python lambda拉姆达表达式「建议收藏」

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>>> lambda x: x + 1
<function <lambda> at 0x000001FC51317840>

看出来，就是一个函数，也是一个表达式。 Because a lambda function is an expression, it can be named. Therefore you could write the previous code as follows:

>>> addone= lambda x:x+1
>>> addone(50)
51

>>> (lambda x:x+1)(50)
51

带2个参数的lambda expression：

>>> sum = lambda x,y :x+y
>>> sum(5,10)
15
>>> 

MAP 是什么 ，WTF is MAP？ map(function, iterable, …) Return an iterator that applies function to every item of iterable, yielding the results.意思就是把这个函数呢应用到每个iterable 的items上。 看下面的例子

def calculateSquare(n):
  return n*n

numbers = (1, 2, 3, 4)
result = map(calculateSquare, numbers)
print(result)
<map object at 0x000001FC51314DD8>

converting map object to set:

numbersSquare = set(result)
print(numbersSquare)
{16, 1, 4, 9}  从结果可见，就是把calculateSquare函数，应用到了每个numbers上。

既然是把函数应用的每个iterable上，自然会想起我们的lambda函数 :

numbers = (1, 2, 3, 4)
result = map(lambda x: x*x, numbers)
print(result)
<map object at 0x00000181A0A280F0>

# converting map object to set
numbersSquare = set(result)
print(numbersSquare)
<map 0x7fafc21ccb00>
{16, 1, 4, 9}

**

Use of lambda() with filter()

**

The filter() function in Python takes in a function and a list as arguments. This offers an elegant way to filter out all the elements of a sequence “sequence”, for which the function returns True. Here is a small program that returns the odd numbers from an input list:

>>> li = [5, 7, 22, 97, 54, 62, 77, 23, 73, 61]
>>> final_list = list(filter(lambda x: (x%2 != 0) , li))
>>> print(final_list)
[5, 7, 97, 77, 23, 73, 61]
>>> 

Use of lambda() with reduce()

The reduce() function in Python takes in a function and a list as argument. The function is called with a lambda function and a list and a new reduced result is returned. This performs a repetitive operation over the pairs of the list. This is a part of functools module. Example: 注意，reduce已经被搬到了functools https://docs.python.org/3/library/functools.html#functools.reduce

>>> from functools import reduce
>>> li = [5, 8, 10, 20, 50, 100]
>>> sum = reduce((lambda x, y: x + y), li)
>>> 
>>> sum
193
>>> 

Here the results of previous two elements are added to the next element and this goes on till the end of the list like (((((5+8)+10)+20)+50)+100).

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