Python程序中while循环用法详解
Python程序中while循环用法详解
用于编写通用迭代结构，顶端测试为真就会执行循环体，并会多次重复测试直到测试为假后才执行循环后的其它语句。
1、while循环语法格式：
1).while bool_expression:
while_suite
else:
else_suite
2).while boolean_expression:
while_suite
if boolean_expression2: continue
if boolean_expression3: break
else:
else_suite
2、用法示例：
例1：while基础格式示例，定义url，并逐行从开头处减少一个字符显示
In [27]: url = 'www.field.com'
In [28]: while url:
....: print url
....: url = url[1:]
....:
www.field.com
ww.field.com
w.field.com
.field.com
field.com
ield.com
eld.com
ld.com
d.com
.com
com
om
m
例2：while基础格式示例，逐行打印“x= 0”到“x = 9”
In [29]: x = 0;y = 10
In [30]: while x < y:
....: print "x = %d"% x
....: x += 1
....:
x = 0
x = 1
x = 2
x = 3
x = 4
x = 5
x = 6
x = 7
x = 8
x = 9
In [39]: x = 0;y = 10
In [40]: while x <y:
print "x = %d" % x,
....: x +=1
....:
# print后用,结尾可以在同一行显示打印结果
x = 0 x = 1 x = 2 x = 3 x = 4 x = 5 x = 6 x= 7 x = 8 x = 9
例3：while基础格式示例，自定义url，并逐行从结尾处减少一个字符显示
In [41]: url = 'www.field.com'
In [43]: while url:
....: print " url is : %s" % url
....: url = url[:-1]
....:
urlis : www.field.com
urlis : www.field.co
urlis : www.field.c
urlis : www.field.
urlis : www.field
urlis : www.fiel
urlis : www.fie
urlis : www.fi
urlis : www.f
urlis : www.
urlis : www
urlis : ww
urlis : w
例4：while-else格式示例，自定义url，并逐行从结尾处减少一个字符显示，且当循环执行结束后打印提示信息“Game over !”
In [44]: url = 'www.field.com'
In [45]: while url:
print " url is : %s " % url
url = url[:-1]
....: else:
....: print "Game over !”
....:
urlis : www.field.com
urlis : www.field.co
urlis : www.field.c
urlis : www.field.
urlis : www.field
urlis : www.fiel
urlis : www.fie
urlis : www.fi
urlis : www.f
urlis : www.
urlis : www
urlis : ww
urlis : w
Game over !
例5：break用法示例：自定义url，逐行从结尾处减少一个字符显示，自定义if条件，当减少5个字符时结束打印。
In [46]: url = 'www.field.com'; x = 0
In [47]: while url:
....: print url
....: url = url[:-1]
....: x += 1
....: if x > 5:
....: break
....: else:
....: print "Reduce 5 chars,gameover!"
....:
www.field.com
www.field.co
www.field.c
www.field.
www.field
www.fiel
# break会跳出最内层的循环，所以else不会执行
例6：continue用法示例：自定义url，逐行从结尾处减少一个字符显示，自定义if条件，当减少5个字符时结束打印，并提示信息“Reduce 5 chars,game over!”
In [47]: while url:
....: print url
....: url = url[:-1]
....: x += 1
....: if x > 5:
....: continue
....: else:
....: print "Reduce 5chars,game over!"
....: break
www.field.com
www.field.co
www.field.c
www.field.
www.field
Reduce 5 chars,game over!
#continue会跳到所处的最近层循环的开始处，即跳到while循环开始执行处。
#else代码块：循环正常终止才会执行；如果循环终止是由break跳出导致的，则else不会执行；
例7、while True语句用法示例：
In [48]: l1=[]
In [49]: while True:
....: x = raw_input('PleaseInput An Char:')
....: if x == 'q' or x =='quit':
....: break
....: else:
....: l1.append(x)
....:
Please Input An Char:a
Please Input An Char:b
Please Input An Char:c
Please Input An Char:d
Please Input An Char:q
In [11]: print l1
['a', 'b', 'c', 'd']
3、while循环练习：
练习1：逐一显示指定列表中的所有元素；
In [54]: l1 = [1,2,3,4,5];count = 0
In [55]: while count < len(l1):
....: print l1[count]
....: count += 1
....: else:
....: print l1
....:
1
2
3
4
5
[1, 2, 3, 4, 5]
In [56]: while l1:
....: print l1[0],
....: l1.pop(0)
....: else:
....: print l1
....:
1 2 3 4 5 []
练习2：求100以内所有偶数之和；
In [70]: count = 2;sum =0
In [71]: while count < 100:
....: sum += count
....: count += 2
....: else:
....: print "sum is%d." % sum
....:
sum is 2450.
练习3：逐一显示指定字典的所有键；并于显示结束后说明总键数；
In [89]: d1 = {'x':11,'y':22,'z':33,'k':44}
In [90]: d1.keys()
Out[90]: ['y', 'x', 'k', 'z']
In [91]: key = d1.keys()
In [92]: while key:
....: print key[0]
....: key.pop(0)
....: else:
....: leng = len(d1)
....: print "Dict d1 Lengis %d." % leng
....:
y
x
k
z
Dict d1 Leng is 4.
练习4：创建一个包含了100以内所有奇数的列表；
In [94]: l1 = [];x = 1
In [95]: while x < 100:
....: l1.append(x)
....: x += 2
....: else:
....: print l1
....:
[1, 3, 5, 7, 9, 11, 13, 15,...,93, 95, 97,99]
In [96]: x = range(1,100,2)
In [97]: print x
[1, 3, 5, 7, 9, 11, 13, 15,..., 93, 95, 97,99]
练习5：逆序逐一显示一个列表的所有元素
In [162]: y = [1,2,3,4]; count = 0
In [163]: y.reverse()
In [164]: while count < len(y):
.....: print y[count]
.....: count += 1
.....: else:
.....: leng = len(y)
.....: print " List yleng is %d,reverse show over." % leng
.....:
4
3
2
1
List y leng is 4,reverse show over.
练习6：列表l1=[0,1,2,3,4,5,6], 列表l2=["Sun","Mon","Tue","Wed","Thu","Fri","Sat"]，以第一个列表中的元素为键，以第二个列表中的元素为值生成字典d1；
In [174]: l1 = [0,1,2,3,4,5,6];l2 = ["Sun","Mon","Tue","Wed","Thu","Fri","Sat"];d1={};count = 0
In [175]: if len(l1) == len(l2):
.....: while count <len(l1):
.....: d1[l1[count]] =l2[count]
.....: count += 1
.....: else:
.....: print "Newdict d1 is %s." % d1
.....: else:
.....: print "listKeyError!"
.....:
New dict d1 is {0: 'Sun', 1: 'Mon', 2:'Tue', 3: 'Wed', 4: 'Thu', 5: 'Fri', 6: 'Sat'}.