Lesson 3 Exercise 2 Solution

Part 1:

import operator 

from functools import reduce 

l = [True, False, True] 

r = reduce(operator.and_, l, True) 

print(r)  #  output will be False in this case 

To check whether or not at least one element is True, the call has to be changed to:

r = reduce(operator.or_, l, False) 

Part 2:

import operator 

from functools import reduce 

l = [-4, 2, 1, -6 ] 

r = reduce(operator.and_, map(lambda n: n > 0, l), True) 

print(r)   # will print False in this case 

We use map(…) with a lambda expression for checking whether or not an individual element from the list is >0. Then we apply the reduce(…) version from part 1 to the resulting list of Boolean values we get from map(…) to check whether or not all elements are True.

Part 3:

import operator

l = [True, False, True] 

def myReduce(f, l, i): 
	intermediateResult = i 
	for element in l: 
		intermediateResult = f(intermediateResult, element) 
	return intermediateResult 

r = myReduce(operator.and_, l, True) 
print(r)  #  output will be False in this case

Maybe you were expecting that an implementation of reduce would be more complicated, but it’s actually quite simple. We set up a variable to always contain the intermediate result while working through the elements in the list and initialize it with the initial value provided in the third parameter i. When looping through the elements, we always apply the function given in parameter f to the intermediate result and the element itself and update the intermediate result variable with the result of this operation. At the end, we return the value of this variable as the result of the entire reduce operation.