How to get the first key and value pair from a hash table in Ruby

Question

I'm trying to get the first key and value key from a hash table in ruby. I don't know the key values of the hash because it is passed to the method. I cant find anywhere online how to find the first key/value as a separate hash table. I think hash[0] will just try to find an element with a name 0 it just returns nil when I run the code.

I know I can find the key name and the value and then create a new hash from them but i wonder if there is an easier way to do this so I get a hash right away.

here is my code:

def rps_game_winner(game)

rock_in_hash = game.invert['R']
paper_in_hash = game.invert['P']
scissors_in_hash = game.invert['S']

if(rock_in_hash)
      if(paper_in_hash)
        return paper_in_hash;
      elsif(scissors_in_hash)
        return rock_in_hash
      end
    elsif(paper_in_hash)
      if(rock_in_hash)
        return paper_in_hash
      elsif(scissors_in_hash)
        return scissors_in_hash
      end
    end
        key = game.keys[-1]
        value = game.values[-1]
            winner = {key => value}
    return winner 
    end

game_one = { "Bob" => 'P', "Jim" => 'P' }

puts rps_game_winner(game_one)

This gets me the correct result the problem is I don't understand why it's -1 instead of zero... And i was hoping there was a better way to get the first key/value pair of a hash table instead of creating new hash table with the key and value you retrieved from the previous table.

Solution 1

You can just do

key, value = hash.first

or if you prefer:

key = hash.keys[0]
value = hash.values[0]

Then maybe:

new_hash = {key => value}

Solution 2

There is a shorter answer that does not require you to use extra variables:

h = { "a" => 100, "b" => 200 , "c" => 300, "d" => 400, "e" => 500}
Hash[*h.first] #=> {"a" => 100}

Or if you want to retrieve a key/value at a any single position

Hash[*h.to_a.at(1)] #=> {"b" => 200}

Or retrieve a key/values from a range of positions:

 Hash[h.to_a[1,3]] #=> {"b"=>200, "c"=>300, "d"=>400}

Solution 3

[hash.first].to_h

Another way to do it.

Solution 4

my_hash = { "a" => "first", "b" => "second" }

{ my_hash.keys.first => my_hash.values.first }

This works too

View more solutions

Comments

[Xitcod13]

your first example trows Line 24:in rps_game_winner': undefined method first' for {"Jim"=>"P", "Bob"=>"P"}:Hash (NoMethodError) from t.rb:31. But what really blew my mind is that when i tried the second method (which I was trying to avoid because i thought there was an easier way) i get the second entry Bob P and whats even stranger when i call it with keys[-1] values[-1] i get the first entry JimP I completly dont understand whats going on. Do you have any ideas. Im using codepad to interpret my script.

[pguardiario]

I think you're using 1.8, hashes were different then.

[Daniel Waltrip]

I just tested this in IRB, and it seems the first method (hash.first) is far better. It doesn't have to construct the array of all keys/values, as it grabs the first one directly. The second method will create arrays holding all keys/values in memory, and then it can grab the first elements from those arrays. If your hash is large, this will take much longer.

[quetzaluz]

Thank you for this much more concise answer! The idea of storing the key and value to variables just for this purpose definitely isn't ideal. This should be the top answer!

[Arnold Roa]

What is the meaning of the *?

[metakungfu]

it's the ruby splat operator. More info here: blog.honeybadger.io/ruby-splat-array-manipulation-destructur‌​ing

[Alfonso Vergara]

How would you return the first ten values of your hash?

[metakungfu]

@AlfonsoVergara check my answer below :)

[Admin]

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