How to build a new Ruby array from a Hash keys

Question

I want to build a new array starting from a hash with the following format:

HashConst = {[120,240] => 60, [240,480]=> 30} #Constant

I need to build a new array and assign as value to a new constant with the following format:

[ [[120,240] ,1], [[240,480], 1] ]

I tried :

NewArrayConst = HashConst.keys.each{ |res| [res, 1]}

but I get instead

[ [120,240], [240,480] ]

Only solution I found is the following:

  tempVar = []
  HashConst.keys.each_with_index{ |res,idx| tempVar [idx] = [res, 1]}
  NewArrayConst = tempVar

Anyone knows a better solution to this and can explain why I cannot get the output I expect from NewArrayConst = HashConst.keys.each{ |res| [res, 1]}. I'm using 2.2.2-p95

Edit:

As many pointed out Hash var name is wrong and misleading, I have updated that to avoid confusion

You are using the very class Hash. Redefining it is a terrible idea. — sawa
– sawa, Commented Jan 6, 2016 at 18:15

Wand Maker · Accepted Answer · 2016-01-06 17:46:47Z

5

You need to use map instead of each.

Array#each method does not return the result of the code executed in the block, but instead just returns the array on which each was called, which in your case is the value of hash.keys.

Array#map collects the values returned by block into an array.

hash = {[120,240] => 60, [240,480]=> 30}
p array = hash.keys.map{ |res| [res, 1]}
#=> [[[120, 240], 1], [[240, 480], 1]]

NOTE: Do not name your variable Hash as it is already a well-known class in Ruby. Use lower case hash if need be. Also, avoid camel case for variable names such as NewArrayConst, as Ruby recommends use of snake_case for naming variables - you can refer Ruby Style Guide for more details.

edited Jan 6, 2016 at 17:46

answered Jan 6, 2016 at 17:41

Wand Maker

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12 Comments

maza Over a year ago

That works and thank you for the explanation. I had the wrong idea that each collected and returned the array of the executed expression

Dave Newton Over a year ago

@mgtemp In what sense? I mean, if you need to create a new array, you need to create a new array.

Aleksei Matiushkin Over a year ago

@mgtemp compared to what? And a prooflink would be great here.

Cary Swoveland Over a year ago

Hash is a "well-known" class? Never heard of it.

Wand Maker Over a year ago

@CarySwoveland May be it should have been named Hush

|

Cary Swoveland · Accepted Answer · 2016-01-06 18:18:30Z

3

h = {[120,240] => 60, [240,480]=> 30}
val = 1

h.keys.product([val])
  #=> [[[120, 240], 1], [[240, 480], 1]]

answered Jan 6, 2016 at 18:18

Cary Swoveland

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1 Comment

Dave Newton Over a year ago

This has the advantage of knowing how large the resulting structure will be "in advance", so if there are memory re-allocations involved during a map as mgtemp suggests, they may be avoidable.

linuxdan · Accepted Answer · 2016-01-06 17:41:29Z

-2

Have you tried Hash.to_a? Sometimes things are easier than you think.

answered Jan 6, 2016 at 17:41

linuxdan

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3 Comments

linuxdan Over a year ago

Why don't you leave that up to @maza, .to_a gives us `[[[120, 240], 60], [[240, 480], 30]]' which is in the format @maza described.

Dave Newton Over a year ago

But not what they're actually asking for. I don't see the confusion.

Cary Swoveland Over a year ago

As usual, @Dave is picky-picky.

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