Add a Python-like slice operator for lists and strings

[justinfagnani]

String.substring() and List.getRange() are somewhat verbose, but very common operations. Python's slice operator provides an easy-to-read notation to replace these.

Examples of substring() and slice [:]

string.substring(a) -> string[a:]

string.substring(a, b) -> string[a:b]

string.substring(0, a) -> string[:a]

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[DartBot]

This comment was originally written by @jinmingjian

---

great. more my idea is to enhance the slicing operator to an general indexing technology for array like objects, like:

1: a:b = a:1:b = a,a+1,...,b
2: a:2:b = a, a+2,...b
3: a:-1:b = a, a-1,...b
4: [:,a:b] = [1,a:b],[2,a:b]...

[DartBot]

This comment was originally written by thebinarysearc...@gmail.com

---

I would argue that it is not easier to read. substring says what it is doing in English, whereas string[3:2] is symbolism.

[DartBot]

This comment was originally written by @jinmingjian

---

yes or no. Yes, because this is just a syntax magic. No, ":" for slicing is the tradition of numerical computing software[1]. My suggestion is more mathematical. That is, it does not pursue symbolism, it pursues intuition. The ":" is used for generating a vectorized index(i.e.,1D array). And,

1. the basic form is [start:step:end];
2. [start:end] is the simplified form of [start:1:end];
3. : is the simplified form of [head:tail];
4. generally, start<=end and step>0, but allow start>=end and step<0 makes reversed indexing easier.
5. get nothing if the index does not exist mathematically.

So,
[3:2] -> [3:1:2], 3 can not reach 2 by step 1 -> nothig. may want [3:-1:2]
[:a] -> not syntax right, may want [0:a]

[1] http://en.wikipedia.org/wiki/Array_slicing

[DartBot]

This comment was originally written by @jinmingjian

---

numerical computing software -> numerical computing language, more exactly.

[DartBot]

This comment was originally written by @seaneagan

---

I think this would require a user-definable operator:

List#getRange becomes:
List<T> operator [:]([int first, int last]);

String#substring becomes:
String operator [:]([int first, int last]);

And it would probably make sense to support slicing then as well, by replacing List#setRange with:

List<T> operator [:]= (List other, [int first, int last]);

List#removeRange also becomes unnecessary, instead of:

list.removeRange(0, 5);

just do:

list[:5] = [];
// or
list[:5] = null;

[floitschG]

Reassigning to language. This requires new syntax and is thus not a library bug.

---

Removed Area-Library label.
Added Area-Language label.

[floitschG]

Issue #8977 has been merged into this issue.

[Zectbumo]

Please support negative indexes (relative from the end) as Python does.
At the very least make String.substring() accept negative indexes.

[gbracha]

Set owner to @gbracha.
Removed Priority-Medium label.
Added Priority-Low, Accepted labels.

[deadsoul44]

It has been 8.5 years. substring method should accept negative values.

[Zectbumo]

Yeah, this is really weird since even Javascript String.prototype.slice() takes negative values.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice

[pauleohare]

Any news on this? Is it ever going to happen? Doesn't sound too difficult. Is it?

[jabbalaci]

Any update? Will it be added to the language?

[mraleph]

Any update? Will it be added to the language?

By default the answer to this sort of question is: there is no update and there are no immediate plans to add it to the language.

If there was any sort of update it would be posted here. Language features which are being worked on are part of the language funnel.

[jabbalaci]

@mraleph Thanks. Then maybe this issue could be closed.