#python学习笔记（五）#循环语句

目录

1 Updating variables

2 The while statement

3 Infinite loops

4 Definite loops using for

5 Examples：counting and summing loops

6 Examples：maximum and minimum loops

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1 Updating variables

▲更新变量的值采用赋值语句

x = x + 1

▲被更新的变量需要有初始值initialize，否则会报错 

>>> x = x + 1
NameError: name 'x' is not defined

2 The while statement

n = 5 #初始化迭代变量interation variable的值
while n > 0: #while+判断语句 为真时执行，否则进行下一个语句print(n) #缩进语句n = n - 1 #更新迭代变量，否则会无限循环成为 infinite loop
#执行一遍后回到while重新判断是否再次循环print('Blastoff!')

3 Infinite loops

This loop is obviously an infinite loop because the logical expression on the while statement is simply the logical constant True

无限循环while的判断语句为常数Ture

在主题语句中采用break来退出循环

while True:#infinite loopline = input('> ')if line == 'done':#如果用户输入'done'break #break退出语句print(line) #否则继续执行print
print('Done!')

 运行结果

> hello there
hello there
> finished
finished
> done   ##输入done时退出循环，运行下一条print语句
Done!

use the continue statement to skip to the next iteration without finishing the body of the loop for the current iteration.

continue用来开始新一轮循环，而不再执行本次循环的后续语句

while True:line = input('> ')if line[0] == '#': continue        ##如果用户输入#开头，重新开始新一轮循环，不执行后续语句if line == 'done':  break           ##如果用户输入done，退出循环print(line)print('Done!')

运行结果

> hello there
hello there 
> # don't print this   ###以#开始，不进行后续print(line)语句
> print this!
print this!
> done
Done!
All

4 Definite loops using for

We call the while statement an indefinite loop because it simply loops until some condition becomes False, whereas the for loop is looping through a known set of items so it runs through as many iterations as there are items in the set.

while语句是不定循环，因为它循环直到判断语句为false

for是定循环，因为它的迭代变量是已知的确定数目的

friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends: #friend为迭代变量，依次遍历friends里的内容print('Happy New Year:', friend)print('Done!')

运行结果

Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Done!

5 Examples：counting and summing loops

###计数循环
count = 0   #初始化计数值
for itervar in [3, 41, 12, 9, 74, 15]:count = count + 1  #每循环一次，count+1print('Count: ', count)###求和循环
total = 0  #初始化
for itervar in [3, 41, 12, 9, 74, 15]:total = total + itervar #每循环一次，total加上新的数值print('Total: ', total)

6 Examples：maximum and minimum loops

#求最大值
largest = None
print('Before:', largest)for itervar in [3, 41, 12, 9, 74, 15]:if largest is None or itervar > largest :largest = itervarprint('Loop:', itervar, largest)print('Largest:', largest)#求最小值
smallest = None
print('Before:', smallest)for itervar in [3, 41, 12, 9, 74, 15]:if smallest is None or itervar < smallest:smallest = itervarprint('Loop:', itervar, smallest)print('Smallest:', smallest)

 可以将该循环语句写成函数重复使用

def min(values): #参数为待比较的数据smallest = Nonefor value in values:if smallest is None or value < smallest:smallest = valuereturn smallest #返回计算出的最小值