# An Introductory Example¶

## Overview¶

We’re now ready to start learning the Julia language itself

### Level¶

Our approach is aimed at those who already have at least some knowledge of programming — perhaps experience with Python, MATLAB, R, C or similar

In particular, we assume you have some familiarity with fundamental programming concepts such as

• variables
• loops
• conditionals (if/else)

If you have no such programming experience, then one option is to try Python first

Python is a great first language and there are many introductory treatments

Otherwise, just dive in and see how you go...

### Approach¶

In this lecture we will write and then pick apart small Julia programs

At this stage the objective is to introduce you to basic syntax and data structures

Deeper concepts—how things work—will be covered in later lectures

Since we are looking for simplicity the examples are a little contrived

### Set Up¶

We assume that you’ve worked your way through our getting started lecture already

For this lecture, we recommend that you work in a Jupyter notebook, as described here

### Other References¶

The definitive reference is Julia’s own documentation

The manual is thoughtfully written but also quite dense (and somewhat evangelical)

The presentation in this and our remaining lectures is more of a tutorial style based around examples

## Example: Plotting a White Noise Process¶

To begin, let’s suppose that we want to simulate and plot the white noise process $$\epsilon_0, \epsilon_1, \ldots, \epsilon_T$$, where each draw $$\epsilon_t$$ is independent standard normal

In other words, we want to generate figures that look something like this:

This is straightforward using Plots.jl, which was discussed in our set up lecture

Fire up a Jupyter notebook and enter the following in a cell

using Plots
ts_length = 100
epsilon_values = randn(ts_length)
plot(epsilon_values, color="blue")


Let’s break this down and see how it works

### Importing Functions¶

The effect of the statement using Plots is to make all the names exported by the Plots module available in the global scope

If you prefer to be more selective you can replace using Plots with import Plots: plot

Now only the plot function is accessible

Since our program uses only the plot function from this module, either would have worked in the previous example

### Arrays¶

The function call epsilon_values = randn(ts_length) creates one of the most fundamental Julia data types: an array

julia> typeof(epsilon_values)
Array{Float64,1}

julia> epsilon_values
100-element Array{Float64,1}:
-0.908823
-0.759142
-1.42078
0.792799
0.577181
1.74219
-0.912529
1.06259
0.5766
-0.0172788
-0.591671
-1.02792
...
-1.29412
-1.12475
0.437858
-0.709243
-1.96053
1.31092
1.19819
1.54028
-0.246204
-1.23305
-1.16484


The information from typeof() tells us that epsilon_values is an array of 64 bit floating point values, of dimension 1

Julia arrays are quite flexible — they can store heterogeneous data for example

julia> x = [10, "foo", false]
3-element Array{Any,1}:
10
"foo"
false


Notice now that the data type is recorded as Any, since the array contains mixed data

The first element of x is an integer

julia> typeof(x[1])
Int64


The second is a string

julia> typeof(x[2])
ASCIIString (constructor with 2 methods)


The third is the boolean value false

julia> typeof(x[3])
Bool


Notice from the above that

• array indices start at 1 (unlike Python, where arrays are zero-based)
• array elements are referenced using square brackets (unlike MATLAB and Fortran)

Julia contains many functions for acting on arrays — we’ll review them later

For now here’s several examples, applied to the same list x = [10, "foo", false]

julia> length(x)
3

julia> pop!(x)
false

julia> x
2-element Array{Any,1}:
10
"foo"

julia> push!(x, "bar")
3-element Array{Any,1}:
10
"foo"
"bar"

julia> x
3-element Array{Any,1}:
10
"foo"
"bar"


The first example just returns the length of the list

The second, pop!(), pops the last element off the list and returns it

In doing so it changes the list (by dropping the last element)

Because of this we call pop! a mutating method

It’s conventional in Julia that mutating methods end in ! to remind the user that the function has other effects beyond just returning a value

The function push!() is similar, expect that it appends its second argument to the array

### For Loops¶

Although there’s no need in terms of what we wanted to achieve with our program, for the sake of learning syntax let’s rewrite our program to use a for loop

using Plots
ts_length = 100
epsilon_values = Array(Float64, ts_length)
for i in 1:ts_length
epsilon_values[i] = randn()
end
plot(epsilon_values, color="blue")


Here we first declared epsilon_values to be an empty array for storing 64 bit floating point numbers

The for loop then populates this array by successive calls to randn()

• Called without an argument, randn() returns a single float

Like all code blocks in Julia, the end of the for loop code block (which is just one line here) is indicated by the keyword end

The word in from the for loop can be replaced by symbol =

The expression 1:ts_length creates an iterator that is looped over — in this case the integers from 1 to ts_length

Iterators are memory efficient because the elements are generated on the fly rather than stored in memory

In Julia you can also loop directly over arrays themselves, like so

words = ["foo", "bar"]
for word in words
println("Hello \$word")
end


The output is

Hello foo
Hello bar


### While Loops¶

The syntax for the while loop contains no surprises

using Plots
ts_length = 100
epsilon_values = Array(Float64, ts_length)
i = 1
while i <= ts_length
epsilon_values[i] = randn()
i = i + 1
end
plot(epsilon_values, color="blue")


The next example does the same thing with a condition and the break statement

using Plots
ts_length = 100
epsilon_values = Array(Float64, ts_length)
i = 1
while true
epsilon_values[i] = randn()
i = i + 1
if i > ts_length
break
end
end
plot(epsilon_values, color="blue")


### User-Defined Functions¶

For the sake of the exercise, let’s now go back to the for loop but restructure our program so that generation of random variables takes place within a user-defined function

using Plots

function generate_data(n)
epsilon_values = Array(Float64, n)
for i = 1:n
epsilon_values[i] = randn()
end
return epsilon_values
end

ts_length = 100
data = generate_data(ts_length)
plot(data, color="blue")


Here

• function is a Julia keyword that indicates the start of a function definition
• generate_data is an arbitrary name for the function
• return is a keyword indicating the return value

#### A Slightly More Useful Function¶

Of course the function generate_data is completely contrived

We could just write the following and be done

ts_length = 100
data = randn(ts_length)
plot(data, color="blue")


Let’s make a slightly more useful function

This function will be passed a choice of probability distribution and respond by plotting a histogram of observations

In doing so we’ll make use of the Distributions package

julia> Pkg.add("Distributions")


Here’s the code

using Plots
using Distributions

function plot_histogram(distribution, n)
epsilon_values = rand(distribution, n)  # n draws from distribution
histogram(epsilon_values)
end

lp = Laplace()
plot_histogram(lp, 500)


The resulting figure looks like this

Let’s have a casual discussion of how all this works while leaving technical details for later in the lectures

First, lp = Laplace() creates an instance of a data type defined in the Distributions module that represents the Laplace distribution

The name lp is bound to this object

When we make the function call plot_histogram(lp, 500) the code in the body of the function plot_histogram is run with

• the name distribution bound to the same object as lp
• the name n bound to the integer 500

#### A Mystery¶

Now consider the function call rand(distribution, n)

This looks like something of a mystery

The function rand() is defined in the base library such that rand(n) returns n uniform random variables on $$[0, 1)$$

julia> rand(3)
3-element Array{Float64,1}:
0.856817
0.981502
0.510947


On the other hand, distribution points to a data type representing the Laplace distribution that has been defined in a third party package

So how can it be that rand() is able to take this kind of object as an argument and return the output that we want?

The answer in a nutshell is multiple dispatch

This refers to the idea that functions in Julia can have different behavior depending on the particular arguments that they’re passed

Hence in Julia we can take an existing function and give it a new behavior by defining how it acts on a new type of object

The interpreter knows which function definition to apply in a given setting by looking at the types of the objects the function is called on

In Julia these alternative versions of a function are called methods

## Exercises¶

### Exercise 1¶

Recall that $$n!$$ is read as “$$n$$ factorial” and defined as $$n! = n \times (n - 1) \times \cdots \times 2 \times 1$$

In Julia you can compute this value with factorial(n)

Write your own version of this function, called factorial2, using a for loop

### Exercise 2¶

The binomial random variable $$Y \sim Bin(n, p)$$ represents

• number of successes in $$n$$ binary trials
• each trial succeeds with probability $$p$$

Using only rand() from the set of Julia’s built in random number generators (not the Distributions package), write a function binomial_rv such that binomial_rv(n, p) generates one draw of $$Y$$

Hint: If $$U$$ is uniform on $$(0, 1)$$ and $$p \in (0,1)$$, then the expression U < p evaluates to true with probability $$p$$

### Exercise 3¶

Compute an approximation to $$\pi$$ using Monte Carlo

For random number generation use only rand()

• If $$U$$ is a bivariate uniform random variable on the unit square $$(0, 1)^2$$, then the probability that $$U$$ lies in a subset $$B$$ of $$(0,1)^2$$ is equal to the area of $$B$$
• If $$U_1,\ldots,U_n$$ are iid copies of $$U$$, then, as $$n$$ gets large, the fraction that falls in $$B$$ converges to the probability of landing in $$B$$
• For a circle, area = pi * radius^2

### Exercise 4¶

Write a program that prints one realization of the following random device:

• Flip an unbiased coin 10 times
• If 3 consecutive heads occur one or more times within this sequence, pay one dollar
• If not, pay nothing

Once again use only rand() as your random number generator

### Exercise 5¶

Simulate and plot the correlated time series

$x_{t+1} = \alpha \, x_t + \epsilon_{t+1} \quad \text{where} \quad x_0 = 0 \quad \text{and} \quad t = 0,\ldots,T$

The sequence of shocks $$\{\epsilon_t\}$$ is assumed to be iid and standard normal

Set $$T=200$$ and $$\alpha = 0.9$$

### Exercise 6¶

Plot three simulated time series, one for each of the cases $$\alpha=0$$, $$\alpha=0.8$$ and $$\alpha=0.98$$

In particular, you should produce (modulo randomness) a figure that looks as follows

(The figure illustrates how time series with the same one-step-ahead conditional volatilities, as these three processes have, can have very different unconditional volatilities.)

## Solutions¶

Solution notebook

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