Filtering basics in MATLAB - Step-by-Step Execution

Concept Flow - Filtering basics

Start with data array

↓

Choose filter condition

↓

Check each element

↓

If element meets condition

↓

Add element to result

↓

Repeat for all elements

↓

Return filtered array

↓

End

Filtering goes through each item in data, checks a condition, and keeps only those that meet it.

Execution Sample

MATLAB

data = [1, 5, 3, 8, 2];
filtered = data(data > 3);

This code keeps only numbers greater than 3 from the data array.

Execution Table

Step	Current Element	Condition (element > 3)	Action	Filtered Array So Far
1	1	False	Skip	[]
2	5	True	Add 5	[5]
3	3	False	Skip	[5]
4	8	True	Add 8	[5, 8]
5	2	False	Skip	[5, 8]

💡 All elements checked, filtering complete.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
data	[1,5,3,8,2]	[1,5,3,8,2]	[1,5,3,8,2]	[1,5,3,8,2]	[1,5,3,8,2]	[1,5,3,8,2]	[1,5,3,8,2]
filtered	[]	[]	[5]	[5]	[5,8]	[5,8]	[5,8]

Key Moments - 2 Insights

Why does the filtered array only contain 5 and 8?

Because only 5 and 8 satisfy the condition element > 3, as shown in steps 2 and 4 of the execution table.

Does the original data array change after filtering?

No, the original data array stays the same throughout, as seen in the variable tracker where 'data' remains unchanged.

Visual Quiz - 3 Questions

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

A [5, 3]

B [5]

C [1, 5]

D []

💡 Hint

Check the 'Filtered Array So Far' column at step 3 in the execution table.

At which step does the condition element > 3 become true for the first time?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Condition' column in the execution table to find the first 'True'.

If the condition changed to element > 5, what would the filtered array be after all steps?

A [8]

B [5, 8]

C [3, 8]

D [1, 2, 3]

💡 Hint

Consider which elements in 'data' are greater than 5.

Concept Snapshot

Filtering basics in MATLAB:
Use logical conditions inside parentheses to select elements.
Example: filtered = data(data > 3);
This keeps elements where condition is true.
Original data stays unchanged.
Result is a new array with filtered elements.

Full Transcript

Filtering basics in MATLAB means selecting elements from an array that meet a condition. The code example uses data = [1, 5, 3, 8, 2] and filters elements greater than 3. Step by step, each element is checked: 1 is skipped, 5 is added, 3 skipped, 8 added, 2 skipped. The filtered array ends up as [5, 8]. The original data array remains unchanged. This process helps keep only the data you want based on simple rules.

Practice

1. What does filtering an array in MATLAB mean?
filtered = array(condition);

easy

A. Selecting elements that meet a condition

B. Sorting the array elements

C. Changing the array size randomly

D. Adding new elements to the array

Solution

1. Step 1: Understand filtering concept

   Filtering means choosing only those elements from an array that satisfy a condition.

2. Step 2: Analyze the MATLAB syntax

   The syntax array(condition) returns elements where the condition is true.

3. Final Answer:

   Selecting elements that meet a condition -> Option A

4. Quick Check:

   Filtering = Selecting elements [OK]

Hint: Filtering picks elements matching condition inside parentheses [OK]

Common Mistakes:

• Thinking filtering sorts the array
• Believing filtering changes array size randomly
• Confusing filtering with adding elements

2. Which of the following is the correct MATLAB syntax to filter array A for values greater than 10?

easy

A. filtered = A > 10;

B. filtered = A{A > 10};

C. filtered = A(A > 10);

D. filtered = filter(A > 10);

Solution

1. Step 1: Identify correct filtering syntax

   In MATLAB, filtering uses parentheses with a logical condition inside to select elements.

2. Step 2: Check each option

   filtered = A(A > 10); uses A(A > 10), which correctly filters elements greater than 10.

3. Final Answer:

   filtered = A(A > 10); -> Option C

4. Quick Check:

   Use parentheses with condition for filtering [OK]

Hint: Use parentheses with condition to filter arrays [OK]

Common Mistakes:

• Using > without parentheses returns logical array, not filtered values
• Using curly braces {} which are for cell arrays
• Using undefined function 'filter'

3. What is the output of this MATLAB code?

A = [3, 12, 7, 15, 9];
filtered = A(A >= 10);
disp(filtered);

medium

A. Error

B. [3 7 9]

C. [3 12 7 15 9]

D. [12 15]

Solution

1. Step 1: Understand filtering condition

   The condition A >= 10 selects elements in A that are 10 or greater.

2. Step 2: Apply condition to array A

   Elements 12 and 15 satisfy this condition, so filtered = [12 15].

3. Final Answer:

   [12 15] -> Option D

4. Quick Check:

   Elements >= 10 = [12 15] [OK]

Hint: Filter keeps elements where condition is true [OK]

Common Mistakes:

• Including elements less than 10
• Expecting original array output
• Confusing logical array with filtered values

4. Identify the error in this MATLAB code for filtering values less than 5:

A = [2, 8, 4, 6];
filtered = A{A < 5};

medium

A. Using curly braces {} instead of parentheses ()

B. Missing semicolon at end of line

C. Incorrect comparison operator

D. Array A is not defined

Solution

1. Step 1: Check filtering syntax

   Filtering arrays uses parentheses (), not curly braces {} which are for cell arrays.

2. Step 2: Identify the error

   Using A{A < 5} causes an error because curly braces cannot index numeric arrays.

3. Final Answer:

   Using curly braces {} instead of parentheses () -> Option A

4. Quick Check:

   Use () for filtering numeric arrays [OK]

Hint: Use parentheses () to filter numeric arrays, not curly braces {} [OK]

Common Mistakes:

• Using {} for numeric array indexing
• Ignoring MATLAB error messages
• Confusing semicolon necessity

5. Given a matrix M = [1 5 9; 4 2 7; 8 3 6], how do you filter all elements greater than 4 and return them as a single column vector?

hard

A. filtered = filter(M > 4);

B. filtered = M(M > 4); filtered = filtered(:);

C. filtered = M{M > 4};

D. filtered = M(:, M > 4);

Solution

1. Step 1: Apply filtering condition

   Use M(M > 4) to select all elements greater than 4 from matrix M.

2. Step 2: Convert result to column vector

   Use filtered(:) to reshape the filtered elements into a single column vector.

3. Final Answer:

   filtered = M(M > 4); filtered = filtered(:); -> Option B

4. Quick Check:

   Filter then reshape to column vector [OK]

Hint: Filter with condition, then reshape using (: ) for column vector [OK]

Common Mistakes:

• Using curly braces {} for filtering
• Trying to filter columns instead of elements
• Using undefined function 'filter'