语法基础
for语句
Python的for语句针对序列(列表或字符串等)中的子项进行循环,按它们在序列中的顺序来进行迭代。
#!python>>> # Measure some strings:... words = ['cat', 'window', 'defenestrate']>>> for w in words:... print(w, len(w))...cat 3window 6defenestrate 12
在迭代过程中修改迭代序列不安全,可能导致部分元素重复两次,建议先拷贝:
#!python>>> for w in words[:]: # Loop over a slice copy of the entire list.... if len(w) > 6:... words.insert(0, w)...>>> words['defenestrate', 'cat', 'window', 'defenestrate']
range()函数
内置函数 range()生成等差数值序列:
#!python>>> range(10)[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
range(10) 生成了一个包含10个值的链表,但是不包含最右边的值。默认从0开始,也可以让range 从其他值开始,或者指定不同的增量值(甚至是负数,有时也称”步长”):
#!python>>> range(5, 10)[5, 6, 7, 8, 9]>>> range(0, 10, 3)[0, 3, 6, 9]>>> range(-10, -100, -30)[-10, -40, -70]>>> range(-10, -100, 30)[]
如果迭代时需要索引和值可结合使用range()和len():
#!python>>> a = ['Mary', 'had', 'a', 'little', 'lamb']>>> for i in range(len(a)):... print(i, a[i])...0 Mary1 had2 a3 little4 lamb
不过使用enumerate()更方便,参见后面的介绍。
break和continue语句及循环中的else子句
break语句和C中的类似,用于终止当前的for或while循环。
循环可能有else 子句;它在循环迭代完整个列表(对于 for)后或执行条件为false(对于 while)时执行,但循环break时不会执行。这点和try…else而不是if…else相近。请看查找素数的程序:
#!python>>> for n in range(2, 10):... for x in range(2, n):... if n % x == 0:... print(n, 'equals', x, '*', n//x)... break... else:... # loop fell through without finding a factor... print(n, 'is a prime number')...2 is a prime number3 is a prime number4 equals 2 * 25 is a prime number6 equals 2 * 37 is a prime number8 equals 2 * 49 equals 3 * 3
continue语句也是从C而来,它表示退出当次循环,继续执行下次迭代。通常可以用if…else替代,请看查找偶数的实例:
#!python>>> for n in range(2, 10):... for x in range(2, n):... if n % x == 0:... print(n, 'equals', x, '*', n//x)... break... else:... # loop fell through without finding a factor... print(n, 'is a prime number')...2 is a prime number3 is a prime number4 equals 2 * 25 is a prime number6 equals 2 * 37 is a prime number8 equals 2 * 49 equals 3 * 3
pass
pass语句什么也不做。它语法上需要,但是实际什么也不做场合,也常用语以后预留以后扩展。例如:
#!python>>> while True:... pass # Busy-wait for keyboard interrupt (Ctrl+C)...>>> class MyEmptyClass:... pass...>>> def initlog(*args):... pass # Remember to implement this!...
循环技巧
在字典中循环时,关键字和对应的值可以使用 items() 方法同时获取:
#!python>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}>>> for k, v in knights.items():... print(k, v)...gallahad the purerobin the brave
在序列中循环时 enumerate() 函数同时得到索引位置和对应值:
#!python>>> for i, v in enumerate(['tic', 'tac', 'toe']):... print(i, v)...0 tic1 tac2 toe
同时循环两个或更多的序列,可以使用 zip() 打包:
#!python>>> questions = ['name', 'quest', 'favorite color']>>> answers = ['lancelot', 'the holy grail', 'blue']>>> for q, a in zip(questions, answers):... print('What is your {0}? It is {1}.'.format(q, a))...What is your name? It is lancelot.What is your quest? It is the holy grail.What is your favorite color? It is blue.
需要逆向循环序列的话,调用 reversed() 函数即可:
#!python>>> for i in reversed(xrange(1, 10, 2)):... print(i)...97531
使用 sorted() 函数可排序序列,它不改动原序列,而是生成新的已排序的序列:
#!python>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']>>> for f in sorted(set(basket)):... print f...applebananaorangepear
若要在循环时修改迭代的序列,建议先复制。
>>> import math>>> raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8]>>> filtered_data = []>>> for value in raw_data:... if not math.isnan(value):... filtered_data.append(value)...>>> filtered_data[56.2, 51.7, 55.3, 52.5, 47.8]
循环
代码:
#!python# -*- coding: utf-8 -*-# Author: xurongzhong#126.com wechat:pythontesting qq:37391319# 技术支持 钉钉群:21745728(可以加钉钉pythontesting邀请加入) # qq群:144081101 591302926 567351477# CreateDate: 2018-6-12 # bowtie.py# Draw a bowtiefrom turtle import *def polygon(n, length): """Draw n-sided polygon with given side length.""" for _ in range(n): forward(length) left(360/n) def main(): """Draw polygons with 3-9 sides.""" for n in range(3, 10): polygon(n, 80) exitonclick() main()
条件循环while
代码:
#!python# -*- coding: utf-8 -*-# Author: xurongzhong#126.com wechat:pythontesting qq:37391319# 技术支持 钉钉群:21745728(可以加钉钉pythontesting邀请加入) # qq群:144081101 591302926 567351477# CreateDate: 2018-6-12 # spiral.py# Draw spiral shapesfrom turtle import *def spiral(firststep, angle, gap): """Move turtle on a spiral path.""" step = firststep while step > 0: forward(step) left(angle) step -= gapdef main(): spiral(100, 71, 2) exitonclick()main()
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