Knowing where you are: custom array indices in Julia (opens in new tab)

(julialang.org)

98 pointsjrevels9y ago28 comments

28 comments

16 comments · 5 top-level

goatlover9y ago· 3 in thread

I really like how Julia handles arrays.

Agreed. One big win here over numpy is that there's the `push!` function on Julia arrays (equivalent to the C++ vector's `push_back`).

Numpy arrays on the other hand are awkward to use in custom algorithms because once the memory is reserved, you can't grow the array at either end.

zeptomu9y ago

I think that is some kind of feature. Numpy tries hard to guarantee efficient access to its ndarrays and this is simpler to implement using fixed arrays and mostly you know your bounds at initialization time anyway.

Btw. you can grow them using hstack and vstack although I am not sure how efficient that is.

2 more replies

rjeli9y ago

Yep, a problem I repeatedly run into with Numpy is that my data is too big to fit in memory as Python data, but can easily fit as Numpy matrices. It seems incrementally building Numpy matrices is very awkward and inefficient

edit: if anyone knows a solution to this, please let me know...

3 more replies

killin_dan9y ago· 3 in thread

This is possible in ruby too, probably in a lot of languages.

I've rewritten someobject#[] plenty of times to have cleaner code.

one-more-minute9y ago

One difference in other languages is that the extra indirection will add overhead. You have to (at the very least) hop through an extra function call, and even that will become significant if you're writing an image kernel.

Julia can inline through all the `getindex!` functions so you get the same performance as with a hand-coded C or Fortran array.

jksmith9y ago

All Wirth languages did this. Great for clarity when a specific mapping is needed. Damn I miss me some Modula 2.

evacchi9y ago

Nim is very much Oberon inspired and supports this

1 more reply

codezero9y ago· 2 in thread

I remember being really intrigued when I ran into Fortran programs that used negative array indices. It's pretty handy in a lot of situations.

Cyph0n9y ago

It's commonly used in Python too:

>> last_elem = arr[-1]

>> rev_arr = arr[::-1] # Reverse array

3JPLW9y ago

That is something completely different. Where Fortran allows negative indexing is when you initialize your array such the the indices are offset in the same manner as described in the linked article.

1 more reply

rch9y ago· 2 in thread

Interesting. It would be nice to surface this approach as a simple spreadsheet app so people could develop an intuitive feel for the concept.

jacobolus9y ago

> surface this approach as a simple spreadsheet app

I don’t understand what you’re proposing. You want a spreadsheet where the row and column headings have arbitrary starting point?

rch9y ago

I'm thinking about snipping out regions, creating views, and propagating indices across sheets.

reggieband9y ago· 1 in thread

This is very interesting to me but not really revolutionary since I am so used to languages like javascript where array indices don't matter the same way they do in C. However, I do see the utility in being able to generate new "views" of an existing array by offsetting all indices by a set amount.

First thing that comes to mind is time-series data - like a sliding window based on some concept of "now" where indices can be offset by the current date/time. (e.g. timeSeries[0] == now, timeSeries[-1] == one second ago, timeSeries[1] == one second in the future, etc.)

I actually find these linear-algebra based languages even more mind-bending than functional based languages.

JeremyBanks9y ago

If you do this in JavaScript, you won't be able to use many standard array operations or get standard array performance. The specification explicitly only supports non nonnegative indices for real arrays.

You're just using Object/map behavior, and if you're doing that with negative indices on an Array Object you're misusing it.

j / k navigate · click thread line to collapse

28 comments

16 comments · 5 top-level

goatlover9y ago· 3 in thread

I really like how Julia handles arrays.

VHRanger9y ago

Agreed. One big win here over numpy is that there's the `push!` function on Julia arrays (equivalent to the C++ vector's `push_back`).

Numpy arrays on the other hand are awkward to use in custom algorithms because once the memory is reserved, you can't grow the array at either end.

zeptomu9y ago

Btw. you can grow them using hstack and vstack although I am not sure how efficient that is.

2 more replies

rjeli9y ago

edit: if anyone knows a solution to this, please let me know...

3 more replies

killin_dan9y ago· 3 in thread

This is possible in ruby too, probably in a lot of languages.

I've rewritten someobject#[] plenty of times to have cleaner code.

one-more-minute9y ago

Julia can inline through all the `getindex!` functions so you get the same performance as with a hand-coded C or Fortran array.

jksmith9y ago

All Wirth languages did this. Great for clarity when a specific mapping is needed. Damn I miss me some Modula 2.

evacchi9y ago

Nim is very much Oberon inspired and supports this

1 more reply

codezero9y ago· 2 in thread

I remember being really intrigued when I ran into Fortran programs that used negative array indices. It's pretty handy in a lot of situations.

Cyph0n9y ago

It's commonly used in Python too:

>> last_elem = arr[-1]

>> rev_arr = arr[::-1] # Reverse array

3JPLW9y ago

That is something completely different. Where Fortran allows negative indexing is when you initialize your array such the the indices are offset in the same manner as described in the linked article.

1 more reply

rch9y ago· 2 in thread

Interesting. It would be nice to surface this approach as a simple spreadsheet app so people could develop an intuitive feel for the concept.

jacobolus9y ago

> surface this approach as a simple spreadsheet app

I don’t understand what you’re proposing. You want a spreadsheet where the row and column headings have arbitrary starting point?

rch9y ago

I'm thinking about snipping out regions, creating views, and propagating indices across sheets.

reggieband9y ago· 1 in thread

I actually find these linear-algebra based languages even more mind-bending than functional based languages.

JeremyBanks9y ago

You're just using Object/map behavior, and if you're doing that with negative indices on an Array Object you're misusing it.

j / k navigate · click thread line to collapse