Python:map () and reduce ()

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

For example, we have a function f (x) =x2, in order to function on a list, you [1, 2, 3, 4, 5, 6, 7, 8, 9] can use the implementation of the map() following:

def f (x):     return x*xmap (f,[1,2,3,4,5,6,7,8,9]

Result

[1, 4, 9, 16, 25, 36, 49, 64, 81]

Reduce functions a function in a sequence [X1, x2, x3 ...] , the function must receive two parameters, and reduce calculates the result and the next element of the sequence, and the effect is:

reduce(f, [x1, x2, x3, x4]) = f(f(f(x1, x2), x3), x4)

Practice:

Exercise 1 uses the map() function to change the non-canonical English name entered by the user into the first capitalization, and the other lowercase canonical names. Input: [‘adam‘, ‘LISA‘, ‘barT‘] , Output: [‘Adam‘, ‘Lisa‘, ‘Bart‘] .

For:

def firsttoupper (x):     = X[:1].upper ()    = x[1:].lower ()    = s1 + s2    return S3
Map (firsttoupper,[' AleN ', ' TOM ', ' hello ')

Explanation: X[:1]: The first character of the x string (starting from 0 to 1 subscript but not 1 subscript); X[1:]: x string of the second word selector until finally, that is, from subscript 1 to the end ...

Results:

[' Alen ', ' Tom ', ' Hello ']

Exercise 2 Python provides sum() functions that accept a list and sum, write a prod() function that accepts a list and takes advantage of the quadrature reduce() .

def prod (x, y    ): return x+yreduce (prod,[1,2,3,4,5,6,7,8100])

Results:

8128

Python:map () and reduce ()