Decomposition

breaking down a complex problem or system into smaller parts

Pattern Recognition

looking for similarities among and within problems

Abstraction

focusing on the important parts only, ignoring irrelevant detail

Algorithm Design

developing a step-by-step solution to the problem

Let’s forget about specific numbers.

In other words, let’s make an abstraction

• corners: How many sides we will draw
• angle: how do you turn after each side
• initial_angle: initial angle
• length: length of each side

Assuming that you know the input values, you need to:

• Point in the direction initial_angle
• repeat corner times
  • draw a line of length length
  • turn right angle degrees

In this case it is easy to see a pattern:

• draw a line of length size
• turn right angle degrees

This pattern is repeated N times

You can do this easily with a loop

We already know an algorithmic structure: loops

repeat (10)

which in R we write as

for(i in 1:10) {
  ...
}

The second algorithmic structure you can use are

Sub-routines

A group of specific steps grouped together

In R we represent sub-routines with functions

In Math and Informatics, a function is a “black box”

A rule to transform the input elements into an output

• e.g. logarithm of a number, length of a vector

The same input should produce always the same output

Notice that there may be more than one input element

newFunc <- function(input) {
  commands
  commands
  return(output)
}

The keyword return can be omitted

In that case the output is the result of the last command

my_func()

## Error in my_func(): argument "a" is missing, with no default

my_func(1,2,3,4)

## Error in my_func(1, 2, 3, 4): unused arguments (3, 4)

Before writing the function we need to decide:

• the name of the function (it’s harder than it seems)
• which are the inputs
• what is the output

Then we can start writing the code

Last semester we built documents, like papers and slides

These are files with .Rmd extension

This semester we build programs and scripts

These are files with .R extension

Question What are “file extensions”?

We have seen two ways to execute multiple lines:

• Select the lines and press the Ctrl+Enter key (or use the Run toolbar button)

• To run the entire document press the Ctrl+Shift+Enter key (or use the Source toolbar button).

Here Source means “run all code from the file”