std.range.countFrom() too

[dlangBugzillaToGithub]

bearophile_hugs reported this on 2012-04-05T17:59:57Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=7839

CC List

• andrei (@andralex)
• greensunny12
• me

Description

I'd like a count() in std.range. It's similar to the generator with the same name in the Python itertools module:
http://docs.python.org/library/itertools.html#itertools.count

a count() is also useful with bigints, where you can't replace it with iota(BigInt.max).


A bare-bones implementation that shows it basic semantics (but it's useful to add few more methods):


struct Count(T) {
    T n;
    this(T n_) { this.n = n_; }
    const bool empty = false;
    @property T front() { return n; }
    void popFront() { /* n++; */ n += 1; }
}

// Two helper functions
Count!T count(T)(T start) { return Count!T(start); }
Count!T count(T)() { return Count!T(cast(T)0); }

[dlangBugzillaToGithub]

bearophile_hugs commented on 2012-04-23T13:54:31Z

And answer to a question by jerro in D.learn:

http://forum.dlang.org/thread/4F949E90.9060700@webdrake.net#post-evdrwckxsqieyrweuxzp:40forum.dlang.org

> Couldn't it just be iota with no parameters?

The Count range has a helper count() function similar to this, that's meant to have an argument that defaults to zero:

Count!T count(T)(T start=0) if (isIntegral!T) { return Count!T(start); }

The argument allows it to start from another starting point, and it allows you to specify the type of the numbers it yields, while in iota() without arguments it's less easy to specify the type of the numbers it yields.

Count(5) is easy to replace with iota(5, int.max), but count(BigInt(0)) is less easy to replace with iota, because it doesn't give you a way to denote a right-open BigInt interval. And using iota(BigInt(0), BigInt(ulong.max)) is not that good. Currently using BigInt in iota seems to not even being supported.

[dlangBugzillaToGithub]

bearophile_hugs commented on 2012-04-24T19:43:36Z

std.algorithm has a function named count(), so this range will need a different name. Maybe countFrom()?

[dlangBugzillaToGithub]

bearophile_hugs commented on 2012-04-24T19:47:13Z

Changed the name of this issue from "std.range.count() too" to "std.range.countFrom() too".

[dlangBugzillaToGithub]

greensunny12 commented on 2017-07-12T17:16:28Z

Wouldn't it make sense to add support for BigInt to iota instead of creating yet another range?

[dlangBugzillaToGithub]

me commented on 2018-01-26T21:03:31Z

(In reply to Seb from comment #4)
> Wouldn't it make sense to add support for BigInt to iota instead of creating
> yet another range?

From what I understand the itertools.count doesn't accetp an upperbound while iota forces the user to specify the upper bound.

Did I understand that correctly?