# Various derivation formulas of Python (list derivation, dictionary derivation, set deduction)

Source: Internet
Author: User
Tags generator

The derivation of comprehensions (also known as analytic) is a unique feature of Python. Derivation is the structure of another new data series that can be built from one data series. There are three kinds of derivation, which are supported in Python2 and 3:

• List-derived
• Dictionary (dict) derivation formula
• Set (set) derivation

First, the list deduction formula

1. Use [] to generate list

Basic format

`variable = [out_exp_res for out_exp in input_list if out_exp = = 2]  out_exp_res: List generates an element expression, which can be a function with a return value. For  Out_exp in input_list: Iteration input_list out_exp incoming out_exp_res expression.  if Out_exp = = 2: Filter which values can be based on conditions.`

Example one:

`multiples = [I for I in range] if I percent 3 is 0]print (multiples) # Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]`

Example two:

`def squared (x):    return x*xmultiples = [Squared (i) for I in range () if I% 3 is 0]print multiples#  Output: [0, 9, 36, 81, 144, 225, 324, 441, 576, 729]`

2. Use () to generate generator

The generator can be obtained by changing the two-table derivation [] to ().

`Multiples = (I for I in range () If I percent 3 is 0) print (type (multiples)) #  Output: <type ' generator ' >`

Second, the dictionary derivation formula

Dictionary derivation and list derivation are used in a similar way, except that the brackets should be changed to curly braces. The direct example illustrates:

Example one: Capitalization key merge

`Mcase = {' A ': ten, ' B ': ", ' a ': 7, ' Z ': 3}mcase_frequency = {    k.lower (): Mcase.get (K.lower (), 0) + mcase.get (K.upper () , 0) for    K in Mcase.keys ()    if K.lower () in [' A ', ' B ']}print mcase_frequency#  Output: {' A ': +, ' B ': 34}`

Example two: Quick change of key and value

`Mcase = {' A ': ten, ' B ': 34}mcase_frequency = {v:k for k, V in Mcase.items ()}print mcase_frequency#  Output: {: ' A ', 3 4: ' B '}`

Three, set deduction formula

They are similar to the list derivation. The only difference is that it uses curly braces {}.

Example one:

`Squared = {x**2 for x in [1, 1, 2]}print (squared) # output:set ([1, 4])`

Various derivation formulas of Python (list derivation, dictionary derivation, set deduction)

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