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Heavy dry goods , First time delivery List yes Python Built in data structures in , It can be used to store any data type or a mixture of different data types . In this paper , We will pass 10 An example to introduce python Medium list.
Let's get started ！
#example 1
import numpy as np
a = [4,6,7,3,2]
b = [x for x in a if x > 5]
b
[6, 7]We iterate through a list ( Iteratable object ) And get greater than 5 The elements of ( Conditions ).
The equivalent of for Cycle is ：
b = []
for x in a:
if x > 5:
b.append(x)
b
[6, 7]We can also do something about elements before they are placed in a new list ：
#example 2
import numpy as np
a = [4,6,7,3,2]
b = [x*2 for x in a if x > 5]
b
[12, 14] We multiply the eligible items by 2, Then put it in a list .
The third example is a string list :
#example 3
names = ['Ch','Dh','Eh','cb','Tb','Td']
new_names = [name for name in names if name.lower().startswith('c')]
new_names
['Ch', 'cb']This time, the condition is to get the string with ”c” Elements that begin with . Because there are cases , So we first convert all the letters to lowercase .
An iteratable object is not necessarily a list , For example, we can iterate over a two-dimensional matrix Numpy Array .
#example 4
import numpy as np
A = np.random.randint(10, size=(4,4))
A
array([[1, 7, 4, 4],
[5, 0, 0, 6],
[7, 5, 8, 4],
[1, 3, 2, 2]])
max_element = [max(i) for i in A]
max_element
[7, 6, 8, 3] We iterate over the matrix A And take the maximum value .
The list can store any data type .
#example 5
vals = [[1,2,3],[4,5,2],[3,2,6]]
vals_max = [max(x) for x in vals]
vals_max
[3, 5, 6]We create a list of maximum values in each list .
There can also be multiple filter criteria .
#example 6
names = ['Ch','Dh','Eh','cb','Tb','Td','Chb','Tdb']
new_names = [name for name in names if
name.lower().endswith('b') and len(name) > 2]
new_names
['Chb', 'Tdb']The above is to get the string with letters ”b” Ends with a length greater than 2.
We can combine multiple conditions with other logical operators ：
#example 7
names = ['chb', 'ydb', 'thd', 'hgh']
new_names = [name for name in names
if name.endswith('b') | name.startswith('c')]
new_names
['chb', 'ydb']Now let's consider the following nested list ：
vals = [[1,2,3],[4,5,2],[3,2,6]] We want to take each element out of the nested list , So the expected output is ：
vals = [1,2,3,4,5,2,3,2,6] This can be done in the following ways ：
#example 8
vals = [[1,2,3],[4,5,2],[3,2,6]]
vals_exp = [y for x in vals for y in x]
vals_exp
[1, 2, 3, 4, 5, 2, 3, 2, 6] Grammar may not seem very intuitive , Equivalent to for Cyclic phase comparison , It will be very intuitive .
For the nested list operation above , We can also add conditions .
text = [['bar','foo','fooba'],['Rome','Madrid','Houston'], ['aa','bb','cc','dd']] We just need a length greater than 3 String in the nested list of .
#example 9
text_1 = [y for x in text if len(x)>3 for y in x]
text_1
['aa', 'bb', 'cc', 'dd'] We put conditions on nested lists , Not on a single element . therefore , equivalent for/if The loop syntax is as follows .
We can also set a condition for a single element .
#example 10
text_2 = [y for x in text for y in x if len(y)>4]
text_2
['fooba', 'Madrid', 'Houston']We are now better at 4 Character string . Because the condition is on a single element , So equivalent nesting for/if loop :
Master this 10 An example will lead us to list To a higher level .
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