Python Advanced Tutorial-map/reduce

Map () and reduce () in Python

Python has the map () and reduce () functions built into it.

Map ()

The map () function receives two parameters, one is a function, the other is a sequence, and map passes the incoming function to each element of the sequence sequentially, returning the result as a new list.

For example, there is a function f (x) =x^2, in order to function on a list[1,2,3,4,5,6,7,8,9], you can use map () to achieve:

def f (x):     return x*x for in range (1,10)] [1, 4, 9, 16, 25, 36, 49, 64, 81]

Map () The first parameter passed in is F, which is the function object itself.

Map () is a high-order function, in fact it abstracts the arithmetic rules, so you can compute any complex function, such as converting all the numbers of a list to a string:

>>> Map (str,[x forXinchRange (1,10)])['1','2','3','4','5','6','7','8','9']

Reduce ()

Reduce puts a function in a sequence [x1,x2,x3, .....] , the function must accept two parameters, and reduce will accumulate the result and the next element of the sequence, the effect is:

Reduce (f, [X1, x2, X3, x4]) = f (f (f (x1, x2), x3), x4)

For example, to sum a sequence, it can be implemented with reduce:

def Add (x, y    ): return x+y for in range (1,101)])

Of course, the sum operation can be built directly into the Python function sum (), no need to use reduce.

But if you convert the sequence [1,3,5,7,9] into an integer 13579,reduce it will come in handy:

def f (x, y    ): return x*10+yforif x%2!=0])13579

This example is not very useful in itself, but if you consider that the string str is also a sequence, make a slight change to the above example, with map (), which is the function that can convert str to int:

>>>deffn (x, y):returnX*10 +y>>>defChar2num (s):return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9': 9}[s]>>> Reduce (Fn,map (Char2num,'13579'))13579

The functions that are organized into a str2int are:

>>>defStr2Int (s):deffn (x, y):returnX*10 +y>>>defStr2Int (s):deffn (x, y):returnX*10 +ydefChar2num (s):return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9': 9}[s]returnReduce (Fn,map (char2num,s))>>> Str2Int ('1234567')1234567

You can also use lambda functions to further simplify:

>>>defChar2num (s):return{'0': 0,'1': 1,'2': 2,'3': 3,'4': 4,'5': 5,'6': 6,'7': 7,'8': 8,'9'8 {}[s]>>>defStr2Int (s):returnReduceLambdax,y:x*10+y, Map (char2num,s))>>> Str2Int ('13579')13579

Python Advanced Tutorial-map/reduce