Computer Science Canada

Hashes
Author:  wtd [ Thu Mar 31, 2005 1:25 pm ]
Post subject:  Hashes
A Hash, or "associative array" can be thought of as very similar to a regular array, except that instead of indexing by numbers we can (in terms of Ruby at least) index by anything. Most commonly we use strings.

So, if I want to have information about an individual, I could write:

code:
info = ["Bob Smith", 42, "129 55th Ave"]


Now, how to I retrieve the name? Oh, right. I need to remember that it's the first element in the array.

code:
name = info[0]


Now I could change the age:

code:
info[1] += 1


But, in these cases I have to go back and look at the original creation of the array to see where each item is. There has to be a better way.

There is. Instead of using an array, use a hash.

code:
info = { "name" => "Bob Smith", "age" => 42, "address" => "129 55th Ave" }


Now, let's get the name again.

code:
name = info["name"]


And let's change the age.

code:
info["age"] += 1


Any other questions? Smile
Author:  Tony [ Thu Mar 31, 2005 3:06 pm ]
Post subject: 
I was actually hoping for a bit more advanced hash handling.

Such as looping through the elements and adding new hashes to the list.
Author:  wtd [ Thu Mar 31, 2005 3:57 pm ]
Post subject: 
Getting all keys:

code:
my_hash = {"foo" => "bar"}
my_hash.keys


Getting all values:

code:
my_hash = {"foo" => "bar"}
my_hash.values


Looping:

code:
my_hash = {"foo" => "bar"}

my_hash.each { |key, value| puts "#{key}: #{value}" }

my_hash.each { |pair| puts "#{pair.first}: #{pair.last}" }

for key, value in my_hash
   puts "#{key}: #{value}"
end

for pair in my_hash
   puts "#{pair.first}: #{pair.last}"   
end


Determining if a hash contains a particular key or value:

code:
my_hash = {"foo" => "bar"}

my_hash.has_key? "foo"

my_hash.has_value? "bar"


Adding to a hash:

code:
my_hash = {"foo" => "bar"}

my_hash["hello"] = "world"
Author:  wtd [ Thu Mar 31, 2005 11:25 pm ]
Post subject: 
More.

Let's start with removing entries from a hash if they meet certain conditions. Ket's say we want to remove any entry where the value is exactly two times the key.

code:
my_hash = {"a" => "aa", "b" => "bbb", 9 => 19, 4 => 8}
new_hash = my_hash.reject { |key, value| value == key * 2 }


new_hash is now:

code:
{"b" => "bbb", 9 => 19}


Next... ever want the keys and values switched?

code:
my_hash = {"a" => "aa", "b" => "bbb", 9 => 19, 4 => 8}
new_hash = my_hash.invert


And one last trick:

Ever want to retrieve multiple entries into an array?

code:
my_hash = {"a" => "aa", "b" => "bbb", 9 => 19, 4 => 8}
my_array = my_hash.indexes("a", 9)
Author:  wtd [ Fri Apr 01, 2005 11:59 pm ]
Post subject: 
Anything else anyone would like to know about Ruby hashes?
Author:  Tony [ Sat Apr 02, 2005 1:06 pm ]
Post subject: 
thx wtd, awesome examples as always Smile

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