this post was submitted on 28 Apr 2024
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Science Memes

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[–] [email protected] 122 points 6 months ago (8 children)

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

[–] [email protected] 72 points 6 months ago (4 children)
[–] [email protected] 101 points 6 months ago (1 children)
[–] [email protected] 40 points 6 months ago (1 children)

So is a wedding gift registry.

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

No, this is Patrick!

[–] [email protected] 80 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 (5 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:

  1. Preallocate the vector if you can guesstimate the size
  2. 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!

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[–] [email protected] 3 points 6 months ago

Many things like each other lined up in a row, and you can take some away or put more in.

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[–] [email protected] 25 points 6 months ago (2 children)

No, it's an n-tuple with certain algebraic properties.

[–] [email protected] 7 points 6 months ago (1 children)

This is such an understated but useful description in this context. It's also how I understood algebra for applied matrix computation.

[–] [email protected] 4 points 6 months ago* (last edited 6 months ago)

I was just coming down from THC when I wrote this, so I'm extra jazzed you liked it. 😁

Edit: also, love the username.

[–] [email protected] 5 points 6 months ago (1 children)

Its the algebraic properties that are important, not all vectors are n-tuples, eg the set of polynomials of degree less than n.

You need a basis to coordinate a vector, you can work with vectors without doing that and just deal with the algebraic properties. The coordinate representation is dependent on the basis chosen and isn't fundamental to the vector. So calling them n-tuples isn't technically correct.

You can turn them into a set of coordinates if you have a basis, but the fact that you can do that is because of the algebraic properties so it's those properties which define what a vector is.

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[–] [email protected] 76 points 6 months ago (4 children)

What do you mean? A vector is a direction and magnitude!

[–] [email protected] 28 points 6 months ago (1 children)

Maybe they mean std::vector in C++?

[–] [email protected] 21 points 6 months ago (2 children)

It's a terrible name. The math answer is what I would give.

[–] expr 4 points 6 months ago (1 children)

It's not a terrible name, since it's derived from the mathematical construct of vectors as n-tuples. In the case of vectors in programming, n relates to the size of the underlying array, and the tuple consists of the elements of the vector.

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[–] [email protected] 3 points 6 months ago

I myself was confused, when I first saw what a vector did in practice.
Really bad name.

But then I didn't take Comp Sci.

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

The only correct answer for a 101 introduction. It’s an incredible powerful intuition even in contexts where vectors are seemingly used as a list of numbers.

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[–] [email protected] 76 points 6 months ago (4 children)

Did nobody else's CS department require a bunch of linear algebra courses? A vector is an element of vector space.

[–] [email protected] 12 points 6 months ago* (last edited 6 months ago)

Sets are just objects in the category of Set.

[–] [email protected] 11 points 6 months ago (2 children)

That is quite possibly the least helpful answer you could give.

[–] [email protected] 9 points 6 months ago

Also the most correct :)

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

Q: what is a vector? A: it is a vector

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

We do and we know this. Maybe programmers would give that answer

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[–] [email protected] 39 points 6 months ago (2 children)

Ooh, do tensors next!

You should ask your biologist friend and your physicist friend and your compsci friend to debate about what vectors are. Singularities, too.

[–] [email protected] 26 points 6 months ago

Singularities, too.

/dev/null

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

It’s just a fancy list of fancy lists! :D

[–] [email protected] 27 points 6 months ago
[–] [email protected] 24 points 6 months ago

you just need to car and cdr your cons cell …

[–] [email protected] 20 points 6 months ago* (last edited 6 months ago) (1 children)

Well mathematically isn't it an n by 1 matrix.

[–] [email protected] 28 points 6 months ago* (last edited 6 months ago) (9 children)

Not always. Any m by n matrix is also a vector. Polynomials are vectors. As are continuous functions.

A vector is an element of a vector space over a field. These are sets which have a few operations, vector addition and scalar multiplication, and obey some well known rules, such as the existence of a zero vector (identity for vector addition), associativity and commutativity of vector addition, distributivity of scalar multiplication over vector sums, that sort of thing!

These basic properties give rise to more elaborate concepts such as linear independence, spanning sets, and the idea of a basis, though not all vector spaces have a finite basis.

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[–] onlinepersona 14 points 6 months ago (3 children)

It's an array.

First time I heard of vectors in comp-sci was in C++. The naming still doesn't make sense to me.

Anti Commercial-AI license

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[–] [email protected] 8 points 6 months ago

As a mathematician this genuinely hurts. Lol.

[–] [email protected] 8 points 6 months ago (1 children)

I asked my math friend. He said a vector is magnitude plus velocity.

[–] [email protected] 42 points 6 months ago

It should be magnitude plus orientation, not velocity. Velocity itself is a vector quantity

[–] [email protected] 4 points 6 months ago (1 children)

A vector is a list of numbers, at its most basic. You can add a lot of extra functionality to it, but at its core, its just a list.

[–] [email protected] 4 points 6 months ago (2 children)

Functions from the reals to the reals are an example of a vector space with elements which can not be represented as a list of numbers.

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