Unexpected Return (LocalJumpError)

问题:

What is the problem with this Ruby 2.0 code?

p (1..8).collect{|denom|
    (1...denom).collect{|num|
        r = Rational(num, denom)
        if r > Rational(1, 3) and r < Rational(1, 2)
            return 1
        else
            return 0
        end
    }
}.flatten

The error is in block (2 levels) in <main>': unexpected return (LocalJumpError). I want to create a flat list containing n ones (and the rest zeroes) where n is the number of rational numbers with denominators below 8 which are between 1/3 and 1/2. (it's a Project Euler problem). So I'm trying to return from the inner block.

回答1:

You can't return inside a block in Ruby^*. The last statement becomes the return value, so you can just remove the return statements in your case:

p (1..8).collect{|denom|
    (1...denom).collect{|num|
        r = Rational(num, denom)
        if r > Rational(1, 3) and r < Rational(1, 2)
            1
        else
            0
        end
    }
}.flatten

^*: You can inside lambda blocks: lambda { return "foo" }.call # => "foo". It has to do with scoping and all that, and this is one of the main differences between lambda blocks and proc blocks. "Normal" blocks you pass to methods are more like proc blocks.