Python Basics (3)--conditional judgment Loop statement and list parsing, generator

To determine the syntax of a looping statement:

1,if

If Boolean_expression:

...

Elif Boolean_expression:

...

Else

...

2,while

While Boolean_expression:

While_suite

If Boolean_expression2:continue

If Boolean_expression3:break

Else

Else_suite

3,for

For expression in object:

For_suite

If Boolean_expression2:continue

If Boolean_expression3:break

Else

Else_suite

While True:

x = Raw_input (' Enter an entry: ')

if x = = ' Q ' or x = = ' Quit ':

Break

Else

Test.append (x)

List resolution and builder expressions:

The difference between the two:

1, list parsing using [], generator using ()

2, list resolution all production, if the volume of data is large, compared to consumption of resources, and the generator each time an entry, this entry yield, high efficiency, low memory consumption. (similar to Range,xrange)

List parsing

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Example 1: list files for all. Log suffix names in the/var/log directory

s1= ' Hello.log '

S1.endswith ('. log ') #endswith () function to determine the suffix name

True

Import OS #加载os模块, there are many

Filelist1 = Os.listdir ('/var/log ') #os. Listdir () function lists all files in the directory

Help (Str.endswith) #查看帮助

Filelist2 = [i-I in Filelist1 if I.endswith ('. log ')]

Print Filelist2

[' Boot.log ', ' pm-powersave.log ', ' anaconda.yum.log ', ' xorg.0.log ', ' anaconda.ifcfg.log ', ' xorg.9.log ', ' Anaconda.storage.log ', ' dracut.log ', ' yum.log ', ' spice-vdagent.log ', ' wpa_supplicant.log ', ' anaconda.log ', ' Anaconda.program.log ']

Example 2: Implementing the L1,L2 list once in the opposite order

L1 = [' x ', ' y ', ' Z ']

L2 = [A]

L3 = [(i,j) for I in L1 for J in L2]

Print L3

[(' X ', 1), (' X ', 2), (' X ', 3), (' Y ', 1), (' Y ', 2), (' Y ', 3), (' Z ', 1), (' Z ', 2), (' Z ', 3)]

Builder expression

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Grammar:

(Expr for Iter_var in iterable)

(Expr for Iter_var in iterable if cond_expr)

exponentiation operation

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Gets the index and the value itself enumerate, this function can get the column index and value, you can generate a dictionary

url = ' www.baidu.com '

in [+]: en

%env Enumerate

in [+]: Enumerate (URL)

OUT[84]: <enumerate at 0x1eb81e0>

In [G1]: = Enumerate (URL)

In [the]: G1.next ()

OUT[86]: (0, ' W ')

In [All]: G1.next ()

OUT[87]: (1, ' W ')

in [+]: G1.next ()

OUT[88]: (2, ' W ')

In []: G1.next ()

OUT[89]: (3, '. ')

Python Basics (3)--conditional judgment Loop statement and list parsing, generator