Python从题目学习：List comprehension

list = [1,2,3,4,5,6,7,8,9]

#打印列表中 所有元素的平方
print [x**2 for x in list] 
#[1, 4, 9, 16, 25, 36, 49, 64, 81]

#打印列表中 大于5的元素的平方 (只需要添加if语句即可)
print [x**2 for x in list if x>5] 
#[36, 49, 64, 81]

#打印列表中 所有元素和元素*10组成的元祖
print [(x,x*10) for x in list]
#[(1, 10), (2, 20), (3, 30), (4, 40), (5, 50), (6, 60), (7, 70), (8, 80), (9, 90)]

#也可以在添加多个for循环
print [(x,y) for x in range(3) for y in range(3)]
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

print [[x,y] for x in range(3) for y in range(3)]
[[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]

#当然，添加多个if循环也可以
[(x, y) for x in range(10) if x % 2 if x > 3 for y in range(10) if y > 7 if y != 8]
#[(5, 9), (7, 9), (9, 9)]

list1 = [2,4,6]
list2 = [3,6,9]
[list1[i]+list2[2] for i in range(len(list1))]
#[11, 13, 15]

>>> print "\n".join([" ".join(["%d*%d=%d"%(secondNum,firstNum,firstNum*secondNum) for secondNum in range(1,firstNum+1)]) for firstNum in range(1,10)])
#1*1=1
#1*2=2 2*2=4
#1*3=3 2*3=6 3*3=9
#1*4=4 2*4=8 3*4=12 4*4=16
#1*5=5 2*5=10 3*5=15 4*5=20 5*5=25
#1*6=6 2*6=12 3*6=18 4*6=24 5*6=30 6*6=36
#1*7=7 2*7=14 3*7=21 4*7=28 5*7=35 6*7=42 7*7=49
#1*8=8 2*8=16 3*8=24 4*8=32 5*8=40 6*8=48 7*8=56 8*8=64
#1*9=9 2*9=18 3*9=27 4*9=36 5*9=45 6*9=54 7*9=63 8*9=72 9*9=81

>>> girls = [‘alice‘,‘bernice‘,‘clarice‘]
>>> boys = [‘chris‘,‘arnold‘,‘bob‘]
>>> [b+‘+‘+g for b in boys for g in girls if b[0]==g[0]]
[‘chris+clarice‘, ‘arnold+alice‘, ‘bob+bernice‘]
#当然这样也可以，脑子不要不转圈
>>> [(b,g) for b in boys for g in girls if b[0]==g[0]]
[(‘chris‘, ‘clarice‘), (‘arnold‘, ‘alice‘), (‘bob‘, ‘bernice‘)]

>>> for girl in girls:
    letterGirls.setdefault(girl[0],[]).append(girl)
>>> [b+‘+‘+g for b in boys for g in letterGirls[b[0]]]
#[‘chris+clarice‘, ‘chris+clarice‘, ‘arnold+alice‘, ‘arnold+alice‘, ‘bob+bernice‘, ‘bob+bernice‘]

#解释一下：
#letterGirls.setdefault(girl[0],[]) 之后
>>> print letterGirls
#{‘a‘: [], ‘c‘: [], ‘b‘: []}

#letterGirls.setdefault(girl[0],[]).append(girl)之后
>>> letterGirls
{‘a‘: [‘alice‘], ‘c‘: [‘clarice‘], ‘b‘: [‘bernice‘]}

#for循环遍历字典，得到的是key值
#setdefault(key,value)，如果没有value值就更新，有的话就不更新值。