Loop structure of program (for loop)

Traversal cycle ：for loop

Traversal cycle refers to the cycle operation mode formed by traversing a certain structure ：

for < Loop variable > in < Traversal structure >:

< Sentence block >

Every time I cycle , Extract one of the elements from the traversal structure , Put it in the previous loop variable , Execute the following statement block , After performing , Then extract the next element from the traversal structure , When all elements in the traversal structure are put into the loop variable , And all have been executed circularly , So the program exits .

Extract elements one by one from the traversal structure , Put it in a loop variable , Then execute the following statement block

By reserved words for and in form , Complete traversal of all elements ends

Each cycle , The resulting element is placed in a loop variable , And execute the statement block once

The traversal structure used is a structure that can contain multiple elements , The so-called traversal structure is not a special type , It contains many types .

Count cycle （N Time ）：

for i in range(N):

< Sentence block >

His execution effect is to traverse the following statement block , perform N Content of secondary statement block .

Traversal by range（） Function produces a sequence of numbers , Create a cycle .

for Sentences can be compared with range() Function collocation ,range() Function can generate an iteratable object composed of integers （ It can be simply understood as supporting the use of for Loop traversed objects ）,range(N) The function generates a sequence of numbers , This sequence contains N Elements , Each element is an integer , Namely 0,1 Until N-1

Count cycle N Time , We can use this variable in the middle i, It is also possible not to use variables i, about for in range It's a cycle N Time , As for not using intermediate loop variables, it is decided according to the logic of the program .

The code example is as follows ：

for i in range(5):
    print(i)
    

The operation interface is as follows ：

  Count cycle （ Specific times ）：

The format is as follows ：

for i in range(M,N,K):

from M Start , Less than N, To N Previous integer , With K Step length .

The code of example 1 is as follows ：

for i in range(1,6):# from 1 Start , To 6 Previous integer 5
    print(i)

The operation interface is as follows ：

The code of example 2 is as follows ：

for i in range(1,6,2):# from 1 Start , To 6 Previous integer 5, With 2 Step length .
    print(i)

The operation interface is as follows ： 

String traversal loop ：

for c in s:

< Sentence block >

s Is string , Traverses each character of the string , Create a cycle .

c Represents each character in the string , It can take each character out of the string in order and put it in c in , And for each extracted character , Execute the statement block once , And create a cycle .

The code example is as follows ：

for c in "Python123":
    print(c,end=",")

The operation interface is as follows ： 

List traversal loop ：

for item in ls:

< Sentence block >

ls It's a list , Traverse every element of it , Create a cycle

The code example is as follows ：

for item in [123,"PY",456]:
    print(item,end=",")

The operation interface is as follows ： 

Tuple traversal loop ：

for i in ( Elements ）：

print(i)

The code is as follows ：

for i in ('python',1,5,6):
    print(i)
    

The operation interface is as follows ：

  Dictionary traversal loop ：

Output all elements in the following format ：

dic={' ' : ,' ': }

for item in dic.items():

print(item)

The code is as follows ：

dic={'python':1,'age':27,5:6}
for item in dic.items():
    print(item)
    

The operation interface is as follows ：

Output all keys in the following format ：

dic={' ' : ,' ': }

for item in dic.keys():

print(item)

The code is as follows ：

dic = {'python': 1, 'age': 27, 5: 6}
for item in dic.keys():
    print(item)

The operation interface is as follows : 

Output the values corresponding to all keys ：

dic={' ' : ,' ': }

for item in dic.values():

print(item)

The code is as follows ：

dic = {'python': 1, 'age': 27, 5: 6}
for item in dic.values():
    print(item)

The operation interface is as follows : 

  The traversal loop uses reserved words for in A cycle formed , It can assign the current loop variable to each element in the traversal structure , And form a cycle .

all for in Such a traversable structure , As long as the following variables or this data type , It is a data type composed of multiple elements , You can use for in The way , Traverse every element of it , And according to the number of elements , Form a corresponding cycle .