Perl Language Primer--5--Hash, hash

First, the form of associative array

% H = (‘a’, 1, ‘b’, 2); a is key 1 is value b is key 2 is value

Starts with%, () is an empty hash

% H = (‘a’ => 1, ’b’ => 2); more obviously, you can also omit the quotes, but the default is all strings. When there are numbers, it is not your original intention

Element form: $ h {‘a’} #Note the use of $

The element is a simple variable, use $. % h {2.5}% {-1} $ {$ ee} are all possible decimals negative numbers simple variables can all be keys

 

Assignment of associative arrays

1, Standard:% a = (‘a’, 1, ‘b’, 2);

2. Assignment between array and hash:% [email protected] The hash is also an array, but there is no order

3, @ a =% h The order of the array elements is random

4, ($ a,% h) [email protected] #Not very likely to put the first value of the array into the simple variable $ a and then put the rest into the hash% h, so @array The number must be odd

5,% h = (% first,% second) #Waste memory

6.% h1 = (% h2, ‘k’, ‘v’) #Add a set of key values, which must appear in pairs

7. The return of a function, such as the result of split (), can also be assigned to a hash:

% H = split (/, /, ‘a, b, c, d’)

8. Snippet: @key = (‘a’, ’b’, ‘c’)

H @hash {@keys} [email protected] {reverse (@keys)}

注意 # Note that @ cannot be changed to $, because the key in $ hash {} is a simple variable, so a number is in parentheses.

现在 Now I want to circle an array, so it needs to be @ before, and @ is inside the brackets.

Should correspond back and forth

If you enter $ hash {@keys} because the key in $ hash braces is a simple variable, this is equivalent to $ hash {@keys length}, which is @hash {3}

Third, element assignment and operation

1, increase: $ h {‘a’} = 1 is created if it does not exist, it is also created if the hash does not exist

2. Delete: delete $ h {‘a’}

3. Do not use order related functions such as push pop shift splice, otherwise the result is unknown

4. Display: Cannot print "% hash"; This only outputs a string:% hash

If print% hash is used, no spaces are displayed

Use traversal method to display one by one

5, $ a =% hash $ a is assigned the value x x is the number of existing elements / number of allocated memory

Fourth, associative array operation functions

Key (% h) or key% h #Return the list of keys

Value (% h) or value% h #Return value list



Each (% h) loop: ($ key, $ value) = each (% hash)

不要 Do not add or delete elements in the loop, because the hash is out of order. The end of the loop returns undef

Exist: See if the keyword exists

Exist $ h {‘key’}

Undef (% h): delete hash equivalent to% = ()

Delete

Introduction to Perl language--5--hashing and hashing