Filtering basics in MATLAB - Mini Project: Build & Apply

Filtering basics

📖 Scenario: You work in a small grocery store. You have a list of product prices, but you want to find only the products that cost more than a certain amount to plan special discounts.

🎯 Goal: Build a MATLAB program that filters a list of product prices to keep only those prices above a given threshold.

📋 What You'll Learn

Create a vector with exact product prices

Create a threshold variable for filtering

Use logical indexing to filter prices above the threshold

Display the filtered prices

💡 Why This Matters

🌍 Real World

Filtering prices helps stores decide which products to discount or promote based on cost.

💼 Career

Data filtering is a common task in data analysis, quality control, and reporting jobs.

1

Create the product prices vector

Create a vector called prices with these exact values: [12.5, 7.8, 15.0, 3.2, 9.9]

MATLAB

% Create the 'prices' vector with the given values
% Your code here

Hint

Use square brackets [] to create a vector in MATLAB.

2

Set the price threshold

Create a variable called threshold and set it to 10

MATLAB

prices = [12.5, 7.8, 15.0, 3.2, 9.9];
% Create the 'threshold' variable and set it to 10
% Your code here

Hint

Just assign the number 10 to the variable threshold.

3

Filter prices above the threshold

Create a new vector called filtered_prices that contains only the values from prices greater than threshold using logical indexing

MATLAB

prices = [12.5, 7.8, 15.0, 3.2, 9.9];
threshold = 10;
% Filter prices greater than threshold into 'filtered_prices'
% Your code here

Hint

Use prices(prices > threshold) to select prices above the threshold.

4

Display the filtered prices

Use disp to display the filtered_prices vector

MATLAB

prices = [12.5, 7.8, 15.0, 3.2, 9.9];
threshold = 10;
filtered_prices = prices(prices > threshold);
% Display the filtered prices
% Your code here

Hint

Use disp(filtered_prices) to show the filtered prices on the screen.

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'