11 questions about hash in Ruby _ruby topics

I've been learning about rails, because learning Java is an introductory language. It's a bit of a bad fit. In particular, Ruby's hash. But it's just a matter of thinking it through. It's basically a key-value. There is no big problem with writing in the beginning. This is a log I saw on the Internet.

1. How do I create a hash?

Copy Code code as follows:

x = hash.new
x = {}
x = {: A => 1,: B => 2}

These are the ways to create a hash.
The first is the same as the second, which indicates the creation of an empty hash.
The third represents the hash created, with 2 pairs of keys/values.

2. What are the types of hash keys?

Can be any target, such as:

Copy Code code as follows:

IRB (main):002:0> x={}
=> {}
IRB (main):003:0> x[1]=2
=> 2
IRB (main):004:0> x["A"]= "B"
=> "B"
IRB (main):005:0> x[:z] = 100
=> 100
IRB (main):006:0> x[[1,2,3]] = [4,5,6]
=> [4, 5, 6]
IRB (main):007:0> X
=> {"A" => "B", [1, 2, 3]=>[4, 5, 6], 1=>2,: z=>100}

There are two points to note:
(1) Any target including array, hash can be key.
(2) When using a character as a key, it is best to use the corresponding symbol. For example, the key "a" can be replaced by: a. This is because each character "a" is a target, and: A is unique,

More memory savings.

3. How do I assign a default value to the hash?

Copy Code code as follows:

IRB (main):003:0> x=hash.new ([])
=> {}
IRB (main):004:0> X[:a]
=> []

Represents the creation of a hash whose default value is an empty array [].
Then access X[:a], although this key is not defined beforehand, there is still a default value.

Note that this method assigns a default value that points to the same target, which changes and affects all default values.

Copy Code code as follows:

IRB (main):005:0> x[:a] << 1
=> [1]
IRB (main):006:0> X[:b]
=> [1]

To make each key correspond to a unique default value, you should create a hash:

Copy Code code as follows:

IRB (main):007:0> y=hash.new do |h,k| H[K] =[] End
=> {}

Let's take another look at:

Copy Code code as follows:

IRB (main):009:0> y[:a] << 1
=> [1]
IRB (main):010:0> Y[:b]
=> []

A change in the value does not affect other default values.

4. How to traverse hash?

Copy Code code as follows:

IRB (main): 018:0* x={1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}

IRB (main):019:0> X.each do |k,v| Puts "#{k} #{v}" end
5 6
1 2
3 4
=> {5=>6, 1=>2, 3=>4}

Use each method of the hash.

5. How to traverse the hash key and value?

Copy Code code as follows:

IRB (main):020:0> X.keys.each do |k| Puts K end

IRB (main):021:0> X.values.each do |v| Puts V end

Using the hash keys and values methods, they return an array.

Of course, you can also use the Each_key and Each_value methods, as the name implies is traversing key and value.

Copy Code code as follows:

IRB (main):016:0> X.each_key do |s| Puts S end
1
3
=> {1=>2, 3=>4}
IRB (main):017:0> X.each_value do |s| Puts S end
2
4
=> {1=>2, 3=>4}

6. Can I sort the hash?

Ruby's hash actually can be sort, returning a sort two-dimensional array.

Copy Code code as follows:

IRB (main):027:0> X
=> {5=>6, 1=>2, 3=>4}

IRB (main):028:0> X.sort
=> [[1, 2], [3, 4], [5, 6]]

Ruby also has the Orderedhash class, which lets you sort the hash elements in the order they are inserted, similar to the ability of an array.

7. How do I add and remove elements from the hash?

Add elements to directly define Key/value:

Copy Code code as follows:

IRB (main):040:0> x[7]=8
=> 8

IRB (main):041:0> X
=> {5=>6, 1=>2, 7=>8, 3=>4}

Delete the element, use the Delete method, and the parameter is the key you want to delete:

Copy Code code as follows:

IRB (main):042:0> X.delete (7)
=> 8

IRB (main):043:0> X
=> {5=>6, 1=>2, 3=>4}

Of course, you can also use delete_if with conditional delete hash elements. For example, delete all keys greater than 3:

Copy Code code as follows:

IRB (main): 074:0* x.delete_if do |k,v| K>3 End
=> {1=>2, 3=>4}

8. How do I find the hash element?

Use Has_key and has_value to determine if there are corresponding keys and values:

Copy Code code as follows:

IRB (main):052:0> X.has_key? 1
=> true

IRB (main):053:0> X.has_value? 4
=> true

IRB (main):054:0> X.has_key? 9
=> false

In the case of default values, it cannot be judged directly with the if Hash[:key, for example:

Copy Code code as follows:

IRB (main):055:0> Y=hash.new (9)
=> {}

IRB (main):056:0> Y.has_key? : A
=> false

IRB (main):057:0> puts 1 if Y[:A]
1
=> Nil

Other search methods, as well as find, select, and so on, such as finding all keys greater than 2 elements:

Copy Code code as follows:

IRB (main):038:0> X.select do |k,v| K>2 End
=> [[5, 6], [3, 4]]

9. Can I reverse hash?

The answer is yes, using the Invert method:

Copy Code code as follows:

IRB (main): 011:0* x={1=>2,3=>4,5=>6}
=> {5=>6, 1=>2, 3=>4}

IRB (main):012:0> X.invert
=> {6=>5, 2=>1, 4=>3}

How to convert hash and array?

Hash conversion to an array is simple:

Copy Code code as follows:

IRB (main):020:0> x.to_a
=> [[5, 6], [1, 2], [3, 4]]

Array conversion to hash trouble, write yourself a method To_hash:

Copy Code code as follows:

IRB (main):023:0> class Array
IRB (main):024:1> def To_hash
IRB (main):025:2> unless size%2 = 0
IRB (main):026:3> raise "array with odd number of elements"
IRB (main):027:3> end
IRB (main):028:2> hash = hash.new
IRB (main):029:2> 0.step (size-1,2) {|x| hash[self[x]] = self[x+1]}
IRB (main):030:2> Hash
IRB (main):031:2> end
IRB (main):032:1> end
=> Nil

IRB (main):033:0> x=[1,2,3,4,5,6]
=> [1, 2, 3, 4, 5, 6]

IRB (main):034:0> X.to_hash
=> {5=>6, 1=>2, 3=>4}

What's the value of the hash loss behavior?

Any time a hash key is modified (remember that the key is a target including an array), the value of the loss behavior occurs.

Copy Code code as follows:

IRB (main):001:0> h=hash.new
=> {}
IRB (main):002:0> x=[1,2,3]
=> [1, 2, 3]
IRB (main):003:0> h[x]=4
=> 4
IRB (main):004:0> H[x]
=> 4
IRB (main):005:0> x<<4
=> [1, 2, 3, 4]
IRB (main):006:0> H[x]

The value of the key X is modified above, and the value corresponding to the key is lost.
The solution is rehash. Any time you change the value of a key, you have to rehash it once.

Copy Code code as follows:

IRB (main):007:0> H.rehash
=> {[1, 2, 3, 4]=>4}
IRB (main):008:0> H[x]
=> 4