[ruby-core:98411] [Ruby master Feature#16899] Proposal: Add Array#both_end method

Issue #16899 has been reported by S_H_ (Shun Hiraoka).

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Feature #16899: Proposal: Add Array#both_end method
https://bugs.ruby-lang.org/issues/16899

* Author: S_H_ (Shun Hiraoka)
* Status: Open
* Priority: Normal
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## About

Add get first & last value method `Array#both_end`.

## Current Status

Somtimes, we want to get both end Array value. But, now no method implemented these behavior.

So, define these code.

```ruby
class Array
  def get_first_and_last(count)
    [self.first(count), self.last(count)]
  end
end

```

## Proposal

Get both end Array value method. `Array#both_end`.

Implment new method `Array#both_end` that get both end value in `Array`.

## Array#both_end behavior

```ruby
# normal Array
ary = [ "w", "x", "y", "z" ]
ary.both_end     #=> ["w", "z"]
ary.both_end(2)  #=> [["w", "x"], ["y", "z"]

# empty Array
[].both_end    #=> [nil, nil]
[].both_end    #=> [nil, nil]

# args num is over Array size
ary = [ "w", "x", "y", "z" ]
ary.both_end(10) #=> [[ "w", "x", "y", "z" ], [ "w", "x", "y", "z" ]]

```

## Implementation

This implmentation used `Array#asscoc`, `Array#first` and `Array#last` in C func.

```c
static VALUE
rb_ary_both_end(int argc, VALUE *argv, VALUE ary)
{
    VALUE first, last;

    rb_check_arity(argc, 0, 1);

    if (RARRAY_LEN(ary) == 0)
        return rb_assoc_new(Qnil, Qnil);

    first = rb_ary_first(argc, argv, ary);
    last = rb_ary_last(argc, argv, ary);

    return rb_assoc_new(first, last);
}
```

## Problem

I'm wondering if I should be these code make error.
And, What type error is best?

```ruby
# args num is over Array size
ary = [ "w", "x", "y", "z" ]
ary.both_end(10) #=> error can't get both end value!

```

Also, I wonder if this method name(Array#both_end) is best?

I would appreciate your feedback. Thanks





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