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Lesson 14 sequence

Lesson 15 Functions

Lesson 16 lambda expression filter and map

1. What is a lambda expression?

I don’t know if any friends have learned Java. Anyone who has learned Java should know what anonymous functions are. In fact, lambda expressions are equivalent to anonymous functions. In fact, it doesn’t matter if you don’t understand it. Simply put, it just simplifies the expression of a function.
It is suitable for replacing functions that are used only once.

2. Usage of lambda expression

      Let me first talk about what the return value of an expression is
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      . It is easy to understand by taking a few simple examples.
      Suppose we define a function as follows:

def fun1(x):
    return x + 2 * x
fun1(2)

You also need to call the function if you want to get the result of the function, but if you use lambda expressions, it doesn't need to be so troublesome.

g = lambda x:x + 2 * x
g(2)

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The result of both is the same. Another example:

through these two examples should be very clear its usage.

3. Exercises of lambda expressions

1: Please use lambda expression to transform the following function into an anonymous function

def fun_A(x,y=3):
	return x * y

I directly posted the answer to the operation method:
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2: Convert the anonymous function below into a normal function

lambda x: x if x % 2 else None

The answer is as follows:
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3: The effect of using lambda expressions:
       ①When writing some execution scripts in Python, using anonymous functions can save the process of defining functions and make the code simpler.
      ②For some functions that are more abstract and only need to call once or twice when the whole program is intelligent, sometimes it is a headache to give the function a function name. When using anonymous functions, you don't need to consider the naming problem.
      ③Simplify the readability of the code, because ordinary functions often jump to the beginning of the def definition part when reading, the use of anonymous functions can save such steps.

4、filter(function or None,iterator)

The filter() method returns an object.
① When the first parameter is None, convert it into a list and output as follows:

list(filter(None,[0,1,False,True]))

The result is: [1,True]
That is, if the first parameter is None, the default output is True data.
②When the first parameter is a function:

def add(x):
	return x % 2
temp = range(10)
show = filter(add,temp)
list(show)

The output result is: [1,3,5,7,9]
That is, in range(10):0-9, output x% 2 has the index of the remainder data.
Combined with the learning of lambda expressions, the above code can be improved as follows:

list(filter(lambda(x:x % 2),range(10)))

4、map(function or None,iterator)

map is a mapping, the usage is the same as filter, but the final output result is the data value, not the key value. The
same code:

list(map(lambda(x:x % 2),range(10)))

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