Select/filter for filtering in Ruby - Step-by-Step Execution

Concept Flow - Select/filter for filtering

Start with array

↓

Check each element

↓

Condition true?

No→Skip element

Yes↓

Add element to new array

↓

More elements?

Yes→Check next element

No↓

Return filtered array

The program checks each item in the array, keeps only those that meet the condition, and returns a new array with them.

Execution Sample

Ruby

numbers = [1, 2, 3, 4, 5]
even_numbers = numbers.select { |n| n.even? }
puts even_numbers

This code selects only even numbers from the list and prints them.

Execution Table

Step	Element (n)	Condition n.even?	Action	Filtered Array
1	1	false	Skip	[]
2	2	true	Add 2	[2]
3	3	false	Skip	[2]
4	4	true	Add 4	[2, 4]
5	5	false	Skip	[2, 4]
6	End	-	Return filtered array	[2, 4]

💡 All elements checked, filtering complete.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
filtered_array	[]	[]	[2]	[2]	[2, 4]	[2, 4]	[2, 4]

Key Moments - 2 Insights

Why is the number 1 not added to the filtered array?

Because at step 1 in the execution_table, the condition n.even? is false for 1, so it is skipped.

Does the original array change after select/filter?

No, the original array stays the same; select/filter creates a new array with only the matching elements, as shown by filtered_array starting empty and filling up.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the filtered array after adding element 4?

A [4]

B [2, 4]

C [2]

D []

💡 Hint

Check the 'Filtered Array' column at step 4 in the execution_table.

At which step does the condition n.even? become false for the first time?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Condition n.even?' column in the execution_table from the start.

If we changed the condition to n > 3, what would be the filtered array after step 5?

A [5]

B [2, 4]

C [4, 5]

D [1, 2, 3]

💡 Hint

Think about which numbers in the original array are greater than 3.

Concept Snapshot

Use select or filter to keep only elements that match a condition.
Syntax: array.select { |item| condition }
Returns a new array with matching items.
Original array stays unchanged.
Common for filtering lists easily.

Full Transcript

This visual execution shows how Ruby's select method filters an array. Starting with the array [1, 2, 3, 4, 5], each element is checked if it is even. If true, the element is added to a new array. Steps show 1 is skipped, 2 added, 3 skipped, 4 added, 5 skipped. The final filtered array is [2, 4]. The original array remains unchanged. This helps beginners see how select works step-by-step.