[Ruby] How to get the value by specifying the key. Differences between hashes, symbols and fetch

This is a personal memo.

In addition to specifying the key by hash or symbol, there is also a method of using the fetch method to fetch the value by specifying the key of the object.

・ Symbol: obj[:a]
・ Hash: obj['a']   （ obj["a"]Is the same)
・ Fetch:    obj.fetch(:a)Or obj.fetch('a')

table of contents

1. Differences between hashes, symbols and fetches (#ハッシュシンボルfetchの違い)
2. (1) Difference depending on the format of the original object
3. ② Do you want to display an error with a key that does not exist?
4. fetch method
5. Exception Handling 1
6. Exception Handling 2
7. When both exception handling 1 and exception handling 2 are described

-① ** What is the original object format **? -② Do you want to display ** error ** with a key that does not exist?

There are two points.

There are ** hash ** and ** symbol ** as the method of specifying the key of the object.

** The hash is a string. The symbol is described with a colon ** in front of it. The specification when retrieving the value must match each format.

Hash format example

obj = {"a"=>1, "b"=>2, "c"=>3}

##Get value
 obj2['b']
=> 2

##NG
obj[:b]
=> nil

Symbol format example

obj = {:a=>1, :b=>2, :c=>3}

##Get value
obj[:b]
=> 2

##NG
obj['b']
=> nil

That is, 'b' and : b are not equal.

The hash is unique to each defined object, but the difference is that all symbols are the same if they have the same key.

** Use fetch ** if you want to display an error.

python

obj = {:a=>1, :b=>2, :c=>3}

##If the value exists
obj.fetch(:b)
=> 2

##If the value does not exist case1
obj.fetch(:g)
KeyError (key not found: :g)

##If the value does not exist case2
obj.fetch("b")
KeyError (key not found: "b")

If you specify a value that does not exist, KeyError (key not found: specified key) is returned.

・object.fetch(Key,Exception handling 1){|Variable 2|Exception handling 2 }

--Exception handling 1 can be specified by either a variable or a value --If both exception handling 1 and exception handling 2 are listed, exception handling 2 takes precedence. --The key name is entered in variable 2 of exception handling 2.

• {}In|variable|If there is no description of, only the processing result is returned.

-Object.fetch (key, exception handling)

** ▼ Specify by character string **

Specified by a character string

obj = {:a=>1, :b=>2, :c=>3}
obj.fetch(:g, "error")

=> "error"

Specified by a character string

obj = {:a=>1, :b=>2, :c=>3}

e = "error"
obj.fetch(:g, e)

=> "error"

Specified by a character string

obj = {:a=>1, :b=>2, :c=>3}
obj.fetch(:g, e = "error")

=> "error"

It is also possible to specify exception handling in the block {}.

python

obj = {:a=>1, :b=>2, :c=>3}
obj.fetch(:g){|x| "#{x}Does not exist"  }

=> "g does not exist"

python

obj = {:a=>1, :b=>2, :c=>3}
obj.fetch(:g){ "It is an error" }

=> "It is an error"

python

obj.fetch(:g, "error1"){ "ERROR2" }
warning: block supersedes default value argument
=> "ERROR2"

supersedes means to replace.