Daily Coding Problem: Problem #5

1 min readJan 13, 2021


Photo by Oskar Yildiz on Unsplash


This problem was asked by Jane Street.

cons(a, b) constructs a pair, and car(pair) and cdr(pair) returns the first and last element of that pair. For example, car(cons(3, 4)) returns 3, and cdr(cons(3, 4)) returns 4.

Given this implementation of cons:

def cons(a, b):
def pair(f):
return f(a, b)
return pair

Implement car and cdr


The cons function returns another function which takes a and b as parameters. The returned function pair takes a function and invokes it with a and b as parameters. So inorder for car and cdr to work as intended, these functions need to take a function as a parameter. Given a pair, car must return the first element and cdr must return the second element. So car and cdr must define a function that will be used by the pair function.

def car(f):  # f is the pair function returned by function cons
return f(lambda a, b: a)

def cdr(f):
return f(lambda a, b: b)
assert car(cons(3, 4)) == 3
assert cdr(cons(3, 4)) == 4

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Software Engineer @ Google