In python reduce / map / filter  The three functions are easy to confuse. Although using functions to operate on elements in iterators or sequences, the applicable scenarios are different;

1, map function

Features of map function: perform the same operation on each element in the iterator or sequence (for example, + 1 for each element, etc.), and return the iterator or list. The example is as follows:

Output results:

x=1 x*x=1
x=2 x*x=4
x=3 x*x=9
x=4 x*x=16
x=5 x*x=25
[1, 4, 9, 16, 25]
************************************************************
[1, 4, 9, 16, 25]

It is worth noting that the return value of the map function is an iterator. Note that the returned result can only be iterated once. If it needs to be used multiple times, please save and process the result in advance, for example:

def func1(x):
    # Square each element
    # print("x=%d x*x=%d"%(x,x*x))
    return x*x
 
if __name__ == "__main__":
    list1 = [1,2,3,4,5]
    value = map(func1,list1) #Returns the map object, which can be forced to list
    print(list(value))
    print(list(value))

Output results:

[1, 4, 9, 16, 25]
[]

It's just silly, isn't it? Obviously, there is no mistake. Why is the second output an empty list? Because the iterator returned by the map function can only be iterated once, the solution is: when getting the result, the iterator is forced to List list The example is as follows:

Output:

[1, 4, 9, 16, 25]
[1, 4, 9, 16, 25]

2, reduce function

Features of reduce function: apply a function with two parameters cumulatively to the items of a sequence from left to right, so as to merge the sequence into a single value (such as accumulation or cumulative multiplication list elements, etc.), and return the final calculation result, which is a value. The example is as follows:

#Python 3 needs to import modules when using the reduce function
 
from functools import reduce # Import module
 
def func1(x,y):
    # Take the result of the last calculation as the input of the next calculation
    print("x=%d y=%d x*y=%d"%(x,y,x*y))
    return x*y
 
if __name__ == "__main__":
    list1 = [1,2,3,4,5]
    #Method 1:
    value = reduce(func1,list1) #equivalence 1*2*3*4*5 = 120
    print(value)
    print(type(value))
 
    print("***"*20)
    #Method two:
    value = reduce(lambda x,y:x*y, list1)  # equivalence 1*2*3*4*5 = 120
    print(value)
    print(type(value))

Output results:

x=1 y=2 x*y=2
x=2 y=3 x*y=6
x=6 y=4 x*y=24
x=24 y=5 x*y=120
120
<class 'int'>
************************************************************
120
<class 'int'>

3, filter function

Features of filter function: filter elements in iteratable objects according to specific conditions (such as filtering all even numbers in the list, etc.), as follows:

lis=[0,1,2,3,4,5,6]
 
#Define common functions for filtering even numbers
def func4(x):
    return x%2==0
 
#First use filter How to function---lambda
res5=filter(lambda x:x%2==0,lis)
print(list(res5))
print(list(res5))
print("***"*20)
 
 
#Second use filter How to function---Ordinary function two
res7=filter(func4,lis)
print(list(res7))
print(list(res7))

Output results:

[0, 2, 4, 6]
[]
************************************************************
[0, 2, 4, 6]
[]

Stupid? It has been proved that the result returned by the filter function is the same as that of the map function, which can only be iterated once. The solution is the same as that of the map solution, and the result is strongly converted to List list Yes.

Guess you like:

    1.python thread mutex Lock

    2.python process mutex Lock

    3.python GIL lock

    4. Difference between Python Gil Lock and mutex Lock

Please indicate: Ape says Python »python reduce/map/filter function differences