Weird behavior in Ruby (on Rails) using Array#find

Question

Take a good look at the snippet below:

a = [1,2,3]
n = 2
puts a.find { |i| i == n }
=> 2

a = [1,2,3]
n = [2]
puts a.find { |i| i == n.shift }
=> nil

Tip: you can see a running version here http://repl.it/OL3

Now explain it. Why the second #find returns nil ?

@majioa That's completely nuts. Just use path[-1] or path.last. — tadman
– tadman, Commented Jan 27, 2014 at 13:36
The snippet is out of context. In the context shift does make sense. — Arthur Corenzan
– Arthur Corenzan, Commented Jan 27, 2014 at 13:37

Charles Caldwell · Accepted Answer · 2014-01-27 13:31:00Z

4

Because Array#shift removes the element from the array.

So, the first time the block executes, it is comparing e['name'] == "pets" but on the next iteration, it is comparing e['name'] == nil. Unless e['name'] is "pets" on the first iteration, the .find will return nil.

answered Jan 27, 2014 at 13:31

Charles Caldwell

17.3k4 gold badges42 silver badges47 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Arthur Corenzan Over a year ago

I think the snippet isn't obvious enough, but that is not the case. You see, I assign a new value to path both times right before i execute #find. First I set it to a string 'pets' then I set it to an array with a string ['pets']. You can even note the lines irb is mentioning: 86 is where I set path, 87 is where I call #find.

tadman Over a year ago

Always be aware of methods that do in-place modification of the object they're called on. Many are identified with ! but there are some like shift that pre-date that convention.

Arthur Corenzan Over a year ago

You both are not paying attention. I DO know that shift mutates the variable. But that is NOT the case.

Charles Caldwell Over a year ago

@ArthurCorenzan .find executes the code in the block once for every element in f.elements. So, if there are 3 elements, your block gets called 3 times. Hence, .shift gets called on the array 3 times. The first time reduces the array to [].

tadman Over a year ago

Maybe you need to amend your question with a more generic example because I'm in complete agreement with @CharlesCaldwell here.

|

Arup Rakshit · Accepted Answer · 2014-01-27 13:53:40Z

0

@Charles Caldwell explained correctly, why you did get that result. Now just use Array#first method as below which is safe :

a.find { |i| i == n.first }

answered Jan 27, 2014 at 13:53

Arup Rakshit

119k30 gold badges270 silver badges328 bronze badges

Collectives™ on Stack Overflow

Weird behavior in Ruby (on Rails) using Array#find

2 Answers 2

7 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related