Filtering basics in MATLAB

Introduction

Filtering helps you keep only the data you want and remove the rest. It makes your data easier to understand and use.

You want to find all numbers bigger than 10 in a list.

You need to select only the even numbers from a group of numbers.

You want to remove empty or zero values from your data.

You want to pick out names that start with a certain letter.

You want to keep only the positive values from a set of measurements.

Syntax

MATLAB

filteredData = data(condition);

The condition is a logical expression that tells MATLAB which elements to keep.

The result is a new array with only the elements that meet the condition.

Examples

Keeps only numbers greater than 10 from the array.

MATLAB

data = [1, 5, 10, 15, 20];
filteredData = data(data > 10);

Keeps only even numbers using the modulo operator.

MATLAB

data = [2, 3, 4, 5, 6];
filteredData = data(mod(data, 2) == 0);

Keeps only names starting with the letter 'A'.

MATLAB

names = {'Anna', 'Bob', 'Alice', 'Bill'};
filteredNames = names(startsWith(names, 'A'));

Sample Program

This program keeps numbers that are 8 or bigger and shows them.

MATLAB

data = [3, 8, 12, 5, 20, 7];
filteredData = data(data >= 8);
disp('Filtered numbers >= 8:');
disp(filteredData);

Important Notes

Filtering does not change the original data unless you save the result back.

You can combine conditions using & (and), | (or), and ~ (not) for more complex filters.

For cell arrays like lists of names, use functions like startsWith or strcmp inside the condition.

Summary

Filtering selects parts of data that meet a condition.

Use logical expressions inside parentheses to filter arrays.

Filtering helps focus on important data and ignore the rest.

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'