Ternary Operator and Arrays

Ruby
1

Given:
ary = Array.new
some_hash = {‘a’ => 1, ‘b’ => 2, ‘c’ => 3}
another_array = [‘a’, ‘c’]

Then:
another_array.each do |x|
ary << some_hash.has_key?(key) ? ’ ’ : some_hash[key]
end

Expected:
ary should be [1, ’ ', 3]

Actual:
[true, false, true]

Question:
Why does that happen?

-Felix

2

Felix D. wrote:

Given:
ary = Array.new
some_hash = {‘a’ => 1, ‘b’ => 2, ‘c’ => 3}
another_array = [‘a’, ‘c’]

Then:
another_array.each do |x|
ary << some_hash.has_key?(key) ? ’ ’ : some_hash[key]
end

Expected:
ary should be [1, ’ ', 3]

Actual:
[true, false, true]

Question:
Why does that happen?

-Felix

It’s probably a precedence issue. Looks like << is binding tighter than
?: .

Anyway, you can just do ary = another_array.collect{|x| some_hash[x] ||
’ '}

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

3

On Thu, Nov 19, 2009 at 7:16 PM, Felix D. [email protected] wrote:

Expected:
ary should be [1, ’ ', 3]

Actual:
[true, false, true]

Question:
Why does that happen?

First of all, I assume you copy/pasted wrong, since I get

NameError: undefined local variable or method `key’ for main:Object

Assuming you want to query the hash for each value of the
another_array, you can do this:

irb(main):025:0> another_array.each do |x|
irb(main):026:1* ary << (some_hash.has_key?(x) ? ’ ’ : some_hash[x])
irb(main):027:1> end
=> [“a”, “c”]
irb(main):028:0> ary
=> [" ", " "]

Please note the parenthesis.
And I have the suspicion that you have the logic backwards, since you
return " " when the hash has the key, and go to the hash when it
doesn’t (which will always return nil or the default value). Maybe you
wanted this:

irb(main):033:0> ary = []
=> []
irb(main):034:0> another_array.each do |x|
irb(main):035:1* ary << (some_hash.has_key?(x) ? some_hash[x]: ’ ')
irb(main):036:1> end
=> [“a”, “c”]
irb(main):037:0> ary
=> [1, 3]

Another way of doing this, more idiomatic would be:

irb(main):038:0> ary = another_array.map {|x| some_hash.has_key?(x) ?
some_hash[x]: ’ '}
=> [1, 3]
irb(main):039:0> ary
=> [1, 3]

But anyway, you won’t get a three element array, since you are
iterating over another_array, which has only two entries. Can you
explain how you should obtain 3 elements in the array? Is it all
elements in the hash that have their key in the another_array? If so:

irb(main):040:0> ary = some_hash.map {|k,v| another_array.include?(k) ?
v : ’ '}
=> [1, " ", 3]

Hope this helps,

Jesus.

4

On Fri, 20 Nov 2009, Felix D. wrote:

Expected:
ary should be [1, ’ ', 3]

Actual:
[true, false, true]

Question:
Why does that happen?

You need parenthesis around the ternary operation.

The reason being is that << has a higher precedence than ?:, so it
evaluates just the some_hash.has_key?(key), giving a true or a false
value.

In general, while parenthesis are often optional in Ruby, if you’re not
sure how something works (or it’s not clear), parenthesis are your
friend.

Matt

5

On Thu, Nov 19, 2009 at 12:29 PM, Marnen Laibow-Koser
[email protected]wrote:

It’s probably a precedence issue. Looks like << is binding tighter than
?: .

Anyway, you can just do ary = another_array.collect{|x| some_hash[x] ||
’ '}

You have to be careful with this, though, because nil and false could be
valid values.

ary = Array.new
some_hash = { ‘a’ => 1 , ‘b’ => 2 , ‘c’ => nil , ‘d’ => false }
another_array = [ ‘a’ , ‘c’ , ‘d’ , ‘e’ ]

#if you want nil and false to be valid values, this doesn’t do what you
expect
another_array.collect{|x| some_hash[x] || ’ ’ } # => [1, " ", " ", " "]

#you can set the default to the string, then it will return that if the
key
doesn’t exist
some_hash.default = ’ ’
another_array.collect{ |x| some_hash[x] } # => [1, nil, false, " "]