LabVIEW
how do i create arrays more efficiently
Yamaeda wrote:
I did a small test program. 10 million random numbers build an array (*2), one through indexing, 1 through preallocation with Initialize array and then replace. Loop and rebuild 100 times and check the result. This is my result!
It might be affected by version and system, but automatic indexing is FASTER than Replacing a preallocated array. 🙂/Y
I have been mislead for a long time. 
I took an intermediate class once, and I'm sure it was after 6.1. The instructor said to pre-allocate the array since it was faster than auto-indexing. Now I know better.
tbob wrote:
Yamaeda wrote:
I did a small test program. 10 million random numbers build an array (*2), one through indexing, 1 through preallocation with Initialize array and then replace. Loop and rebuild 100 times and check the result. This is my result!
It might be affected by version and system, but automatic indexing is FASTER than Replacing a preallocated array. 🙂/Y
I have been mislead for a long time.
Still valid for While loops since they can't anticipate how to size the array.
Ben
Ben wrote:I have been mislead for a long time.
Still valid for While loops since they can't anticipate how to size the array.
Ben
It is also important to note that it depends on how you are determining how many times to iterate the for loop. If you have an array that is created dynamically earlier in your program and you use an array size function wired to the N terminal to determine how many times the for loop will spin, there is no way for the memory to be fully allocated before hand(it doesn't know the array size). However, if you use a constant wired to the N terminal, or array size VI with a constant array known at compile time, then the memory will be preallocated (as far as I understand it). Correct me if I'm wrong. That said, I feel the most efficient way can often depend on the situation. Now because the question specifically says initialize array, this may be a bit out of scope but I feel it is an important point.
for(imstuck) wrote:
Ben wrote:I have been mislead for a long time.
Still valid for While loops since they can't anticipate how to size the array.
Ben
It is also important to note that it depends on how you are determining how many times to iterate the for loop. If you have an array that is created dynamically earlier in your program and you use an array size function wired to the N terminal to determine how many times the for loop will spin, there is no way for the memory to be fully allocated before hand(it doesn't know the array size). However, if you use a constant wired to the N terminal, or array size VI with a constant array known at compile time, then the memory will be preallocated (as far as I understand it). Correct me if I'm wrong. That said, I feel the most efficient way can often depend on the situation. Now because the question specifically says initialize array, this may be a bit out of scope but I feel it is an important point.
Maybe its your understanding of the term "before hand".
If the For Loop indexes off that array that was created earlier
AND
It has an auto-indexing output tunnel then LV will allocate an array for the output tunnel array as indicated by the in-coming array BEFORE the For Loop starts to execute so there is no need to size-up the buffer holding the output tunnel data.
So it is a quick one time operation prior the For loop starting to spin.
Now if you have the luxury to set the buffer size at development time, you do save move that time used to allocate the memory from execution time to load time and you also increase the size of your VI on disk.
Not trying to prove you wrong or similar. Just sharing my understanding too
1) Help you,
2) Get an early warning as to when I should give it up and retire.
Please feel free to Correct me if that seems wrong.
Keeping each other honest,
Ben
Ben wrote:Still valid for While loops since they can't anticipate how to size the array.
Ben
True, but maybe less so than you think. I've added While loops to my test, and Autoindexing (if the design allows it) in a while loop is half as fast as Replacing a preallocated array. It might sound like alot, but Build Array is actually 8 times slower than a autoindexed for-loop!
As many theorize it's probably due to memory management. With autoindex the compiler can try to allocate memory and let the loop fill it (which not surprisingly seems easier with a for-loop). With Initialize Array i suppose it actually fills the array costing time though the result is practically identical between for and while.
Build array increases memory usage is steps, causing several copies/resizes, nullifying the for-while difference. (I assume it works similar to Java, start at 50 bytes and then doubles each time it gets full)
/Y
