exercises.org

About

I came up with these exercises for someone learning to code. But I thought more people might want to do them.

I like functional programming, so the exercises asks you to make functions that are common in functional programming. If you have learned a language, but want to learn more about functional programming, these exercises are for you.

The exercises were originally meant for Python, but doing them in JavaScript, Ruby or any Lisp (Scheme, Clojure, …) should work just as well. It should also be possible to do them in Java and C#, but it will probably not be as easy.

Most of the functions you are asked to create already exist in functional languages, or libraries for most languages. But it can be educational to implement them yourself.

The exercises are just very roughly in order of difficulty. If you get stuck on one, try the next.

Tips/Notes

A “map” is the same as a dictionary or hash table, i.e. a data structure that maps keys to values.

When creating the functions below, think about what happens in edge cases, such as giving zip lists of different lengths. Decide what should happen in these cases!

No function should modify its arguments (as is possible with lists and maps in Python). Always leave the arguments the way they were and create new stuff for the output.

If you find f(arg1)(arg2) confusing, it just means that f(arg1) returns a function that we immediately call with the argument arg2.

If you use Python 3, you might need to wrap your test calls in the list function to get back a list and not an iterator, so you can more easily see the results.

Exercises

Learn about recursion

Recursion is essential for functional programming. Learn about it!

Below are a few pratice problems.

Imagine you have a tree of nested lists, like this:

[1, [[2], 3], [4], 5, [6, 100, [[7], [[8]], 9]], 10]

Write functions to recursively…

• calculate the sum of all the numbers in a tree,
• calculate the depth of a tree (in the tree above, 8 is the deepest element),
• find the largest value in a tree.

You can also try the Towers of Hanoi problem. There are three rods, A, B and C. On rod A there are a stack of 6 disks (with a hole in the middle) of different sizes, sorted by size, so that the biggest disk is on the bottom. Your job is to print out instructions for how to move the stack of disks from rod A to rod C. You can only move one disk at a time, the topmost disk on a rod, and at no point can a bigger disk be on top of a smaller disk. You can of course place disks on rod B temporarily.

So, for example, you can output:

• Move disk from A to B
• Move disk from A to C
• Move disk from B to C
• etc.

It is assumed that you move the only disk you can move, the topmost disk. These instructions would mean that you first take the smallest disk from A to B. Then the next smallest from A to C. Then put the smallest, from B, on top of the next smallest, at C. Print out the instructions needed to move the whole stack of 6 disks from A to C.

Learn about closures

Closures (a kind of function) are also essential. Learn about them!

Learn about map, reduce, filter, apply and anonymous functions

Learn what the functions map, reduce, filter and apply do and about anonymous functions in your language. They are all very useful, and will be helpful in the rest of the exercises.

Note that in JavaScript and Python the ...args and *args syntax respectively can be used instead of apply.

clist

Create a function clist (for create list) that takes an arbitrary number of arguments and creates a list of the arguments given. clist is useful for giving as an argument to other functions.

If you use a Lisp, it already exists in the form of list, and you can skip this exercise.

clist(1, 2, 3) = [1, 2, 3]

add, sub

Create a function add that takes an arbitrary number of arguments, and adds them all. Also create a function sub that subtracts all the arguments but the first from the first. These functions are useful to have, since + and - are not functions that can be passed to other functions. Use reduce to implement them!

You can also make sub negate the argument when it is called with a single argument. Then sub can act as negate.

If you use a Lisp, they already exists in the form of + and -, and you can skip this exercise.

add(1, 2, 3) => 6
sub(5, 1, 2) => 2

compose

Create a function compose that takes 2 functions and does function composition, i.e. compose(double, negate) should return a function that first calls negate and then double on its argument.

compose(double, negate)(3) => -6

compose with var args

Create a compose function again that can take an arbitrary number of arguments, and that returns a function that can take an arbitrary number of arguments.

compose(negate, double, add)(1, 2, 3) => -12
compose(clist, double, sub)(1, 2, 3) => [-8]

zip

Create a function zip that takes an arbitrary number of sequneces, and zips them, i.e. creates a list of lists, where the inner lists consist of the first elements from the given sequences, then the second elements from the given sequences, and so on.

zip([1, 2, 3], [4, 5, 6]) => [[1, 4], [2, 5], [3, 6]]
zip([1, 2, 3], [4, 5, 6], [7, 8, 9]) => [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

zipmap

Create a function zipmap that takes a two seqences, and creates a dictionary from the elements of the first seqence to the elements of the second.

zipmap([1, 2, 3], [4, 5, 6]) => {1: 4, 2: 5, 3: 6}

zipwith

Create a function zipwith that takes a function f and an arbitrary number of sequences, and returns a list of f applied to the first elements of the given seqences, followed by f applied to the second elements of the sequences, an so on.

In some languages, map can act like this. If it does in your language, you can skip this exercise.

zipwith(add, [1, 2, 3], [4, 5, 6]) => [5, 7, 9]
zipwith(add, [1, 2, 3], [4, 5, 6], [1, 1, 1]) => [6, 8, 10]

car and cdr

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.

You are given this implementation of cons:

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

Implement car and cdr.

partial

Create a function partial that takes a function f and an arbitrary number of arguments, and returns a new function that is f partially applied to the arguments.

partial(add, 1, 2)(3, 4) => 10
partial(clist, 1, 2)(3, 4) => [1, 2, 3, 4]
partial(sub, 10)(1, 2) => 7

transpose

Create a function transpose that transposes matrices (lists of lists). Use the things you have learned in previous exercises.

transpose([[1, 2, 3], [4, 5, 6]])
=> [[1, 4], [2, 5], [3, 6]]

flip

Create a function flip that takes a function and flips the first and second argument to it.

flip(clist)(1, 2, 3) => [2, 1, 3]
flip(sub)(10, 1) => -9

flips

Create a function flips that takes a function and reverses the arguments to it.

flips(clist)(1, 2, 3) => [3, 2, 1]
flips(sub)(1, 2, 3) => 0

take

Create a function take that takes a number n and a sequence seq, and returns a list of the first n elements of seq.

take(3, range(10)) => [0, 1, 2]

drop

Create a function drop that takes a number n and a sequence seq, and returns a list with the first n elements of seq dropped.

drop(3, range(6)) => [3, 4, 5]

flatten

Create a function flatten that can flatten a tree.

flatten([1, [2, [3, 4], [5, 6], 7], 8, [9, 10]])
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

interleave

Create a function interleave take takes an arbitrary number of sequences and interleaves them.

interleave([1, 2, 3], [10, 20, 30])
=> [1, 10, 2, 20, 3, 30]
interleave([1, 2, 3], [10, 20, 30], "abc")
=> [1, 10, "a", 2, 20, "b", 3, 30, "c"]

every_pred

Create a function every_pred that takes an arbitrary number of predicates (functions that return a truth value) and returns a function that returns true if and only if all predicates were truthy for the argument.

every_pred(positive, even)(8) => True
every_pred(positive, even)(7) => False

frequencies

Create a function frequencies that takes a sequence and counts how many times the elements appear in the sequence, returns a map.

frequencies("aabcbcac")
=> {'a': 3, 'c': 3, 'b': 2}
frequencies([1, 2, 2, 2])
=> {1: 1, 2: 3}

partition

Create a function partition that takes the arguments n, step, and seq (number, number, seqence). It should take n elements from seq, wrap that in a list, and step forward in seq step steps, then take another n elements, and so on.

partition(3, 1, [1, 2, 3, 4, 5])
=> [[1, 2, 3], [2, 3, 4], [3, 4, 5]]

merge_with

Create a function merge_with that takes a function and an arbitrary number of maps, and merges the maps, combining elements that exist in more than 1 using the given function.

merge_with(add, {"a": 1, "b": 2}, {"b": 2})
=> {"a": 1, "b": 4}
merge_with(clist, {"a": 1, "b": 2}, {"b": 2})
=> {"a": 1, "b": [2 2]}

tree_seq

Create a function tree_seq that takes a functon is_branch, a function children and a tree t, that returns a list of the nodes of the tree, in depth-first order. is_branch should take 1 argument and return true for nodes that can have children. children should take 1 argument and return a sequence of the children for nodes that is_branch returns true for. t is the root of the tree. Look carefully at the examples below. As far as this function is concerned, a tree is not necessarily a list of lists; a tree is whatever the given functions define it to be.

Assume that is_list and is_integer below tests if the argument is a list or integer, respectively, and that identity is the identity function. range takes an integer n an returns a sequence from 0 to n-1 (this function exists in most languages).

t = [1, [2, [3, 5]]]
tree_seq(is_list, identity, t)
=> [[1, [2, [3, 5]]], 1, [2, [3, 5]], 2, [3, 5], 3, 5]
tree_seq(is_integer, range, 3)
=> [3, 0, 1, 0, 2, 0, 1, 0]

memoize

Create a function memoize take takes a function and memoizes it, i.e. it returns a function that does the same thing that the given function, but caches its results in a map.

new_add = memoize(add)
new_add(1, 2, 3) => 6
new_add(1, 2, 3) => 6 (Did not actually compute add(1, 2, 3).)

The second call of new_add should not compute using add again, but instead look up the stored result.

group_by

Create a function group_by that takes a function and a sequence and groups the elements of the sequence based on the result of the given function.

len returns the length of a sequence.

group_by(len, ["hi", "dog", "me", "bad", "good"])
=> {2: ["hi", "me"], 3: ["dog", "bad"], 4: ["good"]}

update

Create a function update that takes a map m, a key k, a function f and an arbitrary number of additional arguments args. Returns a new map, that is like m, but has the value for k replaced by the result of applying f to the previous value and the given args. If the previous value does not exist, create a new entry for k.

bob = {"name": "bob", "hp": 3}
update(bob, "hp", add, 2)
=> {"name": "bob", "hp": 5}
nohp = {"name": "bob"}
update(nohp, "hp", add, 2)
=> {"name": "bob", "hp": 2}

update_in

Create a function update_in that is similar to update, but can update a value inside nested maps.

a = {"a": 1, "b": {"c": 2, "d": {"e": 3}}}
update_in(a, ["b", "d", "e"], add, 10)
=> {"a": 1, "b": {"c": 2, "d": {"e": 13}}}
update_in(a, ["b", "d", "f"], add, 10)
=> {"a": 1, "b": {"c": 2, "d": {"e": 3, "f": 10}}}

balanced

Create a function balanced that takes a string s and returns True if it contains only balanced parens, brackets and braces ((), [], {}), otherwise False.

balanced("abc(def{g}hi[jk]((()))l)m") => True
balanced("a(b") => False
balanced("([)]") => False

postwalk, prewalk

Create two functions postwalk and prewalk, that both take a function f and a tree t. (A tree in this case is a list where the elements can be trees themselves), e.g. [1, [2, 3, [1, 2]], [4, 5, 6]]. In this case 1, [2, 3, [1, 2] and 6 are all elements of the tree (but not the only ones).) The functions should apply f to each element in the tree to create a new tree with the results, like map but for trees instead of lists. The difference between prewalk and postwalk is that prewalk does the replacement on the way “down” and postwalk on the way “up”, so that prewalk can continue down into the new value that was “just” created.

In the following example, imagine that wrap_unless_zero, if the argument is a number, wraps the argument in a list if it is larger than zero, then decrements it by 1.

postwalk(wrap_unless_zero, [1 [2 [3]]]) => [[0] [[1] [[2]]]]
prewalk(wrap_unless_zero, [1 [2 [3]]]) => [[0] [[[0]] [[[[0]]]]]]

Recreate map, filter and reduce

Implement map, filter and reduce from scratch.

@oskarkv great ideas for exercises! I'm starting a new project including interactive exercises and Data Science projects: www.datawars.io. I was wondering if you had thought of any license for these or I could just use them for my students. Thanks!