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73 Quick introduction to Calculus with Julia
The Julia
programming language with a design that makes it well suited as a supplement for the learning of calculus, as this collection of notes is intended to illustrate.
As Julia
is open source, it can be downloaded and used like many other programming languages.
Julia can be used through the internet for free using the mybinder.org service. This link: launch binder will take you to website that allows this. Just click on the CalcululsWithJulia.ipynb
file after launching Binder by clicking on the badge. Binder provides the Jupyter interface.
Here are some Julia
usages to create calculus objects.
The Julia
packages loaded below are all loaded when the CalculusWithJulia
package is loaded.
A Julia
package is loaded with the using
command:
using LinearAlgebra
The LinearAlgebra
package comes with a Julia
installation. Other packages can be added. Something like:
using Pkg
Pkg.add("SomePackageName")
These notes have an accompanying package, CalculusWithJulia
, that when installed, as above, also installs most of the necessary packages to perform the examples.
Packages need only be installed once, but they must be loaded into each session for which they will be used.
using CalculusWithJulia
Packages can also be loaded through import PackageName
. Importing does not add the exported objects of a function into the namespace, so is used when there are possible name collisions.
73.1 Types
Objects in Julia
are “typed.” Common numeric types are Float64
, Int64
for floating point numbers and integers. Less used here are types like Rational{Int64}
, specifying rational numbers with a numerator and denominator as Int64
; or Complex{Float64}
, specifying a complex number with floating point components. Julia also has BigFloat
and BigInt
for arbitrary precision types. Typically, operations use “promotion” to ensure the combination of types is appropriate. Other useful types are Function
, an abstract type describing functions; Bool
for true and false values; Sym
for symbolic values (through SymPy
); and Vector{Float64}
for vectors with floating point components.
For the most part the type will not be so important, but it is useful to know that for some function calls the type of the argument will decide what method ultimately gets called. (This allows symbolic types to interact with Julia functions in an idiomatic manner.)
73.2 Functions
73.2.1 Definition
Functions can be defined four basic ways:
- one statement functions follow traditional mathematics notation:
f(x) = exp(x) * 2x
f (generic function with 1 method)
- multi-statement functions are defined with the
function
keyword. Theend
statement ends the definition. The last evaluated command is returned. There is no need for explicitreturn
statement, though it can be useful for control flow.
function g(x)
= sin(x)^2
a + a^2 + a^3
a end
g (generic function with 1 method)
- Anonymous functions, useful for example, as arguments to other functions or as return values, are defined using an arrow,
->
, as follows:
= x -> sin(2x)
fn fn(pi/2)
1.2246467991473532e-16
In the following, the defined function, Derivative
, returns an anonymously defined function that uses a Julia
package, loaded with CalculusWithJulia
, to take a derivative:
Derivatve(f::Function) = x -> ForwardDiff.derivative(f, x) # ForwardDiff is loaded in CalculusWithJulia
Derivatve (generic function with 1 method)
(The D
function of CalculusWithJulia
implements something similar.)
- Anonymous function may also be created using the
function
keyword.
For mathematical functions \(f: R^n \rightarrow R^m\) when \(n\) or \(m\) is bigger than 1 we have:
- When \(n =1\) and \(m > 1\) we use a “vector” for the return value
r(t) = [sin(t), cos(t), t]
r (generic function with 1 method)
(An alternative would be to create a vector of functions.)
- When \(n > 1\) and \(m=1\) we use multiple arguments or pass the arguments in a container. This pattern is common, as it allows both calling styles.
f(x, y, z) = x*y + y*z + z*x
f(v) = f(v...)
f (generic function with 2 methods)
Some functions need to pass in a container of values, for this the last definition is useful to expand the values. Splatting takes a container and treats the values like individual arguments.
Alternatively, indexing can be used directly, as in:
f(x) = x[1]*x[2] + x[2]*x[3] + x[3]*x[1]
f (generic function with 2 methods)
- For vector fields (\(n,m > 1\)) a combination is used:
F(x,y,z) = [-y, x, z]
F(v) = F(v...)
F (generic function with 2 methods)
73.2.2 Calling a function
Functions are called using parentheses to group the arguments.
f(t) = sin(t)*sqrt(t)
sin(1), sqrt(1), f(1)
(0.8414709848078965, 1.0, 0.8414709848078965)
When a function has multiple arguments, yet the value passed in is a container holding the arguments, splatting is used to expand the arguments, as is done in the definition F(v) = F(v...)
, above.
73.2.3 Multiple dispatch
Julia
can have many methods for a single generic function. (E.g., it can have many different implementations of addition when the +
sign is encountered.) The types of the arguments and the number of arguments are used for dispatch.
Here the number of arguments is used:
Area(w, h) = w * h # area of rectangle
Area(w) = Area(w, w) # area of square using area of rectangle definition
Area (generic function with 2 methods)
Calling Area(5)
will call Area(5,5)
which will return 5*5
.
Similarly, the definition for a vector field:
F(x,y,z) = [-y, x, z]
F(v) = F(v...)
F (generic function with 2 methods)
takes advantage of multiple dispatch to allow either a vector argument or individual arguments.
Type parameters can be used to restrict the type of arguments that are permitted. The Derivative(f::Function)
definition illustrates how the Derivative
function, defined above, is restricted to Function
objects.
73.2.4 Keyword arguments
Optional arguments may be specified with keywords, when the function is defined to use them. Keywords are separated from positional arguments using a semicolon, ;
:
circle(x; r=1) = sqrt(r^2 - x^2)
circle(0.5), circle(0.5, r=10)
(0.8660254037844386, 9.987492177719089)
The main (but not sole) use of keyword arguments will be with plotting, where various plot attribute are passed as key=value
pairs.
73.3 Symbolic objects
The add-on SymPy
package allows for symbolic expressions to be used. Symbolic values are defined with @syms
, as below.
using SymPy
@syms x y z
^2 + y^3 + z x
\(x^{2} + y^{3} + z\)
Assumptions on the variables can be useful, particularly with simplification, as in
@syms x::real y::integer z::positive
(x, y, z)
Symbolic expressions flow through Julia
functions symbolically
sin(x)^2 + cos(x)^2
\(\sin^{2}{\left(x \right)} + \cos^{2}{\left(x \right)}\)
Numbers are symbolic once SymPy
interacts with them:
- x + 1 # 1 is now symbolic x
\(1\)
The number PI
is a symbolic pi
.
sin(PI), sin(pi)
(0, 0.0)
Use Sym
to create symbolic numbers, N
to find a Julia
number from a symbolic number:
1 / Sym(2)
\(\frac{1}{2}\)
N(PI)
π = 3.1415926535897...
Many generic Julia
functions will work with symbolic objects through multiple dispatch (e.g., sin
, cos
, …). Sympy functions that are not in Julia
can be accessed through the sympy
object using dot-call notation:
harmonic(10) sympy.
\(\frac{7381}{2520}\)
Some Sympy methods belong to the object and a called via the pattern object.method(...)
. This too is the case using SymPy with Julia
. For example:
= [x 1; x 2]
A det() # determinant of symbolic matrix A A.
\(x\)
73.4 Containers
We use a few different containers:
- Tuples. These are objects grouped together using parentheses. They need not be of the same type
= (1, "two", 3.0) x1
(1, "two", 3.0)
Tuples are useful for programming. For example, they are used to return multiple values from a function.
- Vectors. These are objects of the same type (typically) grouped together using square brackets, values separated by commas:
= [1, 2, 3.0] # 3.0 makes theses all floating point x2
3-element Vector{Float64}:
1.0
2.0
3.0
Unlike tuples, the expected arithmetic from Linear Algebra is implemented for vectors.
- Matrices. Like vectors, combine values of the same type, only they are 2-dimensional. Use spaces to separate values along a row; semicolons to separate rows:
= [1 2 3; 4 5 6; 7 8 9] x3
3×3 Matrix{Int64}:
1 2 3
4 5 6
7 8 9
- Row vectors. A vector is 1 dimensional, though it may be identified as a column of two dimensional matrix. A row vector is a two-dimensional matrix with a single row:
= [1 2 3.0] x4
1×3 Matrix{Float64}:
1.0 2.0 3.0
These have indexing using square brackets:
1], x2[2], x3[3] x1[
(1, 2.0, 7)
Matrices are usually indexed by row and column:
1,2] # row one column two x3[
2
For vectors and matrices - but not tuples, as they are immutable - indexing can be used to change a value in the container:
1], x3[1,1] = 2, 2 x2[
(2, 2)
Vectors and matrices are arrays. As hinted above, arrays have mathematical operations, such as addition and subtraction, defined for them. Tuples do not.
Destructuring is an alternative to indexing to get at the entries in certain containers:
= x2 a,b,c
3-element Vector{Float64}:
2.0
2.0
3.0
73.4.1 Structured collections
An arithmetic progression, \(a, a+h, a+2h, ..., b\) can be produced efficiently using the range operator a:h:b
:
5:10:55 # an object that describes 5, 15, 25, 35, 45, 55
5:10:55
If h=1
it can be omitted:
1:10 # an object that describes 1,2,3,4,5,6,7,8,9,10
1:10
The range
function can efficiently describe \(n\) evenly spaced points between a
and b
:
range(0, pi, length=5) # range(a, stop=b, length=n) for version 1.0
0.0:0.7853981633974483:3.141592653589793
This is useful for creating regularly spaced values needed for certain plots.
73.5 Iteration
The for
keyword is useful for iteration, Here is a traditional for loop, as i
loops over each entry of the vector [1,2,3]
:
for i in [1,2,3]
println(i)
end
1
2
3
Technical aside: For assignment within a for loop at the global level, a global
declaration may be needed to ensure proper scoping.
List comprehensions are similar, but are useful as they perform the iteration and collect the values:
^2 for i in [1,2,3]] [i
3-element Vector{Int64}:
1
4
9
Comprehesions can also be used to make matrices
1/(i+j) for i in 1:3, j in 1:4] [
3×4 Matrix{Float64}:
0.5 0.333333 0.25 0.2
0.333333 0.25 0.2 0.166667
0.25 0.2 0.166667 0.142857
(The three rows are for i=1
, then i=2
, and finally for i=3
.)
Comprehensions apply an expression to each entry in a container through iteration. Applying a function to each entry of a container can be facilitated by:
- Broadcasting. Using
.
before an operation instructsJulia
to match up sizes (possibly extending to do so) and then apply the operation element by element:
= [1,2,3]
xs sin.(xs) # sin(1), sin(2), sin(3)
3-element Vector{Float64}:
0.8414709848078965
0.9092974268256817
0.1411200080598672
This example pairs off the value in bases
and xs
:
= [5,5,10]
bases log.(bases, xs) # log(5, 1), log(5,2), log(10, 3)
3-element Vector{Float64}:
0.0
0.43067655807339306
0.47712125471966244
This example broadcasts the scalar value for the base with xs
:
log.(5, xs)
3-element Vector{Float64}:
0.0
0.43067655807339306
0.6826061944859854
Row and column vectors can fill in:
= [4 5] # a row vector
ys g(x,y) = (x,y)
g.(xs, ys) # broadcasting a column and row vector makes a matrix, then applies f.
3×2 Matrix{Tuple{Int64, Int64}}:
(1, 4) (1, 5)
(2, 4) (2, 5)
(3, 4) (3, 5)
This should be contrasted to the case when both xs
and ys
are (column) vectors, as then they pair off (and here cause a dimension mismatch as they have different lengths):
g.(xs, [4,5])
LoadError: DimensionMismatch: arrays could not be broadcast to a common size; got a dimension with lengths 3 and 2
- The
map
function is similar, it applies a function to each element:
map(sin, [1,2,3])
3-element Vector{Float64}:
0.8414709848078965
0.9092974268256817
0.1411200080598672
Many different computer languages implement map
, broadcasting is less common. Julia
’s use of the dot syntax to indicate broadcasting is reminiscent of MATLAB, but is quite different.
73.6 Plots
The following commands use the Plots
package. The Plots
package expects a choice of backend. We will use gr
unless, but other can be substituted by calling an appropriate command, such as pyplot()
or plotly()
.
using Plots
The plotly
backend and gr
backends are available by default. The plotly
backend is has some interactivity, gr
is for static plots. The pyplot
package is used for certain surface plots, when gr
can not be used.
73.6.1 Plotting a univariate function \(f:R \rightarrow R\)
- using
plot(f, a, b)
plot(sin, 0, 2pi)
Or
f(x) = exp(-x/2pi)*sin(x)
plot(f, 0, 2pi)
Or with an anonymous function
plot(x -> sin(x) + sin(2x), 0, 2pi)
The time to first plot can be lengthy! This can be removed by creating a custom Julia
image, but that is not introductory level stuff. As well, standalone plotting packages offer quicker first plots, but the simplicity of Plots
is preferred. Subsequent plots are not so time consuming, as the initial time is spent compiling functions so their re-use is speedy.
Arguments of interest include
Attribute | Value |
---|---|
legend |
A boolean, specify false to inhibit drawing a legend |
aspect_ratio |
Use :equal to have x and y axis have same scale |
linewidth |
Integers greater than 1 will thicken lines drawn |
color |
A color may be specified by a symbol (leading : ). |
E.g., :black , :red , :blue |
- using
plot(xs, ys)
The lower level interface to plot
involves directly creating x and y values to plot:
= range(0, 2pi, length=100)
xs = sin.(xs)
ys plot(xs, ys, color=:red)
- plotting a symbolic expression
A symbolic expression of single variable can be plotted as a function is:
@syms x
plot(exp(-x/2pi)*sin(x), 0, 2pi)
- Multiple functions
The !
Julia convention to modify an object is used by the plot
command, so plot!
will add to the existing plot:
plot(sin, 0, 2pi, color=:red)
plot!(cos, 0, 2pi, color=:blue)
plot!(zero, color=:green) # no a, b then inherited from graph.
The zero
function is just 0 (more generally useful when the type of a number is important, but used here to emphasize the \(x\) axis).
73.6.2 Plotting a parameterized (space) curve function \(f:R \rightarrow R^n\), \(n = 2\) or \(3\)
- Using
plot(xs, ys)
Let \(f(t) = e^{t/2\pi} \langle \cos(t), \sin(t)\rangle\) be a parameterized function. Then the \(t\) values can be generated as follows:
= range(0, 2pi, length = 100)
ts = [exp(t/2pi) * cos(t) for t in ts]
xs = [exp(t/2pi) * sin(t) for t in ts]
ys plot(xs, ys)
- using
plot(f1, f2, a, b)
. If the two functions describing the components are available, then
f1(t) = exp(t/2pi) * cos(t)
f2(t) = exp(t/2pi) * sin(t)
plot(f1, f2, 0, 2pi)
- Using
plot_parametric
. If the curve is described as a function oft
with a vector output, then theCalculusWithJulia
package providesplot_parametric
to produce a plot:
r(t) = exp(t/2pi) * [cos(t), sin(t)]
plot_parametric(0..2pi, r)
The low-level approach doesn’t quite work as easily as desired:
= range(0, 2pi, length = 4)
ts = r.(ts) vs
4-element Vector{Vector{Float64}}:
[1.0, 0.0]
[-0.6978062125430444, 1.2086358139617603]
[-0.9738670205273388, -1.6867871593690715]
[2.718281828459045, -6.657870280805568e-16]
As seen, the values are a vector of vectors. To plot a reshaping needs to be done:
= range(0, 2pi, length = 100)
ts = r.(ts)
vs = [vs[i][1] for i in eachindex(vs)]
xs = [vs[i][2] for i in eachindex(vs)]
ys plot(xs, ys)
This approach is facilitated by the unzip
function in CalculusWithJulia
(and used internally by plot_parametric
):
= range(0, 2pi, length = 100)
ts plot(unzip(r.(ts))...)
- Plotting an arrow
An arrow in 2D can be plotted with the quiver
command. We show the arrow(p, v)
(or arrow!(p,v)
function) from the CalculusWithJulia
package, which has an easier syntax (arrow!(p, v)
, where p
is a point indicating the placement of the tail, and v
the vector to represent):
plot_parametric(0..2pi, r)
= pi/8
t0 arrow!(r(t0), r'(t0))
73.6.3 Plotting a scalar function \(f:R^2 \rightarrow R\)
The surface
and contour
functions are available to visualize a scalar function of \(2\) variables:
- A surface plot
f(x, y) = 2 - x^2 + y^2
= ys = range(-2,2, length=25)
xs surface(xs, ys, f)
The function generates the \(z\) values, this can be done by the user and then passed to the surface(xs, ys, zs)
format:
f(x, y) = 2 - x^2 + y^2
= ys = range(-2,2, length=25)
xs surface(xs, ys, f.(xs, ys'))