Ruby nested block iteration fail

Question

I'm newish to ruby, used to JS and C#, dealing with nested block and i have 2 'loops' here which should print the exact same thing.

    page.search( "//div[@id='mw-content-text']" ).search("p").find do |p| 
        puts p.inner_text.gsub(/[^a-z ]/i, '').split( ' ' )
    end

    page.search( "//div[@id='mw-content-text']" ).search("p").find do |p| 
    p.inner_text.gsub(/[^a-z ]/i, '').split( ' ' ).each do |word|
        puts word
        end
    end

They both start by getting the all the paragraph tags in a page, then iterating through them. The first acts as expected, but when i try to iterate through each word with a nested block, i then only get one result from the outer block. it's as if the first 'end' is breaking the outer block or something. Is this normal ruby behaviour? have i missed something obvious?

Thanks for your help.

Simon.

Can you paste the exception message

Ishank Gupta
– Ishank Gupta

2014-01-10 09:51:21 +00:00
Commented Jan 10, 2014 at 9:51 — Ishank Gupta
– Ishank Gupta, Commented Jan 10, 2014 at 9:51

Matt · Accepted Answer · 2014-01-10 10:01:43Z

1

If you want to loop over all the p elements, you probably want to use each and not find. The find method "passes each entry in enum to block. Returns the first for which block is not false."

answered Jan 10, 2014 at 10:01

Matt

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1 Comment

theSiberman Over a year ago

Thanks Matt, this did the trick, and with Timo's explanation I can really see what was going wrong. Cheers.

Timo · Accepted Answer · 2014-01-10 11:05:29Z

1

Matt is correct. Use each. However, considering your novice status, I think you might benefit from a bit more detailed explanation of what is going on there.

In Ruby every method call or block returns what ever the last executed line of code in it returned (or nil if it's empty). You do not need to call return (and for blocks you can't call it anyway) explicitly unless you wish to prematurely stop the execution. Now keeping that in mind we can test that puts("something").nil? #=> true. In conditional statements nil is considered to be false, which is why the first call for find runs through the entire set. For every p tag you just call puts which returns nil and tells find that this is not the element we are looking for. However, the each method returns whatever it was called for (so you can chain calls) as illustrated by [].each {}.class #=> Array, thus indicating to the find method that whatever you were looking for was found and we can stop iterating the set. And finally the find method then returns the first p element on the search results.

answered Jan 10, 2014 at 11:05

Timo

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2 Comments

theSiberman Over a year ago

Thanks for the explanation Timo, i did have to read it twice, but i understand what's happening now. Ruby seems to be a very different animal to languages i have used before. ps. Don't have the rep to vote up your answer. Cheers

Timo Over a year ago

It is very different beast in deed. I suggest that when ever your in doubt of what some method does check apidock.com and/or experiment in the rails console or plain old irb.

chuang wang · Accepted Answer · 2014-01-10 11:09:09Z

1

page.search( "//div[@id='mw-content-text']" ).search("p")

gain Enumerable.

Enumerable#find

Passes each entry in enum to block. Returns the first for which block is not false. If no object matches, calls ifnone and returns its result when it is specified, or returns nil otherwise. Example.

(1..10).find {|item| p item}
# 1.

you can use Enumerable#find_all, Enumerable#collect, map

Example.

(1..10).find_all {|item| p item}
# 1 2 3 4 ... 10
[1,2,3,4,5,6,7,8,9,10]

Hope that it can help you.

answered Jan 10, 2014 at 11:09

chuang wang

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2 Comments

theSiberman Over a year ago

Hey Chuang, thanks for the answer, i've got it working using .each, but it's good to know there is another approach.

Timo Over a year ago

Your find_all example may be a little bit misguiding. It's purpose is to filter an array to contain only the elements which in fact do contain the objects for which the block returns something non nil and non false (as oppose to find that returns the first found, it returns all found and consequentially has to go through the entire set, at least for your usual arrays and such). Map and collect which are synonyms are for mapping data to something else, typically used e.g. to reduce an array of objects to an array of some attribute of those objects. Use each for just iterating through a set.

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