I’m trying to wrap my head around arrays in Julia. In Java, if you have an array (int[] arr, for example) with 100 elements, then to add another element, you have to make another array with size 101 and copy all the old elements over. In Julia, push! doubles the size of allocated memory each time …

push! will not necessarily double the allocated memory for the array every time it is called, but it does need to make room for whatever you are appending to the array. It’s worth noting that in some cases push! in Julia can actually be surprisingly efficient (I think this happens because often the…

If you have a rough idea of the upper bound on size, you can first allocate an array of that size, and then empty! it. Julia won’t deallocate the reserved space, and push! into that array won’t allocate new memory.

The preferred way to do this is with sizehint!. E.g. v = Int[]; sizehint!(v, 10_000) will tell julia to make space such that v has room to store 10_000 integers.

@squirrel , some array classes in Java also double: java - Expanding an Array? - Stack Overflow I think what you’re missing is the following: the capacity is different from the size. It’s the capacity that doubles, not the size. Just try push! and see for yourself—the length is always equal to the n…

You’re right. “Capacity” is exactly what I was reaching for. I like that Julia doubles the capacity each time it needs to expand its capacity, I just would like to pre-define a large capacity for my array so that I can skip the growing pains.

That’s a clever idea. Are you sure that Julia will NEVER deallocate the space, or is there a possibility that the program might hit some sort of limitation and deallocate space (assumes) it no longer needs?

[image] ExpandingMan: imagine filling your array with other arrays [image] ExpandingMan: to use isassigned when populating an array is if it was already partially populated by something else Ironically, this is my use case on both counts. :sweat_smile: :upside_down_face: :sob: Thank…

[image] squirrel: I just would like to pre-define a large capacity for my array so that I can skip the growing pains. I know you saw it, but @Mason ’s suggestion to use sizehint! is what you’re looking for.

[image] squirrel: Are you sure that Julia will NEVER deallocate the space, or is there a possibility that the program might hit some sort of limitation and deallocate space (assumes) it no longer needs? In the current implementation, the only way to shrink the capacity is with sizehint!.

[image] squirrel: Are you sure that Julia will NEVER deallocate the space, or is there a possibility that the program might hit some sort of limitation and deallocate space (assumes) it no longer needs? In addition to the others said: worrying about the details of GC is usually a distraction …

Best Way to Declare Arrays

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squirrel January 16, 2020, 4:20pm 12

That’s a really good point. I don’t think I’ve every really considered programming in that sort of abstraction context before.

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Best Way to Declare Arrays

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