Daily Coding Problem – cons(a, b) constructs a pair, and car(pair) and cdr(pair) returns the first and last element of that pair.

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Problem

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.

Context

So what is car and cdr anyway? You can check out some more details about car and cdr in Wikipedia.

A cons cell is composed of two pointers; the car operation extracts the first pointer, and the cdr operation extracts the second.

Solution

For the solution for this problem, I reference the code from this github.

Code Example

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

def car(cons):
    return cons(lambda a, b: a)

def cdr(cons):
    return cons(lambda a, b: b)

print(car(cons(3, 4)))
print(cdr(cons(3, 4)))

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