for循环

例1 for打印数组

for i in range(5) :
    print(i)

结果

C:\Users\lixiang\AppData\Local\Programs\Python\Python35\python.exe C:/Users/lixiang/PycharmProjects/py_s15/day2.py

0

1

2

3

4

 

Process finished with exit code 0

例2 for+if elif

oldboy_age = 30
for i in range(5):
    guess_age = int(input("age:"))
    if guess_age == oldboy_age:
        print("correct!!!")
    elif guess_age > oldboy_age:
            print("try smaller..")
    else:
            print("try bigger...")

结果

age:23

try bigger...

age:25

try bigger...

age:28

try bigger...

age:30[l2] 

correct!!!

age:31

try smaller..

例3 for+break

oldboy_age = 30
for i in range(5):
    guess_age = int(input("age:"))
    if guess_age == oldboy_age:
        print("correct!!!")
        break
    elif guess_age > oldboy_age:
            print("try smaller..")
    else:
            print("try bigger...")

结果

age:22

try bigger...

age:39

try smaller..

age:30

correct!!!

 

例4 for循环结束后进行下一段语句的开始

oldboy_age = 30
for i in range(5):
    guess_age = int(input("age:"))
    if guess_age == oldboy_age:
        print("correct!!!")
        break
    elif guess_age > oldboy_age:
            print("try smaller..")
    else:
            print("try bigger...")

print("welcome enter to the second level...")

结果

try bigger...

age:1

try bigger...

age:1

try bigger...

age:1

try bigger...

age:1

try bigger...

welcome enter to the second level...

这段语句的目的是运行正确后进入到下一段语句。如果直接在for循环外直接print，若输入5次错误了还会是进行到下一段语句。所以需要加入else exit（）作为输入错误的结束。

例5 for else的用法

1 不加else

for i in range(10):
    print(i)
    if i == 5:
        break
print("循环顺利结束")

结果

0

1

2

3

4

5

循环顺利结束

此结果是不正确的，因为for循环并没有顺利结束

2 for+else

for i in range(10):
    print(i)
    if i == 5:
        break
else:
    print("循环顺利结束")

结果

0

1

2

3

4

5

没有    if i == 5:的结果
        break

0

1

2

3

4

5

6

7

8

9

循环顺利结束

例6 for循环嵌套

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

结果

0 0

0 1

0 2

0 3

0 4

1 0

1 1

1 2

1 3

1 4

2 0

2 1

2 2

2 3

2 4

3 0

3 1

3 2

3 3

3 4

4 0

4 1

4 2

4 3

4 4

若在j的循环里加入if判断并break跳出的话，只跳出j的循环

for i in range(5):
    for j in range(5):
        print(i,j)
        if j > 2:
            break

结果

0 0

0 1

0 2

0 3

1 0

1 1

1 2

1 3

2 0

2 1

2 2

2 3

3 0

3 1

3 2

3 3

4 0

4 1

4 2

4 3

例 6 for+continue的用法

for i in range(5):
    for j in range(5):
        if j < 3:
            break  #跳出整个当前层循环，只跳一层
            print(i,j)

结果

Process finished with exit code 0

若想实现只打印j大于3的结果就得用continue

for i in range(5):
    for j in range(5):
        if j < 3:
            continue    #跳出当次循环，执行下一次循环
        print(i,j)

结果

0 3

0 4

1 3

1 4

2 3

2 4

3 3

3 4

4 3

4 4

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