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Explain yourselves, comp sci. (mander.xyz)

submitted 6 months ago by [email protected] to c/[email protected]

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[–] [email protected] 81 points 6 months ago* (last edited 6 months ago) (1 children)

It's a dynamically-sized list of objects of the same type stored contiguously in memory.

dynamically-sized: The size of it can change as needed.

list: It stores multiple things together.

object: A bit of programmer defined data.

of the same type: all the objects in the list are defined the same way

stored contigiously in memory: if you think of memory as a bookshelf then all the objects on the list would be stored right next to each other on the bookshelf rather than spread across the bookshelf.

[–] [email protected] 15 points 6 months ago (4 children)

Dynamically sized but stored contiguously makes the systems performance engineer in me weep. If the lists get big, the kernel is going to do so much churn.

[–] [email protected] 15 points 6 months ago (1 children)

Contiguous storage is very fast in terms of iteration though often offsetting the cost of allocation

[–] [email protected] 7 points 6 months ago

Modern CPUs are also extremely efficient at dealing with contiguous data structures. Branch prediction and caching get to shine on them.

Avoiding memory access or helping CPU access it all upfront switches physical domain of computation.

[–] [email protected] 7 points 6 months ago* (last edited 6 months ago)

Which is why you should:

Preallocate the vector if you can guesstimate the size
Use a vector library that won't reallocate the entire vector on every single addition (like Rust, whose Vec doubles in size every time it runs out of space)

Memory is fairly cheap. Allocation time not so much.

[–] [email protected] 4 points 6 months ago

matlab likes to pick the smallest available spot in memory to store a list, so for loops that increase the size of a matrix it's recommended to preallocate the space using a matrix full of zeros!

[–] [email protected] 2 points 6 months ago (1 children)

Is that churn or chum? (RN or M)

[–] [email protected] 5 points 6 months ago

Churm