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How to select an ideal array size in C?

14 points by AlbertJWilliams ↗ HN
So far I've seen so many pieces of code where arrays are declared with a seemingly random size, I mean, the size doesn't seem to have a specific reason, but it seems more like a magic number. How do I select the size for my arrays? Is it just about putting on a big number? I still don't get it, and I feel like if I knew that, my code would get a lot better. Thanks in advance.

13 comments

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nuclx
In common user-space non-realtime environments only use arrays, if you have known bounds for the amount of elements. If an arbitrary number of elements is needed, you need dynamic memory management anyways, so use linked lists. If your requirements prohibit malloc (at all or after initialization), you need to work with the available RAM as it suits your needs. Use fixed upper bounds, but check for full arrays and report errors, if array bounds are exceeded. This can be a bit of an art, especially when it comes to deciding what data to keep on the stack, in global arrays or on the heap as well as choosing approriate array sizes. Profile the RAM you need against what is available to you and optimize array sizes to make best use of the available RAM.
mathieubordere
Dynamic memory management in no way implies linked lists.
nuclx
Fair point. There are certainly other options like realloc'ing the memory for the array, if you want to keep constant time indexing.
asdjw0efjlka
He's a C programmer and they don't have generics, so they don't take advantage of vectors, but when you point this out they will display a macro hack or void* hack, then continue to use linked lists.
nuclx
Did you see the OP ask any questions about C++?
AlbertJWilliams
Is dynamic localization a valid and elegant solution or is it just an ugly hack? I want to use it but I also want to keep my code as flawless as possible (even if I'm still learning, I'd like to write high quality code).
agitator
I assume you meant to say dynamic allocation?

It depends on your application.

If your application is fine with returning failed allocations and can handle the case, then it's okay. But if you have a real-time or safety critical system, for example, something running on a vehicle, robot, or flying device, you usually can't risk failed allocations. In that case the better bet is to reserve memory on the heap for your array and ensure that you are within bounds or within a certain threshold by checking on push data to your array or using a circular buffer.

There are many approaches to solving a problem, and many ways to implement the solution, but the path you take usually depends on your application and your constraints. Think about which set of tradeoffs with respect to performance, safety, and memory usage makes the most sense for what you are trying to do.

meh2frdf
Your best guess +1 ;)
meh2frdf
Really a down vote for a C joke, or did you not get it?
jventura
What do you want to insert on your arrays?! Numbers, characters? Could you provide a specific example of what you want?
krapp
This is my heuristic - make your best guess (you should have some idea), double it if you're uncertain, allocate the nearest power of 2 above that. If during testing it turns out not to be large enough, realloc as necessary.
SamReidHughes
Generally speaking, you don't do this. You figure out the size you need and allocate it dynamically. If you don't know the size, you use a dynamically growing array type (that does the same thing as C++'s std::vector). E.g. use asprintf (or implement your own using vsnprintf) as necessary. What you don't do is emulate the behavior of janky C programs that allocate a fixed size array. That's bad.

(Sometimes you have arrays where the size doesn't affect the correctness of your program. An example is if you're processing a file in chunks (e.g. to compute ROT-13), then chunk size will affect performance, but not correctness. You pick the size appropriate for your use case.)

swah
The good enough algorithm is "take a guess, then double every time a new element doesn't fit". Really, just watch the whole Handmade Hero series..