Higher-Order Functions

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Class outline:

  • Iteration example
  • Designing functions
  • Generalization
  • Higher-order functions
  • Lambda expressions
  • Conditional expressions

Iteration example

Virahaṅka-Fibonacci numbers

Discovered by Virahanka in India, 600-800 AD, later re-discovered in Western mathematics and commonly known as Fibonacci numbers.

0 1 1 2 3 5 8 13 21 34 …
+ =
+ =
+ =
+ =
+ =
+ =
+ =
+ =

Virahanka's question

How many poetic meters exist for a total duration?

S = short syllable, L = long syllable

Duration Meters Total
1 S 1
2 SS, L 2
3 SSS, SL, LS 3

The So-called Fibonacci Numbers in Ancient and Medieval India

Fibonacci's question

How many pairs of rabbits can be bred after N months?

Attribution: Fschwarzentruber, Wikipedia

Virahanka-Fibonacci number generation

                    VF 0   1   1   2   3   5   8   13   21   34   55 …
                    N  0   1   2   3   4   5   6   7    8    9    10 …

                    def vf_number(n):
                        """Compute the nth Virahanka-Fibonacci number, for N >= 1.
                        >>> vf_number(2)
                        >>> vf_number(6)
                        prev = 0  # First Fibonacci number
                        curr = 1  # Second Fibonacci number
                        k = 1
                        while k < n:
                            (prev, curr) = (curr, prev + curr)
                            k += 1
                        return curr

Golden spiral

The Golden spiral can be approximated by Virahanka-Fibonacci numbers.

Golden spiral overlaid on grid

Go bears!

The Golden spiral is found everywhere in nature...

Photo of a bear with a Golden spiral overlaid

Designing Functions

Describing Functions

                    def square(x):
                        """Returns the square of X."""
                        return x * x
Aspect Example
A function's domain is the set of all inputs it might possibly take as arguments. x is a number
A function's range is the set of output values it might possibly return. square returns a non-negative real number
A pure function's behavior is the relationship it creates between input and output. square returns the square of x

Designing a function

Give each function exactly one job, but make it apply to many related situations.

                    round(1.23)     # 1
                    round(1.23, 0)  # 1
                    round(1.23, 1)  # 1.2
                    round(1.23, 5)  # 1.23

Don't Repeat Yourself (DRY): Implement a process just once, execute it many times.


Generalizing patterns with arguments

Geometric shapes have similar area formulas.

Shape Diagram of square Diagram of circle Diagram of hexagon
Area $$\colorbox{#f8e9eb}{$1$} * r^2$$ $$\colorbox{#f8e9eb}{$\pi$} * r^2$$ $$\colorbox{#f8e9eb}{$\dfrac{3\sqrt{3}}{2}$} * r^2$$

A non-generalized approach

                    from math import pi, sqrt

                    def area_square(r):
                        return r * r

                    def area_circle(r):
                        return r * r * pi

                    def area_hexagon(r):
                        return r * r * (3 * sqrt(3) / 2)

How can we generalize the common structure?

Generalized area function

                    from math import pi, sqrt

                    def area(r, shape_constant):
                        """Return the area of a shape from length measurement R."""
                        if r < 0:
                            return 0
                        return r * r * shape_constant

                    def area_square(r):
                        return area(r, 1)

                    def area_circle(r):
                        return area(r, pi)

                    def area_hexagon(r):
                        return area(r, 3 * sqrt(3) / 2)

Higher-order functions

What are higher-order functions?

A function that either:

  • Takes another function as an argument
  • Returns a function as its result

All other functions are considered first-order functions.

Generalizing over computational processes

$$\sum\limits_{k=1}^5 \colorbox{#f8e9eb}{$k$} = 1 + 2 + 3 + 4 + 5 = 15$$ $$\sum\limits_{k=1}^5 \colorbox{#f8e9eb}{$k^3$} = 1^3 + 2^3 + 3^3 + 4^3 + 5^3 = 225$$ $$\sum\limits_{k=1}^5 \colorbox{#f8e9eb}{$\dfrac{8}{(4k - 3)\cdot(4k - 1)}$} = \dfrac{8}{3} + \dfrac{8}{35} + \dfrac{8}{99} + \dfrac{8}{195} + \dfrac{8}{323} = 3.04$$

The common structure among functions may be a computational process, not just a number.

Functions as arguments

                    def cube(k):
                        return k ** 3

                    def summation(n, term):
                        """Sum the first N terms of a sequence.
                        >>> summation(5, cube)
                        total = 0
                        k = 1
                        while k <= n:
                            total = total + term(k)
                            k = k + 1
                        return total

Functions as return values

Locally defined functions

Functions defined within other function bodies are bound to names in a local frame.

                    def make_adder(n):
                        """Return a function that takes one argument k
                           and returns k + n.
                        >>> add_three = make_adder(3)
                        >>> add_three(4)
                        def adder(k):
                            return k + n
                        return adder

Call expressions as operator expressions

make_adder(1)( 2 ) Operator Operand make_adder(1) func make_adder... 1 make_adder(n) def adder(k): return k + nreturn adder func adder(k) func adder(k) 2 3

Lambda expressions

Lambda syntax

A lambda expression is a simple function definition that evaluates to a function.

The syntax:

                    lambda <parameters>: <expression>

A function that takes in parameters and returns the result of expression.

A lambda version of the square function:

                    square = lambda x: x * x

A function that takes in parameter x and returns the result of x * x.

Lambda syntax tips

A lambda expression does not contain return statements or any statements at all.


                    square = lambda x: return x * x

                    square = lambda x: x * x

Def statements vs. Lambda expressions

                                    def square(x):
                                        return x * x

                                    square = lambda x: x * x
screenshot of PythonTutor calling a square function defined with def screenshot of PythonTutor calling a square function defined with lambda
Both create a function with the same domain, range, and behavior.
Both bind that function to the name square.
Only the def statement gives the function an intrinsic name, which shows up in environment diagrams but doesn't affect execution (unless the function is printed).

Lambda as argument

It's convenient to use a lambda expression when you are passing in a simple function as an argument to another function.

Instead of...

                    def cube(k):
                        return k ** 3

                    summation(5, cube)

We can use a lambda:

                    summation(5, lambda k: k ** 3)

Conditional expressions

Conditional expressions

A conditional expression has the form:

                    <consequent> if <predicate> else <alternative>

Evaluation rule:

  • Evaluate the <predicate> expression.
  • If it's a true value, the value of the whole expression is the value of the <consequent>.
  • Otherwise, the value of the whole expression is the value of the <alternative>.

Lambdas with conditionals

This is invalid syntax:

                    lambda x: if x > 0: x else: 0

Conditional expressions to the rescue!

                    lambda x: x if x > 0 else 0