B4J Question Compiler question

[Ray Y]

Given:
For I = 0 To somelist.Size - 1

Is the expression "somelist.Size - 1" recomputed every loop iteration?

 

[Ed Brown]

It should be - yes. This is because the list 'somelist' can change in size while inside the loop.

If the list does not change size then it is more efficient (with large lists) to set the another variable to the size of the list before the loop.

 

[KMatle]

Nope. Example: If you remove items from a list while processing you need to do it backwards:

For list.Size-1 To 0 Step -1

otherwiese you get an outofbounds exeption.

 

[Ed Brown]

Nope.

Hmm, you're right. But this is also not the way it's meant to be. Why is B4J optimising the for/loop by assigning a variable (as my original suggestion)? There are times where this is not the desired behaviour and for B4J to simply just do this is WRONG and not the normal behaviour.

 

[KMatle]

B4J to simply just do this is WRONG and not the normal behaviour

In VB.net it's the same. The FOR statement is initialized ONCE with the given values. Changing it later will not cause the FOR loop to finish (e.g. if it was 10 at start and now it's 5 it will not change the terminator of the FOR statement).

 

[Ray Y]

Programming 101: Don't use a FOR loop unless you know the number of times you want the loop executed. And certainly don't change the loop variables inside the loop. I scream every time I see:

x = 10
For i = 1 to x
   do something
   if WeAreDone then x = 1
next i

Lazy lazy lazy. This is stuff C programmers do all the time. Drives me crazy. Plus, it makes debugging big loops a nightmare. This is what WHILE is for.

Anyway, I was just asking to know if I had to use a temp variable to store the computed result, or if it works the way _I_ think it should and optimizes it.

And I was too lazy to write my own test.

Thanks all.

 

[Erel]

But this is also not the way it's meant to be. Why is B4J optimising the for/loop by assigning a variable (as my original suggestion)?

There is no right or wrong here. This is how For loops behave in B4X.
https://www.b4x.com/android/forum/pages/results/?query=for

 

[KMatle]

From the documentation:

Note that the loop limits will only be calculated once before the first iteration.

 

[Ray Y]

From the documentation:

Ah...RTFM. My apologies for not seeing that. Where is that phrase actually located (i.e. which "documentation"?)

 

[Erel]

See the description before the search results:
https://www.b4x.com/android/forum/pages/results/?query=for

 

[Ray Y]

See the description before the search results:
https://www.b4x.com/android/forum/pages/results/?query=for

Yes, I see it there, but where did that description come from? Not a forum post. Wherever it is, I obviously need to take a look at the rest of what's there to avoid posting another RTFM question in the future.

 

[Erel]

Keywords documentation: https://www.b4x.com/android/help/core.html#keywords

You can also see it in the IDE itself:

 

[Ray Y]

Much thanks. Is the IDE stuff a duplicate of that Keywords core help documentation?

 

[Erel]

They both extract the information from the libraries XML files.

There are also several "help viewers" tools that will show you the same information from your list of libraries.