Filtering basics in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter the vector x using the filter coefficients b and a.

MATLAB

y = filter([1], a, x);

Drag options to blanks, or click blank then click option'

Ay

Ba

Cx

Db

Attempts:

3 left

💡 Hint

Common Mistakes

Using a as the first argument instead of b.

Swapping the order of arguments.

The filter function in MATLAB uses the syntax filter(b, a, x), where b and a are the filter coefficients. Here, b is the numerator coefficient vector.

2fill in blank

medium

Complete the code to create a low-pass FIR filter with cutoff frequency 0.4 (normalized) and order 20.

MATLAB

b = fir1(20, [1]);

Drag options to blanks, or click blank then click option'

A0.2

B0.6

C0.4

D0.8

Attempts:

3 left

💡 Hint

Common Mistakes

Using a cutoff frequency higher than 0.5 (Nyquist frequency).

Confusing filter order with cutoff frequency.

The cutoff frequency for the low-pass filter is given as 0.4 (normalized frequency). The fir1 function takes the order and cutoff frequency as arguments.

3fill in blank

hard

Fix the error in the code to apply a moving average filter of length 5 to the signal data.

MATLAB

b = ones(1, [1]) / 5;
y = filter(b, 1, data);

Drag options to blanks, or click blank then click option'

A3

B5

C4

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using a length different from 5 for the averaging vector.

Dividing by a number other than 5.

The moving average filter length is 5, so the vector of ones should have length 5.

4fill in blank

hard

Fill both blanks to create a bandpass filter with order 30 and cutoff frequencies 0.3 and 0.6.

MATLAB

b = fir1([1], [[2] 0.6], 'bandpass');

Drag options to blanks, or click blank then click option'

A30

B20

C0.3

D0.4

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping cutoff frequencies or using wrong order.

Using cutoff frequencies outside the valid range.

The filter order is 30 and the lower cutoff frequency is 0.3. These fill the blanks in the fir1 function call.

5fill in blank

hard

Fill all three blanks to create a high-pass filter with order 25, cutoff frequency 0.5, and apply it to signal sig.

MATLAB

b = fir1([1], [2], 'high');
y = filter(b, [3], sig);

Drag options to blanks, or click blank then click option'

A25

B0.5

C1

D0.7

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong cutoff frequency or order.

Using denominator coefficients other than 1 for FIR filters.

The filter order is 25, cutoff frequency is 0.5, and the denominator coefficients for filter are 1 for FIR filters.

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'