Reduce,Map and Filter examples in the most simplest way

Map:

def to_gram(K):
    return (K * 1000)

def to_kilogram(G):
    return G/1000
   
grams=[5000,6000,7000]
kilograms=[1,2,3]

G=map(to_gram,kilograms)
K=map(to_gram,grams)

print "Grams ""+ G
print "Kgs" + K

Filter:
def div_by_2(x):
    return x % 2 == 0

arr = [1, 2, 3, 4]
new_arr = filter(div_by_2, arr)
new_arr will contain [2, 4]

reduce:

def add_arr(x, y):
    return x + y

arr = [1, 2, 3]

sum = reduce(add_arr, arr)