Compact for removing nil values in Ruby - Step-by-Step Execution

Concept Flow - Compact for removing nil values

Start with array

↓

Check each element

↓

Is element nil?

Yes→Skip element

No↓

Keep element

↓

Build new array without nils

↓

Return new array

The method goes through each item in the array, skips nil values, and keeps all others to create a new array without nils.

Execution Sample

Ruby

arr = [1, nil, 2, nil, 3]
new_arr = arr.compact
puts new_arr.inspect

This code removes all nil values from the array and prints the new array.

Execution Table

Step	Current Element	Is nil?	Action	New Array State
1	1	No	Keep element	[1]
2	nil	Yes	Skip element	[1]
3	2	No	Keep element	[1, 2]
4	nil	Yes	Skip element	[1, 2]
5	3	No	Keep element	[1, 2, 3]
6	-	-	Return new array	[1, 2, 3]

💡 All elements checked; nil values removed; new array returned

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
arr	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]	[1, nil, 2, nil, 3]
new_arr	[]	[1]	[1]	[1, 2]	[1, 2]	[1, 2, 3]	[1, 2, 3]

Key Moments - 2 Insights

Why does the new array not include nil values even though the original array has them?

Because at each step (see execution_table rows 2 and 4), the method checks if the element is nil and skips it, so nils are never added to the new array.

Does the original array change after using compact?

No, the original array stays the same (see variable_tracker for 'arr'), compact returns a new array without nils.

Visual Quiz - 3 Questions

Look at the execution table, what is the new array state after step 3?

A [1, nil, 2]

B [1, 2]

C [1]

D [1, nil]

💡 Hint

Check the 'New Array State' column at step 3 in the execution_table.

At which step does the method skip a nil element for the first time?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Is nil?' and 'Action' columns in the execution_table.

If the original array had no nil values, how would the new array compare?

A It would have only nil values

B It would be empty

C It would be the same as the original

D It would have fewer elements

💡 Hint

Think about what compact does when no nils are present, check variable_tracker for 'new_arr'.

Concept Snapshot

Array#compact removes all nil values from an array.
It returns a new array without changing the original.
Each element is checked; nils are skipped.
Useful to clean arrays before processing.
Syntax: new_arr = old_arr.compact

Full Transcript

The Ruby method compact goes through each element of an array and removes all nil values. It creates a new array with only the non-nil elements. The original array stays unchanged. For example, given [1, nil, 2, nil, 3], compact returns [1, 2, 3]. This is useful when you want to clean your data by removing empty or missing values represented by nil. The execution table shows each step checking elements and building the new array. The variable tracker confirms the original array does not change, while the new array grows only with non-nil values.