Template [Julia]

1. Data types, data structures and indexing

1.1 Basics

Object, assignment, functions, how to comment and get help

# We can use Julia as a calculator!

# Functions help us execute things; we usually have to provide arguments

# Don't know how a function works? Ask for help

1.2 Data types

Numeric (integers or doubles)

String

Boolean

1.3 Data classes

Arrays

Arrays are ordered collection of elements.

A 1-dimensional array is a vector.

What happens when we add 1 to a logical vector?

In Julia, arrays can contain elements of different types.

A 2-dimensional array is a matrix.

reshape(my_vector, (lines, columns))			# reshapes my_vector

Arrays can also contain other arrays.

Tuples

(name_element1 = element1, name_element2 = element2, name_element3 = element3)

Data frames

DataFrame(vector1, vector2)	# bind vectors with the same length

1.4 Dimensions

size(matrix) # returns the dimensions

length(object) # returns the number of elements

Can you guess the value of the length of my_array_of_arrays?

size(my_array_of_arrays)

length(my_array_of_arrays)

1.5 Indexing

# 1-dimensional

vector[index]

# 2-dimensional

matrix[row, column]

dataframe[row, column]

# Multi-dimensional

array[element][...]

tuple[element][...]

1.6 Slicing

2. Files

Absolute paths

C:/Users/RonBumblefootThal/Documents/RFolder/MyFirstProject/Draft/IDon'tKnowWhatI'mDoing/etc.R

Relative Paths

~/I_love_my_project/CoolCode.R

2.1 Working directories

2.2 Save/write files

Save a dataframe

soa_tour = DataFrame(country = ["USA", "UK", "FRA", "GER", "BRA"], 

                     frequency = [34, 9, 6, 5, 3], 

                     continents = ["north_america", "europe", "europe", 

                                   "europe", "south_america"])

# Function structure

CSV.write(path, object)

2.3 Load/read files

From your PC

object = CSV.read(path)

From url to your PC, then read

download("http://remote.repo/data/file.csv", path)

object = CSV.read(path)

Metabolic rates data: http://sciencecomputing.io/data/metabolicrates.csv

3. Control Flow

You already apply control flow when you decide how to go to work during winter.

For example:

• You take the metro if it's snowy

• You take the metro if it's cold

• You walk every other time

Now, let's put that into code!

3.1 Conditional evaluation

Simple if statement

Structure:

if (condition is true)

	do expression

end

Example:

If/else statement

Structure:

if (condition)

	expression 1

else 

 	expression 2

end

Example:

Nested if /else statement

if (condition 1)

	expression 1

  if (condition 2)

    expression 2

  end

end

Example:

Simpler alternative:

Adding a condition:

3.2 For loops

Simple for loops

Using for loops, you can then plan your schedule for a few days.

What we had:

weather = "snowy"

temperature = -15

But what about this?

weather_vec = ["snowy", "cloudy", "snowy", "clear", "rainy"]

temperature_vec = [-15, -23, -2, -40, 5]

Does the same code work?

if weather_vec == "snowy" || temperature_vec < -20

  println("Take the metro!")

else

  println("Let's walk!")

end

Iterations

for i in iterations

	expression(i)

end

More generally:

If statement inside for loops

Structure:

for i in iterations

	if (condition)

 	 	expression1

  else

  	expression2

  end

end

Example:

# Previous statement

if weather_vec == "snowy" || temperature_vec < -20

  println("Take the metro!")

else

  println("Let's walk!")

end

# Will this work?

	# Previous statement

	if weather_vec == "snowy" || temperature_vec < -20

	  println("Take the metro!")

	else

	  println("Let's walk!")

	end

3.3 Extras

Some logical operators

Comparisons:

• less than (<)

• more than (>)

• less than or equal to (<=)

• more than or equal to (>=)

• equal to (==)

• not equal to (!=)

Logic:

• not x (!x)

• x or y (x || y)

• x and y (x && y)

4. Functions

4.1 Syntax and arguments

Basic syntax

function functionname(argument1, argument2)		# Name and arguments 

	result = expression		# What the function does

  return result # What the function returns

end

# Define a function

# Apply on values

# Apply on variables

Calling (personal) functions within functions

4.2 Scope

Variables can exist either in global or local scope.

Remember, the element to return in our function was called abs_result.

# What will this return, outside the function?

result

Here is a second example for ecologists who like to count living things:

# Global variables

4.3 Integration

Combining functions and control flow

Let's come back to our previous example about transportation according to the weather.

Here is the forecast for the week and the weekend:

# Week forecast

weather_week = ["snowy", "cloudy", "snowy", "clear", "rainy"]

temperature_week = [-15.0, -23.0, -2.0, -40.0, 5.0]

# Weekend forecast

weather_weekend = ["snowy", "rainy"]

temperature_weekend = [-3.0, 2.0]

Now, let's build a function that will work with either the week or weekend forecasts.

It will look like:

function transportation

	for all days of the week/weekend

  	if (snowy or cold)

    	take metro

    else

    	walk

    end

  end

end

# Create a new function

	# Previous for loop

	for i in 1:length(weather_vec)

  	if weather_vec[i] == "snowy" || temperature_vec[i] < -15.0

    	println("Take the metro")

  	else

    	println("Just walk")

  	end

	end

# Plan for the week

# Plan for the weekend

4.4 Exercise - Planning the week

Exercise to integrate the following:

• Functions

• Control flow

• Files

1. Write a function to read a file if it exists, downloading it first if it does not exist.

2. Apply the choose_transportation function to the data in the file

# Pseudocode

function (file, url)

	if (file exists)

  	read file

  else

  	download file

    read file

end

# Useful functions

?isfile

?CSV.read

?download

Forecast data url: https://docs.google.com/spreadsheets/d/e/2PACX-1vSdqGDzfGPygowYRaffMTsVHQz1nejPPyjE2Q1yYIRKPUfhayMTcCMhdzqfbea5IeYKi82aW4NDas_G/pub?gid=0&single=true&output=csv

4.5 Extras

Default values

# Define function

function add_and_multiply(var1, var2; var3 = 1)

  result = (var1 + var2) * var3

  return result

end

add_and_multiply(1.0, 2.0) # multiplies by 1, as default

add_and_multiply(1.0, 2.0, 2.0) # returns error

add_and_multiply(1.0, 2.0, var3 = 2.0) # proper syntax

Controlling return

function add_and_multiply(var1, var2; var3 = 1)

  addition_result = var1 + var2

  multiplication_result = addition_result * var3

  return (added = addition_result, multiplied = multiplication_result)

end