index <= end value>
<statement block>
<index += step value>
Loop
That CONST is effectively what the referenced article is talking about:
With a VB.NET For ... Next loop, the end value is evaluated ONCE, and
for the whole time the loop is looping, the loop continuation condition
is 'index <= end value'. With a C# for() loop, the actual *expression*
supplied as the loop continuation condition is evaluated EVERY TIME
around.
Example:
VB.NET:
Dim endvalue as integer = 10
dim i as integer
for i = 1 to endvalue
msgbox(i.tostring)
endvalue \= 2
next
'Because endvalue is evaluated ONCE at the start of the loop,
'you will see TEN messageboxes
C#:
int ev=10;
int i;
for(i=1; i<=ev; i++)
{
MessageBox.Show (i.ToString());
ev /=2;
}
// because "i<=ev" is evaluated EVERY TIME ROUND,
// you will see only TWO message boxes
Make sense?
