Array performance

General Programming
1

I’ve been reading about dynamic arrays in C. Fun bed time reading right? It has got me thinking about arrays in Xojo however.

Let’s say I have a need for a dynamically-growable array of UInt8.

// Define
Var bytes() As UInt8
// Add a new byte
bytes.AddRow(10)

Is it more performant to allocate a certain amount of elements up front (accepting that uses more memory) and insert into an empty index or call AddRow?

// Sloppy code but you get the gist. 
Var bytes(1000) As UInt8
Var pos As Integer = 0
Var lastIndex As Integer = 999

bytes(pos) = 10
pos = pos + 1
If pos > lastIndex Then
  bytes.ResizeTo(2000)
lastIndex = 1999
End If
2

I believe that you will find pre-allocating the array is quicker

As far as I recall append will grow the array in “chunks” but it has to do it by using realloc (I think thats what it uses) which grows it in place (again I think this is the case)
And if it cant grow it in place it may perform even worse

Pre-allocating skips all those reallocs

1 Like
3

From experience I can tell you that dimming the array to the preferred size is quicker than appending during population.

For arrays to which I don’t know the size to begin with, I over dim, then redim at the end.

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4

Thanks guys, that confirms my suspicions. Man, writing a compiler in Xojo is hard. All these little performance hacks add up.

5

One reason Xojo’s compiler wssnt written in Xojo
Plus LLVM makes it way simpler to target difference CPU’s etc