Ruby: How can I insert a single value between each item in an array?

Question

I have an array [1,2,3] and I want to insert the same value (true) between each item so that it becomes:

#=> [1, true, 2, true, 3, true]

My current method is a little long-winded:

[1,2,3].zip(Array.new(3, true)).flatten

Can anyone suggest a more elegant way to do this?

What are going to do with this array after? Maybe there's an easier way to do that than going through a temp array. — Sergio Tulentsev
– Sergio Tulentsev, Commented Jul 31, 2019 at 13:16
Your text and your example do not match up. In your text, you say you want to insert a single value between items, whereas in your code example, you are adding a value after each item. Also, your question is confusing since you are asking about inserting a value between each item, which doesn't make sense, since you cannot insert something between one thing, you can only insert it between two things. — Jörg W Mittag
– Jörg W Mittag, Commented Jul 31, 2019 at 17:28

Sebastián Palma · Accepted Answer · 2019-07-31 13:19:03Z

5

You can try using flat_map, and add after each element a true object:

p [1, 2, 3].flat_map { |e| [e, true] } # [1, true, 2, true, 3, true]

Another way would be to get the product of [1,2,3] and [true], and flatten the result:

p [1, 2, 3].product([true]).flatten # [1, true, 2, true, 3, true]

edited Jul 31, 2019 at 13:19

answered Jul 31, 2019 at 13:13

Sebastián Palma

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Peter Camilleri · Accepted Answer · 2019-07-31 13:27:57Z

3

Only a small tweak: Your code suffers from having to know the number of elements in the array being processed. This takes the form of the magic number 3 used to make the true array. Here is an alternative. Better? Dunno, but at least no magic numbers to break.

[1,2,3].zip([true].cycle).flatten

yields

[1, true, 2, true, 3, true]

Curious note: Adding a space between the "zip" and the opening "(" will cause the interpreter (version 2.3.3 in my tests) to generate an error:

Error NoMethodError: undefined method `flatten' for #<Enumerator: [true]:cycle>

This may be more robust as it avoids ambiguity in Ruby's "friendly" parser:

([1,2,3].zip([true].cycle)).flatten

answered Jul 31, 2019 at 13:27

Peter Camilleri

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