Lecture 9

Recursion - Approaching Complex Problems in a Different Way

Difficulty curve of the course

Writing

functions

Lists and

higher order functions

Recursive functions

Imperative programming

and object-oriented programming

Programming as com/position

• Expression composition
• Primitive expressions: Constants, variables
• Compound expressions: Function calls, special forms
• Function composition
• Primitive functions:�+,-,*,/, rectangle, overlay, etc.
• Compound functions:�Created by λ expressions
• Image composition
• Primitive images: Rectangles, ellipses
• Compound images: Overlays, iterated-overlays

Levels of abstraction

• Functions are defined in terms of simpler functions
• That are defined in terms of simpler ones
• That are defined in terms of simpler ones
• And so on

​

​

• You can never completely describe a function

computing tip

multiplication

addition

single-digit addition

counting

…

The story so far

We’ve been focusing on

• Building complicated constructions
• From simple prefab primitives

​

Algorithm design

• Now we're going to go in the other direction
• Reduce some of our “primitive” components to even simpler ones
• Recursion
• General technique for building algorithms
• Really just means “function calls”, which you already know about
• Minimalism
• Important theme in computer science
• What's the simplest, most versatile set of primitive components we can start from?

​

How do we write an iterator?

• We've used a lot of iterators
• iterated-beside, map, filter, etc.
• And we’ve defined new iterators in terms of old ones
• But it turns out our “primitive” iterators aren’t primitive

Universality

• Today, we'll show how
• To build iteration
• From non-iterative primitives
• Turing-complete system
• Function calls
• λ expressions
• If expressions

​

(Actually, Church proved you don't even need if expressions!)

Summing a list

Simple example: summing a list

• Let's start with something simpler than foldl or iterated-beside

​

• Summing a list

​

> (sum-list (list 1 2 3))

6

​

Simple list accessors

(first list), (second list), …, (eighth list)�;; first, etc. : (listof T) -> T��Extracts the specified element of list

​

(rest list)�;; rest: (listof T) -> (listof T)

�Returns all but the first element of list

Summing a 2-element list

• Let's start by assuming the list only has two elements
• Then we can just add them
• Easy!

;; sum-list-2:

;; (list number number) -> number

;; Sums a two-element list

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list)))

​

(check-expect (sum-list-2 (list 1 2))

3)

​

Summing a 3-element list

• What about a three element list?
• Same thing!

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(second a-list)

(third a-list))))

Summing a 3-element list

• But remember, we want to be lazy!
• Why rewrite the part of our program that calculates 2-element lists…while writing the program for 3-element lists?
• But we can also define sum-list-3 in terms of sum-list-2

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2 (rest

a-list)))))

Summing a 4-element list

• And we can use the same trick to sum a 4 element list
• Add the first element
• To the sum of the other three

​

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2

(rest a-list)))))

​

(define sum-list-4

(lambda (a-list)

(+ (first a-list)

(sum-list-3

(rest a-list)))))

Summing a 5-element list

• And the same again for a 5-element list
• Add the first element
• To the sum of the rest of the elements

​

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2

(rest a-list)))))

​

(define sum-list-4

(lambda (a-list)

(+ (first a-list)

(sum-list-3

(rest a-list)))))

​

(define sum-list-5

(lambda (a-list)

(+ (first a-list)

(sum-list-4

(rest a-list)))))

​

We can sum any number of elements this way

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(second a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2

(rest a-list)))))

​

(define sum-list-4

(lambda (a-list)

(+ (first a-list)

(sum-list-3

(rest a-list)))))

​

(define sum-list-5

(lambda (a-list)

(+ (first a-list)

(sum-list-4

(rest a-list)))))

(define sum-list-6

(lambda (a-list)

(+ (first a-list)

(sum-list-5

(rest a-list)))))

​

(define sum-list-7

(lambda (a-list)

(+ (first a-list)

(sum-list-6

(rest a-list)))))

​

(define sum-list-8

(lambda (a-list)

(+ (first a-list)

(sum-list-7

(rest a-list)))))

​

(define sum-list-9

(lambda (a-list)

(+ (first a-list)

(sum-list-8

(rest a-list)))))

​

Summing a list under the substitution model

A quick aside: quoting lists

• Double-quotes let you type a string as a constant in your code

​

• "this is a test string"
• Means a string
• Whose characters are t,h,i,s, space, i, s, etc.

• A single quote lets you type a list as a constant�
• '(1 2 3 4)
• Means a list
• Whose elements are: 1,2,3, & 4

​

• Note that we don't need a close quote because the parens tell Racket where the list ends

​

Note: I'm going to use quoting to simplify the following slides, it's never required or necessary to quote.

​

Warning: this works with CONSTANTS but provides strange behavior with more complex expressions.

Execution under the substitution model

(sum-list-4 (list 1 2 3 4))

• (sum-list-4 '(1 2 3 4))
• (+ (first '(1 2 3 4))

(sum-list-3 (rest '(1 2 3 4))))

• (+ 1

(sum-list-3 '(2 3 4)))

• (+ 1

(+ (first '(2 3 4))

(sum-list-2 (rest '(2 3 4)))))

• (+ 1

(+ 2

(sum-list-2 '(3 4))))

​

• (+ 1

(+ 2

(+ (first '(3 4))

(sum-list-1 (rest '(3 4)))))

​

​

• (+ 1

(+ 2

(+ 3

(sum-list-1 '(4)))))

​

• (+ 1

(+ 2

(+ 3

(+ (first '(4))

(sum-list-0

(rest '(4)))))))

• (+ 1

(+ 2

(+ 3

(+ 4

(sum-list-0

'())))))

​

Execution under the substitution model

• (+ 1

(+ 2

(+ 3

(+ 4

0))))

​

• (+ 1

(+ 2

(+ 3

4)))

​

• (+ 1

(+ 2

7))

​

• (+ 1

9)

​

• 10

But this is still a dumb way to write it

• Have to write a separate function for each length
• That's a lot of typing…
• Might not know the length of the list in advance
• So how do we know which function to call?
• Code for most the functions is nearly the same

(define sum-list-0

(lambda (a-list)

0))

​

(define sum-list-1

(lambda (a-list)

(+ (first a-list)

(sum-list-0 (rest a-list)))))

​

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(sum-list-1 (rest a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2 (rest a-list)))))

Recursion

Recursion

• Writing functions that call themselves
• A recursive function is just one where, in its definition, it calls itself.
• Basic idea
• Most problems can be broken down into smaller problems
• Often those subproblems are simpler or smaller versions of the original problem

General outline

• Check if it's a hard problem or an easy problem
• Hard problem?
• Break it down into simpler versions of itself
• Call myself on the simpler versions
• Easy problem?
• Just do it

Recursively�summing a list

We can sum any number of elements this way

(define sum-list-0

(lambda (a-list)

0))

​

(define sum-list-1

(lambda (a-list)

(+ (first a-list)

(sum-list-0 (rest a-list)))))

​

(define sum-list-2

(lambda (a-list)

(+ (first a-list)

(sum-list-1 (rest a-list))))

​

(define sum-list-3

(lambda (a-list)

(+ (first a-list)

(sum-list-2 (rest a-list)))))

​

(define sum-list-4

(lambda (a-list)

(+ (first a-list)

(sum-list-3 (rest a-list)))))

​

(define sum-list-5

(lambda (a-list)

(+ (first a-list)

(sum-list-4 (rest a-list)))))

​

(define sum-list-6

(lambda (a-list)

(+ (first a-list)

(sum-list-5 (rest a-list))))

​

(define sum-list-7

(lambda (a-list)

(+ (first a-list)

(sum-list-6 (rest a-list)))))

A better way to write it

• Just one function
• Calls itself on the rest of the list
• And adds the first element
• Unless the list is empty
• empty? returns true if the length of a list is 0

;; sum-list:

;; (listof number) -> number

;; Total of elements of a list

(define sum-list

(lambda (a-list)

(if (empty? a-list)

0

(+ (first a-list)

(sum-list (rest a-list))))))

​

(check-expect

(sum-list (list 1 2 3 4))

10)

(check-expect (sum-list (list)) 0)

Note: in the video, this function call was missing

Execution under the substitution model

• (sum-list (list 1 2 3 4))

​

• (sum-list '(1 2 3 4))

​

• (if (empty? '(1 2 3 4))

0

(+ (first '(1 2 3 4))

(sum-list (rest '(1 2 3 4)))))

​

• (if false

0

(+ (first '(1 2 3 4))

(sum-list (rest '(1 2 3 4)))))

​

• (+ (first '(1 2 3 4))

(sum-list (rest '(1 2 3 4)))

​

(+ 1

(sum-list '(2 3 4)))

​

• (+ 1

(if (empty? '(2 3 4))

0

(+ (first '(2 3 4))

(sum-list (rest '(2 3 4))))))

​

• (+ 1

(if false

0

(+ (first '(2 3 4))

(sum-list (rest ''(2 3 4))))))

​

• (+ 1

(+ (first '(2 3 4))

(sum-list (rest '(2 3 4)))))

​

• (+ 1

(+ 2

(sum-list '(3 4))))

​

Execution under the substitution model

• (+ 1

(+ 2

(sum-list '(3 4))))

​

• (+ 1

(+ 2

(if (empty? '(3 4))

0

(+ (first '(3 4))

(sum-list (rest '(3 4)))))))

​

• (+ 1

(+ 2

(if false

0

(+ (first '(3 4))

(sum-list (rest '(3 4)))))))

​

• (+ 1

(+ 2

(+ (first '(3 4))

(sum-list (rest '(3 4))))))

​

• (+ 1

(+ 2

(+ 3

(sum-list '(4)))))

​

• (+ 1

(+ 2

(+ 3

(if (empty? '(4))

0

(+ (first '(4))

(sum-list

(rest '(4))))))))

​

Execution under the substitution model

• (+ 1

(+ 2

(+ 3

(+ 4

(sum-list '())))))

​

• (+ 1

(+ 2

(+ 3

(+ 4

(if (empty? '())

0

(+ (first '())

(sum-list

(rest '()))))))))

​

• (+ 1

(+ 2

(+ 3

(if false

0

(+ (first '(4))

(sum-list

(rest '(4))))))))

​

• (+ 1

(+ 2

(+ 3

(+ (first '(4))

(sum-list

(rest '(4)))))))

​

Execution under the substitution model

• (+ 1

(+ 2

(+ 3

(+ 4

(if true

0

(+ (first '())

(sum-list

(rest '()))))))))

​

• (+ 1

(+ 2

(+ 3

(+ 4

0))))

​

• (+ 1

(+ 2

(+ 3

(+ 4

0))))

​

• (+ 1

(+ 2

(+ 3

4)))

​

• (+ 1

(+ 2

7))

​

• (+ 1

9)

​

• 10

Basic template�For recursion

Basic template for recursion

• Recursion is about
• Solving a complicated problem
• by solving a simpler version of the problem

​

• But you have to stop sometime
• Stop when you get to “easy” problem

​

• You need to write code for
• Recognizing easy cases
• Solving easy (base) cases
• Simplifying the problem (get one step closer to the base/easy cases)
• Fixing the simplified solution into a solution to the full problem

​

(define function

(lambda (args)� (if easy-case?� solve-easy-case� (fix-solution� (function simplified))))

Basic template for recursion

• You often simplify the problem by splitting it
• At least one of them will be a recursive call
• Then the fixup step consists of combining the answers to the two problems

(define function

(lambda (args …) � (if easy-case?� solve-easy-case� (combine solve-one-part

(function

other-part)))))

Look familiar?

• Just one function
• Calls itself on the rest of the list
• And adds the first element
• Unless the list is empty
• empty? returns true if the length of a list is 0

;; sum-list:

;; (listof number) -> number

;; Total of elements of a list

(define sum-list

(lambda (sum-list)

(if (empty? list)

0

(+ (first list)

(sum-list (rest list))))))

​

(check-expect

(sum-list (list 1 2 3 4))

10)

(check-expect (sum-list (list)) 0)

​

Correction! (missing function call)

Writing foldl/r

Using recursion

How do we write foldr?

• sum-list just adds
• How do we make something more general like foldl/r?

(define sum-list

(lambda (a-list)

(if (empty? a-list)

0

(+ (first a-list)

(sum-list (rest a-list))))))

​

;; foldr: (X X -> X) X (listof X) -> X

;; Aggregates elements of list

;; using proc.

(define foldr

(lambda (func start list)

???))

​

How do we write foldr?

• This is the same pattern as sum-list!
• Foldr is just further abstracted
• Change + to func
• Change 0 to start value
• We’ll show how to write foldl next time

(define sum-list

(lambda (a-list)

(if (empty? a-list)

0

(+ (first a-list)

(sum-list (rest a-list))))))

​

;; foldr: (X X -> X) X (listof X) -> X

;; Aggregates elements of list

;; using func.

(define foldr

(lambda (func start list)

(if (empty? list)

start

(func (first list)

(foldr func

start

(rest list))))))

​

Writing length

Using recursion

Example

• Write the length function using empty?, +, and rest

​

;; length: (listof any) -> number�;; Returns number of elements in list

Example

• Write the length function using empty?, +, and rest

​

;; length: (listof any) -> number�;; Returns number of elements in list

(define length�(λ (list) � (if (empty? list)� 0� (+ 1� (length (rest list))))))

​

(check-expect (length (list 1 2 3))� 3)

Execution in the substitution model

• (length (list 1 2 3))

​

• (if (empty? '(1 2 3))� 0� (+ 1 (length (rest '(1 2 3)))))
• (if false� 0� (+ 1 (length (rest '(1 2 3)))))
• (+ 1 (length (rest '(1 2 3))))
• (+ 1 (length '(2 3)))
• (+ 1 (if (empty? '(2 3))� 0� (+ 1 (length (rest '(2 3))))))
• (+ 1 (if false� 0� (+ 1 (length (rest '(2 3))))))
• (+ 1 (+ 1 (length (rest '(2 3))))))
• (+ 1 (+ 1 (length '(3))))

• (+ 1 (+ 1 (if (empty? '(3))� 0� (+ 1(length(rest '(3)))))))
• (+ 1 (+ 1 (if false� 0� (+ 1 (length (rest '(3)))))))
• (+ 1 (+ 1 (+ 1 (length (rest '(3))))))
• (+ 1 (+ 1 (+ 1 (length '()))))
• (+ 1 (+ 1 (+ 1 (if (empty? '())� 0� (+ 1 (length

(rest '())))))))

• (+ 1 (+ 1 (+ 1 (if true� 0� (+ 1 (length

(rest '())))))))

• (+ 1 (+ 1 (+ 1 0)))
• (+ 1 (+ 1 1))
• (+ 1 2)
• 3

Sussman-form definitions

This is a good time to mention Sussman-form definitions

(define (function-name inputs …)�output)

​

• Used extensively in the books
• Sussman form is a more "readable shorthand" for:� (define function-name� (λ (inputs …) output))

​

• Makes the part after define look like a call to the function so just beware…
• It means exactly the same thing
• The computer processes it by first translating it into the long form, and then executing it.

Writing iterated-beside

Using recursion

Example

• Write iterated-beside as a recursion (without using apply)

;; iterated-beside: (number -> picture) number -> picture

;; Makes a horizontal series of pictures using�;; generator function

Example

• Easy case:
• When count is zero, make a blank picture

​

(define (iterated-beside func count)�(if (= count 0)� empty-image ; this is a magic var with a blank image

… to be filled in …))

Example

• Recursive case:
• Make one picture

​

(define (iterated-beside func count)�(if (= count 0)� empty-image� (func count)))

Example

• Recursive case:
• Make the rest of the pictures

​

(define (iterated-beside func count)�(if (= count 0)� empty-image

(combine (func count)� (iterated-beside func (- count 1)))))

Example

• Recursive case:
• Make the rest of the pictures
• And combine them

​

(define (iterated-beside func count)�(if (= count 0)� empty-image

(beside (func (- count 1)) ; we need to subtract 1 here� (iterated-beside func (- count 1)))))

Execution is recursive

Example: running code

• Executing ISL code is a recursive process too
• Primitive expressions are easy:
• Constants are their own values
• Variables you look up in the dictionary
• function calls involve recursively executing all the subexpressions

Is it a primitive expression?

• Yes (simple case)
• Look up its value
• No (recursive case)
• Find the values of all its subexpressions
• Call
• The value of the first subexpression
• With the values of the other subexpressions as inputs

Some code that almost works

(define interpreter

(λ (expression)

(if (number? expression)

expression

(if (name? expression)

(look-it-up expression)

(local [(define values

(map interpreter expression))]

(apply (first values)

(rest values)))))))

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