Python程序中for循环用法详解「建议收藏」

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Python程序中for循环用法详解

一个通用的序列迭代器，用于遍历任何有序的序列对象内的元素，可用于字符串、元组、列表和其它内置可迭代对象，以及通过类所创建的新对象。

1、for循环语法格式：

for expression in object:

for_suite

else:

else_suite

2、for语法格式扩展：

for expression in object:

for_suite

if boolean_expression2: continue

if boolean_expression3: break

else:

else_suite

3、用法示例：

例1：for循环基本用法示例

In [1]: url = ‘web2.field.com’

In [2]: for x in url:

…..: print x,

…..:

w e b 2 . f i e l d . c o m

In [3]: l1 = [1,2,3,4]

In [4]: for i in l1:

…..: print i,

…..:

1 2 3 4

例2：for-else用法示例

In [5]: for x in url:

print x,

else:

print “\n” +”Game Over!”

….:

w e b 2 . f i e l d . c o m

Game Over!

例2：for-if-else-else用法示例

In [19]: for i in l1:

print i

if i == 3:

print “i = 3”

else:

print “i != 3”

else:

print “Show End”

….:

1

i != 3

2

i != 3

3

i = 3

4

i != 3

Show End

4、for循环练习

练习1：逐一分开显示指定字典d1中的所有元素

In [214]: print d1

{0: ‘Sun’, 1: ‘Mon’, 2: ‘Tue’, 3: ‘Wed’, 4:’Thu’, 5: ‘Fri’, 6: ‘Sat’}

In [215]: d1.items()

Out[215]:

[(0, ‘Sun’),

(1,’Mon’),

(2,’Tue’),

(3,’Wed’),

(4,’Thu’),

(5,’Fri’),

(6,’Sat’)]

In [216]: for (k,v) in d1.items():

…..: print k,v

…..:

0 Sun

1 Mon

2 Tue

3 Wed

4 Thu

5 Fri

6 Sat

练习2：逐一显示列表中l1=[“Sun”,”Mon”,”Tue”,”Wed”,”Thu”,”Fri”,”Sat”]中的索引为奇数的元素；

In [219]: l1=[“Sun”,”Mon”,”Tue”,”Wed”,”Thu”,”Fri”,”Sat”]

In [220]: range(1,10,2)

Out[220]: [1, 3, 5, 7, 9]

In [221]: for i in range(1,len(l1),2):

…..: print l1[i]

…..:

Mon

Wed

Fri

练习3：将属于列表l1=[“Sun”,”Mon”,”Tue”,”Wed”,”Thu”,”Fri”,”Sat”]，但不属于列表l2=[“Sun”,”Mon”,”Tue”,”Thu”,”Sat”]的所有元素定义为一个新列表l3;

In [225]:l2=[“Sun”,”Mon”,”Tue”,”Thu”,”Sat”]

In [226]:l2=[“Sun”,”Mon”,”Tue”,”Thu”,”Sat”]

In [227]: for i in l1:

…..: if i not in l2:

…..: l3.append(i)

…..:

In [228]: print l3

[‘Wed’, ‘Fri’]

练习4：已知列表namelist=[‘stu1′,’stu2′,’stu3′,’stu4′,’stu5′,’stu6′,’stu7’]，删除列表removelist=[‘stu3’, ‘stu7’, ‘stu9’]；请将属于removelist列表中的每个元素从namelist中移除（属于removelist，但不属于namelist的忽略即可)；

In [229]: namelist=[‘stu1′,’stu2′,’stu3′,’stu4′,’stu5′,’stu6′,’stu7’]

In [230]: removelist=[‘stu3’, ‘stu7′,’stu9’]

In [231]: for i in namelist:

…..: if i in namelist:

…..: namelist.remove(i)

…..:

In [232]: print namelist

[‘stu2’, ‘stu4’, ‘stu6’]

练习5、自定义整数列表，修改列表，令列表中每个元素加1

In [235]: l1 = [3,4,56,8,2,9]

In [236]: for i in range(len(l1)):

…..: l1[i]+=1

In [237]: print l1

[4, 5, 57, 9, 3, 10]

练习6：l1 = [1,2,3,4,5,6,7];l2 = [“Sun”,”Mon”,”Tue”,”Wed”,”Thu”,”Fri”,”Sat”]，构造字典d1= {…}

In [238]: l1 = [1,2,3,4,5,6,7];l2 =[“Sun”,”Mon”,”Tue”,”Wed”,”Thu”,”Fri”,”Sat”]

In [239]: zip(l1,l2)

Out[239]:

[(1, ‘Sun’),

(2,’Mon’),

(3,’Tue’),

(4,’Wed’),

(5,’Thu’),

(6,’Fri’),

(7,’Sat’)]

In [240]: for (k,v) in zip(l1,l2):

…..: d1[k] = v

…..:

In [241]: print d1

{0: ‘Sun’, 1: ‘Sun’, 2: ‘Mon’, 3: ‘Tue’, 4:’Wed’, 5: ‘Thu’, 6: ‘Fri’, 7: ‘Sat’}

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