Using lambda inside Ruby method

Question

I'm just starting with Ruby and I hit an issues which I feel is important for the language so I don't want to just pass it. I would really appreciate answer which includes not only a working example but at least brief explanation where I went wrong with mine.

So first step is having this method:

def filter (arr)
    arr.each do |e|
        puts e if e % 2 != 0
    end
end 

filter [1, 2, 3, 4, 5, 6]

And the result as expected is:

1 3 5 [Finished in 0.2s]

Second I tried this one:

def filter (arr)
    arr.each do |e|
        puts e if yield(e)
    end
end 

filter ([1, 2, 3, 4, 5, 6]) { |n| n.odd? }

And I got the same result:

1 3 5 [Finished in 0.2s]

Third I want to do this using a lambda. Ultimately I want ti invoke the filter method like so filter([1, 2, 3, 4, 5, 6], &is_odd). But since I can't still figure it out I am currently stuck at this:

is_odd = lambda { |n| puts n if n.odd? }

def filter ()
    arr = [1, 2, 3, 4, 5, 6]
    arr.each do |e|
        is_odd(e)
    end
end 

filter &is_odd

And I got the following error:

block in filter': undefined methodis_odd' for main:Object (NoMethodError)

It kind of makes sense to me that this is not working since, if I define the lambda inside the filter function and use it like so:

def filter ()
    is_odd = lambda { |n| puts n if n.odd? }
    arr = [1, 2, 3, 4, 5, 6]
    arr.each &is_odd
end 

filter

I'm again getting the expected behavior, but I am following a tutorial and it seems like it should be possible to declare is_odd outside the filter method and invoke filter like so filter([1, 2, 3, 4, 5, 6], &is_odd).

I would like to know if indeed it is possible to do use lambda this way and if yes, where did my logic fail?

Sergio Tulentsev · Accepted Answer · 2018-09-01 21:44:56Z

6

I would like to know if indeed it is possible to do use lambda this way

Yes.

and if yes, where did my logic fail?

By passing the lambda as &is_odd, you're turning it into a block of the method. So, use it as a block. In the snippet above you show that you know about yield. Here's another way of calling the passed block.

is_odd = lambda { |n| puts n if n.odd? }

def filter(&block)
    arr = [1, 2, 3, 4, 5, 6]
    arr.each do |e|
        block.call(e)
    end
end 

filter(&is_odd)
# >> 1
# >> 3
# >> 5

answered Sep 1, 2018 at 21:44

Sergio Tulentsev

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4 Comments

Leron Over a year ago

Thanks, I was wondering about this option. Is it OK to include lambdas in method definition?

Sergio Tulentsev Over a year ago

@Leron: the &block? It's not your lambda anymore. In this case, it's a Proc object, which is made from the method's block (which, in turn, comes from your lambda). And yes, it's a common thing to see in ruby code.

mu is too short Over a year ago

Probably worth noting that you don't use parentheses like that when calling a lambda, hence the NoMethodError. You use brackets (some_lamba[arg]), #call (some_lambda.call(arg)), or dot-parentheses (some_lambda.(arg)).

Sergio Tulentsev Over a year ago

@muistooshort: ah yes, totally.

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