`map` based on condition

Question

I have a struct like this:

Struct.new("Test", :loc, :type, :hostname, :ip)

clients = [
Struct::TestClient.new(1, :pc, "pc1", "192.168.0.1")
Struct::TestClient.new(1, :pc, "pc2", "192.168.0.2")
Struct::TestClient.new(1, :tablet, "tablet1", "192.168.0.3")
Struct::TestClient.new(1, :tablet, "tablet2", "192.168.0.3")
and etc...
]

If I want to get the IP address of all devices, I can use test_clients.map(&:ip). How do I select the IP addresses of specific devices, say all device types called "tablet"? How can I do that with map?

Solution 1

Do a select first

clients.select{|c| c.type == 'tablet'}.map(&:ip)

Solution 2

Ruby 2.7+

Ruby 2.7 is introducing filter_map for this exact purpose. It's idiomatic and performant, and I'd expect it to become the norm very soon.

For example:

numbers = [1, 2, 5, 8, 10, 13]
enum.filter_map { |i| i * 2 if i.even? }
# => [4, 16, 20]

Here's a good read on the subject.

Hope that's useful to someone!

Solution 3

Answer is as simple as that:

clients.map { |client| client.ip if client.type == 'tablet' }.compact

Mapping with condition will give nils for clients which failed the condition, for that only we kept compact, which will actually flush the nil values.

Solution 4

An alternate to Sergio Tulentsev's method using #collect. I think using #collect is semantically correct here. I know the O.P. asked how to use #map, but it's my two cents.

clients.collect { |c| c.ip if c.type == "tablet" } # will return nils for clients where the type is not "tablet"

# or

clients.select { |c| c.type == "tablet" }.collect(&ip)

View more solutions

Comments

[silasjmatson]

Sorry about the duplicate answers, hit the backspace key one too many times and went back to a create rather than edit.

[PJP]

I'm curious, how do you differentiate a difference between collect and map? Also, does collect(&ip) return what you expect?

[silasjmatson]

I usually use collect when I'm just grabbing values from a set of objects, and map when I'm doing a bunch of logic in the block, or returning an array that has changed values. Am I wrong on that?

[mu is too short]

The select-less version will leave you with nils.

[silasjmatson]

Oh, true. I didn't think about that too much.

[PJP]

collect is an alias to map; It doesn't matter which you use because they're identical. Any difference is most likely due to languages you've used in the past because of familiarity with one or the other. And, as a result, I use map because it's easier to type.

[Sk. Irfan]

No need of using, select and map as mentioned in the Sergio Tulentsev's answer, as it will result in two loops and comparatively slower.

[Nick]

compact also iterates over the array and checks each value for nil. Sergio's answer could actually be faster. In your example, both iterations are over the full length of clients, whereas in Sergio's example, the second iteration may be over a smaller set.

[theartofbeing]

This is the correct answer for 2.7 and above