Introduction to the use of Pythont special syntax filter, map, reduce, apply

(1)lambda

lambda is a very useful syntax in Python, which allows you to quickly define a single-line minimal function. Similar to macros in C language, they can be used wherever a function is required.

The basic syntax is as follows:

Function name = lambda args1,args2,...,argsn : expression

For example:

add = lambda x,y : x + y
print add(1,2)

(2) filter

The filter function is equivalent to a filter. The function prototype is: filter(function, sequence), which means sequence sequence Each element in the sequence executes the function in turn. The function here is a bool function. For example:

sequence = [1,2,3,4,5,6,7,8,9,10]
fun = lambda x : x % 2 == 0
seq = filter(fun,sequence)
print seq

The following code means to filter out the elements in the sequence. All even numbers.

The filter function prototype is roughly as follows:

def filter(fun,seq):
    filter_seq = []
    for item in seq:
        if fun(item):
            filter_seq.append(item)
    return filter_seq

(3) map

The basic form of map is: map(function, sequence), which applies the function function to the sequence sequence and then returns a final result sequence. For example:

seq = [1,2,3,4,5,6]
fun = lambda x : x << 2

print map(fun,seq)

The function source code of map is roughly as follows:

def map(fun,seq):
    mapped_seq = []
    for item in seq:
        mapped_seq.append(fun(item))
    return mapped_seq

(4)reduce

The form of the reduce function is: reduce(function, sequence, initVal), function represents a binary function, sequence represents the sequence to be processed, and initVal represents processing initial value. For example:

seq = [1,2,3,4,5,6,7,8,9,10]
fun = lambda x,y: x + y

print reduce(fun,seq,0)

means accumulating each element in the sequence seq starting from the initial value 0, so the result is 55

def reduce(fun,seq,initVal = None):
    Lseq = list(seq)
    if initVal is None:
        res = Lseq.pop(0)
    else:
        res = initVal
    for item in Lseq:
        res = fun(seq,item)
    return res

(5) apply

def say(a,b):
    print a,b

apply(say,(234,'Hello World!'))