Python程序中while循环用法详解

约定不等于承诺〃 2022-05-23 07:42 242阅读 0赞

**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

**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

....:

\[1, 2, 3, 4, 5\]

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

....:

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

.....:

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'\}.

Python程序中while循环用法详解

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