Python程序中for循环用法详解

妖狐艹你老母 2022-05-23 07:42 270阅读 0赞

**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"

....:

i != 3

i = 3

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