Python中的for循环用法详解，一文搞定它_python for循环

for循环

在Python中，for循环是一种迭代结构，用于遍历可迭代对象（如列表、元组、字符串、字典、集合等）中的元素。

for循环可以解决while循环的局限性
#while循环在遍历数据时的局限性

如下for循环可以实现

lst = [1,2,3,4,5]  # ok
i = 0
while i < len(lst):
    print(lst[i])
    i+=1
1
2
3
4
5

用while循环无法实现：

setvar = {"a","b","c"} # not ok
i = 0
while i < len(setvar):
    print(setvar[i])
    i+=1
1
2
3
4
5

1.for循环的基本语法

Iterable 可迭代性数据：1.容器类型数据 2.range对象 3.迭代器

for 变量 in Iterable:
    code1
1
2

#字符串
container = "北京和深圳温差大概20多度"
#列表
container = [1,2,3,4,4,5]
#元组
container = ("孙开洗","孙健","孙悟空")
#集合 
container = {"陈璐","曹静怡","王志国","邓鹏","合力"}
#字典
container = {"cl":"风流倜傥","cjy":"拳击选手","wzg":"寻花问柳","dp":"帅气,祖国的栋梁","hl":"你是个好人"}
1
2
3
4
5
6
7
8
9
10

#遍历数据

for i in container:
    print(i)
1
2

（1）遍历不等长多级容器

container = [1,2,3,4,("嗄","234",{"马春配","李虎凌","刘子涛"})]
for i in container:
    # 判断当前元素是否是容器,如果是,进行二次遍历,如果不是,直接打印
    if isinstance(i,tuple):
        # ("嗄","234",{"马春配","李虎凌","刘子涛"})
        for j in i:
            # 判断当前元素是否是集合,如果是,进行三次遍历,如果不是,直接打印
            if isinstance(j,set):
                # j = {"马春配","李虎凌","刘子涛"}
                for k in j :
                    print(k)
            else:
                print(j)
                
    # 打印数据
    else:
        print(i)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

（2）遍历不等长多级容器

container = [("刘玉波","历史源","张光旭"), ("上朝气","于朝志"),("韩瑞晓",)]
for i in container:
    for j in i:
        print(j)
1
2
3
4

（3）遍历等长的容器

container = [("马云","小马哥","马春配") , ["王健林","王思聪","王志国"],{"王宝强","马蓉","宋小宝"}]
for a,b,c in container:
    print(a,b,c)
1
2
3

2.变量的解包

a,b,c = "poi"
a,b = (1,2)
a,b = 1,2
a,b,c = [10,11,12]
a,b = {"林明辉","家率先"}
a,b = {"lmh":"林明辉","jsx":"家率先"}
a,b,c = ("马云","小马哥","马春配")
print(a,b,c)
1
2
3
4
5
6
7
8

3.for…else【详细讲解】

for 临时变量 in 序列:
    重复执行的代码
    ......
else:
    循环正常结束后要执行的代码
1
2
3
4
5

所谓else指的是循环正常结束后要执行的代码，即如果是break终止循环的情况。else下方缩进的代码将不执行。

4.range对象

range([开始值,] 结束值 [,步长])
取头舍尾,结束值本身获取不到,获取到它之前的那一个数据

#range(一个值)  默认从0开始
for i in range(5): # 0 ~ 4
    print(i)
1
2
3

#range(二个值)
for i in range(3,8): # 3 4 5 6 7 
    print(i)

#range(三个值) 正向的从左到右
for i in range(1,11,3): # 1 4 7 10 
    print(i)

#range(三个值) 逆向的从右到左
for i in range(10,0,-1): # 10 9 8 7 ... 1 
    print(i)
1
2
3
4
5
6
7
8
9
10
11

5.总结

while 一般用于处理复杂的逻辑关系
for 一般用于迭代数据
部分情况下两个循环可以互相转换;

i = 1
while i <= 9:
    j = 1
    while j <= i:
        print("%d*%d=%2d " % (i,j,i*j) ,end="" )
        j+=1
    print()    
    i +=1

for i in range(1,10):
    for j in range(1,i+1):
        print("%d*%d=%2d " % (i,j,i*j) ,end="" )
    print()
1
2
3
4
5
6
7
8
9
10
11
12
13

for循环乘法口诀

方向二：

方向三：

方向四：

6.打印 1 ~ 10 跳过5

i = 1
while i <= 10:    
    if i == 5:
        i += 1
        continue
    print(i)
    i +=1
    
for i in range(1,11):
    if i == 5:
        continue
    print(i)
1
2
3
4
5
6
7
8
9
10
11
12

7.打印菱形小星星

     *
    ***
   *****
  *******
 *********
***********
***********
 *********
  *******
   *****
    ***
     *
1
2
3
4
5
6
7
8
9
10
11
12

空格 + 星星 + 换行

总行数:
对于最大行任意个星星n ,总行数: n // 2 + 1
13 -> 7
11 -> 6
9 -> 5
7 -> 4

空格:
对于当前行i , 空格数量 = 总行数 - 当前行
1 => 5
2 => 4
3 => 3
4 => 2
5 => 1
6 => 0

星星:
对于当前行i , 星星数量 = 2 * 当前行 - 1
1 => 1
2 => 3
3 => 5
4 => 7

n = 13
hang = n // 2 + 1
i = 1
while i <= hang:
    # 打印空格
    kongge = hang - i
    print(" " * kongge, end="")

    # 打印星星
    xingxing = 2 * i - 1
    print("*" * xingxing, end="")

    # 打印换行
    print()
    i += 1

i = hang
while i >= 1:
    # 打印空格
    kongge = hang - i
    print(" " * kongge, end="")

    # 打印星星
    xingxing = 2 * i - 1
    print("*" * xingxing, end="")

    # 打印换行
    print()
    i -= 1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29

方法二：
#打印菱形

     *
    ***
   *****
  *******
 *********
***********
***********
 *********
  *******
   *****
    ***
     *
1
2
3
4
5
6
7
8
9
10
11
12

最大行星星数n

n = 2*hang - 1
kongge = hang - i
xingxing = 2*i -1
1
2
3

n = 13
hang = int((n + 1) / 2)
i = 1
while i <= hang:
    kongge = hang - i
    print(" " * kongge,end='')
    xingixng = 2*i - 1
    print(f"*"* xingixng,end='')
    print()
    i += 1


i = hang
while i >= 1:
    kongge = hang - i
    print(" " * kongge,end='')
    xingixng = 2*i - 1
    print(f"*"* xingixng,end='')
    print()
    i -= 1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

菱形二

   *
  * *
 * * *
* * * * 
* * * * 
 * * *
  * *
   *
1
2
3
4
5
6
7
8

行数 n 星星 n

kongge = n - i

hang = 5
i = 1
while i <= hang:
    kongge = hang - i
    print(f" " * kongge,end='')
    xingxing = i
    if i == 1:
        print("*",end='')
    else:
        j = 1
        while j <= 2*i -1:
            if j % 2 == 1:
                print("*",end='')
            else:
                print(" ",end='')
            j += 1
    print()
    i += 1

i = hang
while i >= 1:
    kongge = hang - i
    print(f" " * kongge,end='')
    xingxing = i
    if i == 1:
        print("*",end='')
    else:
        j = 1
        while j <= 2*i -1:
            if j % 2 == 1:
                print("*",end='')
            else:
                print(" ",end='')
            j += 1
    print()
    i -= 1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36