table of Contents
Three, two kinds of cycle comparison
Preface
In python, to achieve "repetitive and automatic code execution", there are two loop statements for us to choose from:
One is a for...in...loop statement, and the other is a whileloop statement.
One, for loop:
For loop format:
Code example
for i in [1,2,3,4,5]:
print(i)Running effect chart:
Of course, the loop here can not only be lists, but also dictionaries and strings, not integers, floating-point numbers,
If it is a dictionary, it will print out all the 【keys】 in a loop; if it is a string, each string will be printed out in order
For example, in the above example, [1,2,3,4,5] is a list, we can also try a dictionary
ages = {'张三':18,'李四':19,'王五':20}
for i in ages:
print(i)
names = '赵六'
for j in names:
print(j)running result
Can not be integer and floating point examples
a = 50
b = 0.5
for i in a:
print(i)
for i in b:
print(i) 
In addition to the three data types of lists, dictionaries, and strings, we can also traverse other data sets. For example, it is often used with for loops: the range() function.
Use of range() function
Using the range(x) function, you can generate a sequence of integers from 0to x-1.
range(a,b), you can generate a sequence of integers from a to b-1. (It is a function that takes left but not right)
range(a,b,n), you can generate an entire sequence from a to b-1, and the interval is n
range(a,b) is actually a special range(a,b,n), if n is not filled in, it defaults to 1
such as
for i in range(5):
print(i)
print('------------------------')
for j in range(5,8):
print(j)
for i in range(0,7,2):
print(i)
print('--------------')
for j in range(0,6,2):
print(j)
Two, while loop:
while loop format
Code example
The while statement can enter the code inside the while and execute it only when the conditions behind whlle are met, otherwise it won’t be entered nor executed
a = 0
while a < 5:
print(a)
a=a+1
We can also use the while and input() functions to achieve continuous input, and only when the input result meets certain conditions, it ends
password = '' # 变量password用来保存输入的密码
while password !='789':
password = input('请输入您正确的密码：')
print('恭喜你登陆成功！')
Three, two kinds of cycle comparison
According to the above code example, we can know that the for loop is used for a relatively small number or some fixed values, we can use the for loop to print
If there is a large number, and there is a certain pattern or a certain condition is met, we can use the while loop to print
But in most cases, for and while achieve the same effect, you can use it appropriately.
We can use for and while codes to print the number from 1 to 7, and not the number 4
Code example
for i in range(1,8):
if i!=4:
print(i)
print('----------------')
a = 1;
while a<=7:
if a!=4:
print(a)
a=a+1