Python traversal skills

Dictionary traversal : keywords and corresponding values ​​can be interpreted at the same time using the items() method;

eg：

knights = {'gallahad': 'the pure', 'robin': 'the brave'}

for k, v in knights.items():

    print(k, v)

gallahad the pure
robin the brave

Sequence traversal : the index position and corresponding value can be obtained at the same time using the enumerate() function;

eg：

for i,v in enumerate([i for i in range(2,5)]):

    print(i,v)

0 2
1 3
2 4

To traverse two or more sequences at the same time , you can use zip() combination;

eg：

questions = ['name', 'quest', 'favorite color']

answers = ['lancelot', 'the holy grail', 'blue']

for q, a in zip(questions, answers):

    print('What is your {0}?  It is {1}.'.format(q, a)) 

What is your name?  It is lancelot.
What is your quest?  It is the holy grail.
What is your favorite color?  It is blue.

To traverse a sequence in reverse, first specify the sequence, and then call the reversed() function;

eg：

for i in reversed(range(1, 10, 2)):

    print(i)

9
7
5
3
1

To traverse a sequence in order, use the sorted() function to return a sorted sequence without modifying the original value:

eg：

basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']

for f in sorted(set(basket)):

     print(f)

apple
banana
orange
pear

str(): Convert dictionaries, tuples, and lists to strings;

eval(): convert string to dictionary, tuple, list

1、简单表达式
 
 
print(eval('1+2'))
 
 
输出结果：3
 
 
2、字符串转字典
 
 
print(eval("{'name':'linux','age':18}")
 
 
输出结果：{'name':'linux','age':18}
 
 
3、传递全局变量
 
 
print(eval("{'name':'linux','age':age}",{"age":1822}))
 
 
输出结果：{'name': 'linux', 'age': 1822}
 
 
4、传递本地变量
 
 
age=18
 
 
print(eval("{'name':'linux','age':age}",{"age":1822},locals()))
 
 
输出结果：{'name': 'linux', 'age': 18}

The zip function for loop uses:

The zip() function in the for loop is used to traverse the list in parallel and output the data

 

A = ['python','java','c++','abc']
B = ['a','b','c','d']
for i,j in zip(A,B):
    print(i,j)

enumerate() function:

 The enumerate() function in the for loop is an enumeration function, which is used to combine a traversable data object (such as a list, tuple or string) into an index sequence, and list data and data subscripts at the same time.

A = ['python','java','c++','abc']
for i,val in enumerate(A):
    print(i,val)