python中lambda表達式的用法-排序、複合函數

對於一個函數，只有一句話表示，那麼就可以用lambda表達式表示，如：

def f(x):
	return x * x
print(f(5))

out: 25

可以寫爲：

f = lambda x: x*x # 冒號左邊爲輸入，右邊是返回值，f是函數名
print(f(5))

out: 25

對於多個形式參數：

g = lambda x,y: x+y # 冒號左邊爲輸入，右邊是返回值，f是函數名
print(g(4,5))

out: 9

lambda用到比較多的地方是排序，如：

def get_four(my):
	return my[2]
tuple_my = []
file = open("file.csv", "r")
for line in file:
	Line = line.strip()
	arr = line.split(",")
	one = arr[1]
	three = arr[3]
	four = int(arr[4])
	tuple_my.append( (one, three, four) )
tuple_my.sort(key=get_four)
for my in tuple_my:
	print(my)

可以寫爲：

get_four = lambda my: my[2]
tuple_my = []
file = open("file.csv", "r")
for line in file:
	Line = line.strip()
	arr = line.split(",")
	one = arr[1]
	three = arr[3]
	four = int(arr[4])
	tuple_my.append( (one, three, four) )
tuple_my.sort(key=get_four)
for my in tuple_my:
	print(my)

tuple_my = []
file = open("file.csv", "r")
for line in file:
	Line = line.strip()
	arr = line.split(",")
	one = arr[1]
	three = arr[3]
	four = int(arr[4])
	tuple_my.append( (one, three, four) )
tuple_my.sort(key=lambda my: my[2])
for my in tuple_my:
	print(my)

lambda也經常用在複合函數下，如：

def quadratic(a, b, c):
	return lambda x: a*x*x*x + b*x*x + c*x
f = quadratic(3, -2, 4)
print(f(5))

345

def quadratic(a, b, c):
	return lambda x: a*x*x*x + b*x*x + c*x
print(quadratic(3, -2, 4)(5))

345