Programonic

Before you start

Ruby basics and arrays.

Define a method and transform an array with map.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A method starts with def and ends with end. The value of its last expression is returned automatically, so return is optional. Ruby does not infer a float from integer division, so the example calls to_f on the size to keep the fraction.

map calls a block on each element and collects the results into a new array, leaving the original unchanged. inspect shows an array in its source-like form, with brackets and commas.

Code example

Ruby

def average(values)
  values.sum / values.size.to_f
end

puts average([82, 91, 76, 95])
puts [1, 2, 3].map { |n| n * n }.inspect

Output

86.0
[1, 4, 9]

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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Guided walkthroughReady

1def average(values)

2 values.sum / values.size.to_f

3end

5puts average([82, 91, 76, 95])

6puts [1, 2, 3].map { |n| n * n }.inspect

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Define a method with one parameter.	`—`	—
344 divided by 4.0; the last expression is the return value.	`return = 86.0`	—
Print the average.	`return = 86.0`	86.0
Square each element and print the new array.	`return = 86.0`	86.0 [1, 4, 9]

Behind the scenes

How Ruby processes this code

Ruby parses and executes the script. Blocks run when their methods invoke them.

Cost and common mistakes

Both operations are O(n) time.

344 / 4 with both integers would give an integer; at least one side must be a Float for decimals.
map returns a new array; use map! only if you really want to change the original.

Try it yourself

Predict [1, 2, 3].map { |n| n + 10 }.

Reveal answer and reasoning

It returns [11, 12, 13].

Ruby Methods and map