Object#tap (was Nice algorithm for 'spreading' indexes across an array?)

Ruby
1

On Wed, Sep 2, 2009 at 1:41 AM, Max
Williams[email protected] wrote:

arr = (1…12).to_a
It feels like there should be a simple solution for this but i can’t
think of a nice way. Anyone?

thanks
max

Thanks to a post by David M. in a recent thread, I learned about
Object#tap.
He used it on a hash.

I’ve used it here on an array.
Is there any problem using Object#tap in this way?
I saw some examples, but in the examples the object was not modified
in the block.

class Array
def spread(n)
dup.tap{|a| (size-n).downto(1).map{|b| size*b/(size-n+1)}.each{|c|
a.delete_at(c)}}
end
end

arr = (1…12).to_a
(3…12).each{|t| p arr.spread(t)}

###Output

[1, 6, 12]
[1, 4, 8, 12]
[1, 3, 6, 9, 12]
[1, 3, 5, 8, 10, 12]
[1, 2, 4, 6, 8, 10, 12]
[1, 2, 4, 6, 7, 9, 11, 12]
[1, 2, 3, 5, 6, 8, 9, 11, 12]
[1, 2, 3, 4, 6, 7, 8, 10, 11, 12]
[1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

Harry

2

On Wednesday, July 28, 2010 09:03:18 am Harry K. wrote:

Is there any problem using Object#tap in this way?
I saw some examples, but in the examples the object was not modified
in the block.

Oh, you mean modifying the array?

class Array
def spread(n)
dup.tap{|a| (size-n).downto(1).map{|b| size*b/(size-n+1)}.each{|c|
a.delete_at©}}
end
end

Not particularly. The thing you have to be careful of is reassigning it
– for
example, this wouldn’t work:

1.tap {|x| x+=1}

You have to actually modify the object. For example, if we’re talking
about
arrays, instead of this:

string.tap {|x| x + ‘foo’}

Do this:

string.tap {|x| x << ‘foo’}

It seems like << returns self, so that’s completely unnecessary, you
could
just do this:

(string << ‘foo’)

…but hopefully it illustrates the point.

I have a feeling I’m overcomplicating things, though. Here’s one
possible
implementation of Object#tap:

class Object
def tap
yield self
self
end
end

Nothing mysterious about it at all. (MRI might do it in C, I’m not sure,
but
the above works.)

3

On Thu, Jul 29, 2010 at 12:40 PM, David M. [email protected]
wrote:

Do this:

string.tap {|x| x << ‘foo’}

A nice use of #tap (IMO, and these things are very much a matter of
opinion):

message = String.new.tap { |s|
s << “…”
s << “…”
if some_condition?
s << “…”
else
s << “…”
end
end

Gavin