RFC: Python-like slices in vec

[nikomatsakis]

I would like to modify the vector (and string, I suppose) slicing routines to be more "Python-like". This means that they are more lenient of invalid indices and they consider negative numbers to count from the right.

All in all I propose six functions:

vec::islice(v, from, to) // equivalent to v[from:to] in Python
vec::islice_from(v, from) // equivalent to v[from:] in Python, or vec::slice(v, from, vec::len(v))
vec::islice_to(v, to) // equivalent to v[:to] in Python, or vec::slice(v, 0, to)

vec::uslice(v, from, to)
vec::uslice_from(v, from, to)
vec::uslice_to(v, from, to)

The i family accepts signed integers: negative inputs are considered as counting from the right. The u family takes unsigned integers. All of them are tolerant of invalid indices, returning empty list rather than failing, as Python does. In my experience this is usually what you want.

[BrendanEich]

+1 all around. Thanks,

/be

Niko Matsakis wrote:

I would like to modify the vector (and string, I suppose) slicing routines to be more "Python-like". This means that they are more lenient of invalid indices and they consider negative numbers to count from the right.

All in all I propose six functions:

 vec::islice(v, from, to) // equivalent to v[from:to] in Python
 vec::islice_from(v, from) // equivalent to v[from:] in Python, or vec::slice(v, from, vec::len(v))
 vec::islice_to(v, to) // equivalent to v[:to] in Python, or vec::slice(v, 0, to)

 vec::uslice(v, from, to)
 vec::uslice_from(v, from, to)
 vec::uslice_to(v, from, to)

The i family accepts signed integers: negative inputs are considered as counting from the right. The u family takes unsigned integers. All of them are tolerant of invalid indices, returning empty list rather than failing, as Python does. In my experience this is usually what you want.

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#1799

[BrendanEich]

Oops, replied to the list. For the record, +1 all around.

/be

[killerswan]

Looks good. Why not make islice into slice?

[brson]

I agree with killerswan. Just call islice slice. I'm not crazy about accepting bad input though.

[marijnh]

I'd make the unsigned-integer version fail on out-of-range indices. So you'd have the choice of a liberal version and a strict version.

Also, yes, name one of them simply slice.

[nikomatsakis]

I'd be ok with uslice being precisely what slices are today (strict as can be) and slice being the convenient, Python-like form.

[qwertie]

As a general principle, I think that for performance reasons, there should always be a "never-fails" version of all important language primitives. The reason is that it is common for developers to want a version that can't fail, and if the language/library doesn't offer one then the developer has to write a function to do it himself. Then what happens is, the developer checks the arguments, and then the compiler or std lib does exactly the same check a second time, which is just wasteful in an inner loop. Plus, the compiler has to spend a little extra time inlining this little function everywhere.

Of course, a version that fails on unexpected input is equally useful and should also be provided. IMO, then, there should be three slice functions: two fast (unsigned with fail, unsigned without fail) and one slower and more flexible (python style). But then you would have more arguing to do about naming. Mind you, I didn't catch how slices work in Rust -- I'm assuming it's like Go where a slice is merely a pointer and a length, no copying involved. If a slice actually copies the whole range then it was silly of me to worry about the speed of the range checks.

[Dretch]

One thing I like about python is that it has a single slice operator (albeit with optional arguments). I find an API made up of lots of very similar functions to be confusing.

[bstrie]

Python's slicing mechanism really is one of my favorite aspects of the language. +1

Add my voice to the chorus of those who'd love to see the v[from:to] syntax be supported.

It looks like Go handles this by making slices a separate type entirely: http://blog.golang.org/2011/01/go-slices-usage-and-internals.html

This issue is also related to #555.

[nettok]

Don't forget about Python's extended slice notation v[from:to:step].

With this is possible to reverse a list doing this v[::-1].

[darkf]

+1 for [from:to:step] syntax.

[killerswan]

Why not just call vec::reverse?

[nettok]

vec::reverse would be equivalent to v[::-1], but it is just a subset of what would be possible with this notation.

For example, take only pair indices v[::2], and reversed pair indices v[::-2], etc...

[graydon]

There was some gesture towards supporting this long ago, but I removed it out of lack of effort. If someone wants to champion it, I'm fine with that.

[killerswan]

@nikomatsakis, slice_to and slice_from could also be called take and drop, instead.

[amunra]

+1

I'd love to see the slicing notation

[from:to:step]

Makes for very intuitive code

[catamorphism]

Underway, but not for 0.3 and possibly not with step, as per @graydon .

[nikomatsakis]

withdrawing in favor of a newer proposal based on our current slices

[bstrie]

I take it that means this proposal:

http://smallcultfollowing.com/babysteps/blog/2012/05/14/vectors/

Not sure if there's an issue for it yet.