Logical indexing in MATLAB - Step-by-Step Execution

Concept Flow - Logical indexing

Create array A

↓

Create logical mask L

↓

Use L to select elements from A

↓

Output selected elements

↓

End

Logical indexing uses a true/false mask to pick elements from an array.

Execution Sample

MATLAB

A = [10, 20, 30, 40, 50];
L = A > 25;
B = A(L);

Selects elements from A that are greater than 25 using logical indexing.

Execution Table

Step	Variable	Value	Action	Output
1	A	[10 20 30 40 50]	Create array A
2	L	[false false true true true]	Compare A > 25 to create logical mask
3	B	[30 40 50]	Use L to select elements from A	[30 40 50]
4	-	-	End of execution

💡 All steps completed; logical indexing selected elements > 25.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
A	undefined	[10 20 30 40 50]	[10 20 30 40 50]	[10 20 30 40 50]	[10 20 30 40 50]
L	undefined	undefined	[false false true true true]	[false false true true true]	[false false true true true]
B	undefined	undefined	undefined	[30 40 50]	[30 40 50]

Key Moments - 3 Insights

Why does the logical mask L have true only for some elements?

Because L is created by comparing each element of A to 25, only elements greater than 25 get true (see step 2 in execution_table).

What happens if we use L to index A?

Only elements where L is true are selected from A, producing B with those elements (see step 3 in execution_table).

Can logical indexing change the size of the output array?

Yes, the output array B only contains elements where L is true, so it can be smaller than A (see step 3 output).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of L?

A [true true false false false]

B [false false true true true]

C [true false true false true]

D [false true false true false]

💡 Hint

Check the 'Value' column for L at step 2 in the execution_table.

At which step does B get its value assigned?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for when B is created and assigned in the execution_table.

If A was [5 15 25 35 45], what would be the value of L after step 2?

A [false true true true true]

B [true true true false false]

C [false false false true true]

D [true false false false true]

💡 Hint

Compare each element of A to 25 to create L as in step 2.

Concept Snapshot

Logical indexing in MATLAB:
- Create a logical mask by comparing array elements.
- Use mask to select elements: B = A(L).
- Output contains only elements where mask is true.
- Useful for filtering arrays easily.

Full Transcript

Logical indexing in MATLAB lets you pick elements from an array by using a true/false mask. First, you create an array A. Then you make a logical mask L by comparing A to a condition, like A > 25. This mask has true where the condition is met and false elsewhere. Using A(L) selects only the elements where L is true. For example, if A = [10 20 30 40 50], then L = A > 25 gives [false false true true true]. Using B = A(L) results in B = [30 40 50]. This method is simple and powerful for filtering data.