MATLAB Practice Problems for Paper Exam Review

MATLAB Practice Problems for Paper Exam Review#

Week 10B: From Python to MATLAB - Syntax and Plotting#

Basic Syntax Differences#

Problem 1: Without using MATLAB, predict the output of the following code:

x = 5;
y = x * 2
z = y + 3;

Problem 2: What is the difference between these two indexing operations?

Python: my_list[0]
MATLAB: my_array(1)

Problem 3: Given vec = [2, 4, 6, 8, 10], what would be the result of:

a) vec(3)
b) vec(end)
c) vec(2:4)

Problem 4: Write MATLAB code to create the following vectors:

a) A row vector with values 3, 6, 9, 12, 15 using the colon operator
b) A row vector with values from 10 to 50 with a step of 5
c) A vector with 5 evenly spaced points from 0 to 20 using linspace

Plotting#

Problem 5: Write the MATLAB code to:

a) Plot y = x² for x values from 0 to 10
b) Add a title “Quadratic Function”
c) Label the x-axis “x values” and y-axis “y values”
d) Add a grid to the plot

Problem 6: What line style, color, and marker would the following create?

a) plot(x, y, 'r--o')
b) plot(x, y, 'b:*')
c) plot(x, y, 'k-s')
d) plot(x, y, 'g-.^')

Problem 7: Write code to create a figure with 2 rows and 1 column of subplots. In the first subplot, plot sin(x), and in the second, plot cos(x), for x from 0 to 2π.

Problem 8: Given the following code, what would happen?

x = 0:0.5:5;
y1 = x;
y2 = x.^2;
plot(x, y1, x, y2)
legend('Linear', 'Quadratic')

Problem 9: Write the code to create a scatter plot of 50 random points and give it an appropriate title.

Week 11: Debugging and Loops#

Loop Syntax#

Problem 10: Convert this Python loop to MATLAB:

for i in range(1, 11):
    print(i * 2)

Problem 11: What does this MATLAB loop output?

sum = 0;
for i = 2:2:10
    sum = sum + i;
end
disp(sum)

Problem 12: Convert this Python while loop to MATLAB:

count = 10
while count > 0:
    print(count)
    count = count - 2

Problem 13: What will be the final value of result after this loop executes?

result = 1;
for i = 1:5
    result = result * 2;
end

Debugging Concepts#

Problem 14: Given this code:

numbers = [10, 25, 30, 15, 40];
threshold = 20;
count = 0;

for i = 1:length(numbers)
    if numbers(i) > threshold
        count = count + 1;
    end
end

If you set a breakpoint at line 3 (count = 0;) and step through, what would be the value of count after:

a) The 1st iteration?
b) The 3rd iteration?
c) The loop completes?

Problem 15: This code has a bug. Identify the problem:

data = [5, 10, 15, 20];
total = 0;

for i = 1:length(data)
    total = total + data(i);
end

average = total / count;

Problem 16: What error will occur when running this code, and why?

values = [8, 12, 6, 15];
limit = 20;

position = 1;
while values(position) < limit
    position = position + 1;
end

Performance Concepts#

Problem 17: Which of these two code snippets would run faster, and why?

Version A:

result = [];
for i = 1:1000
    result = [result, i^2];
end

Version B:

result = zeros(1, 1000);
for i = 1:1000
    result(i) = i^2;
end

Problem 18: Rewrite this loop using vectorization:

x = 1:100;
y = zeros(1, 100);
for i = 1:length(x)
    y(i) = 3 * x(i) + 5;
end

Week 12A: Creating Vectors and Matrices#

Creating Matrices#

Problem 19: Write MATLAB code to create this matrix:

2  3  4
6  7  8
10 11 12

Problem 20: What’s the difference between creating these two vectors?

a) vec1 = [1, 2, 3, 4]
b) vec2 = [1; 2; 3; 4]

Problem 21: Write MATLAB code to create a column vector with values 2, 4, 6, 8 using:

a) Direct entry with semicolons
b) The colon operator and transpose

Accessing Matrix Elements#

Problem 22: Given this matrix:

mat = [1  2  3  4
       5  6  7  8
       9  10 11 12]

What value would be returned by:

a) mat(2,3)
b) mat(1,:)
c) mat(:,4)
d) mat(2:3, 2:3)
e) mat(end, end)

Special Matrix-Creating Functions#

Problem 23: Write the MATLAB code to create:

a) A 3×3 matrix of all zeros
b) A 2×4 matrix of all ones
c) A 4×4 identity matrix
d) A 3×2 matrix of random integers between 5 and 15

Problem 24: What’s the difference between:

a) rand(3, 3)
b) randn(3, 3)
c) randi(10, 3, 3)

Concatenating Matrices#

Problem 25: Given A = [1 2; 3 4] and B = [5 6; 7 8], what would be the result of:

a) [A, B] (horizontal concatenation)
b) [A; B] (vertical concatenation)

Problem 26: Given C = [1 2 3] and D = [4 5 6], can you perform:

a) [C, D]
b) [C; D] For each, explain why it works or doesn’t work, and give the result if it works.

Problem 27: What would happen if you tried to concatenate these matrices?

A = [1 2 3; 4 5 6];
B = [7 8; 9 10];
C = [A, B]

Modifying Matrices#

Problem 28: Given vec = [10 20 30 40 50], write code to:

a) Change the third element to 99
b) Change elements 2 through 4 to all be 0

Problem 29: Given this matrix:

mat = [1 2 3
       4 5 6
       7 8 9]

Write code to:

a) Replace the second row with [40 50 60]
b) Replace the first column with [10; 20; 30]
c) Replace the 2×2 submatrix in the lower-right corner with zeros

Problem 30: Given vec = [5, 12, 8, 3, 15, 9], write code to:

a) Replace all values greater than 10 with 10
b) Replace all values less than 5 with 5

Reshaping Arrays#

Problem 31: Given A = [1 2 3 4 5 6 7 8 9 10 11 12], what would be the result of:

a) reshape(A, [3, 4])
b) reshape(A, [2, 6])
c) reshape(A, [4, 3])

Problem 32: You have a 2×6 matrix and want to reshape it into a 3×4 matrix. Is this possible? Why or why not?

Week 12B: Vector and Matrix Operations#

Scalar Operations#

Problem 33: Given vec = [2, 4, 6, 8], calculate by hand:

a) vec * 3
b) vec + 10
c) vec - 5
d) vec / 2

Problem 34: Given this matrix:

mat = [2 4 6
       1 3 5]

Calculate by hand:

a) mat * 2
b) mat + 10

Array Operations#

Problem 35: Given vec = [2, 4, 6, 8], calculate by hand:

a) vec .^ 2
b) vec .* vec

Problem 36: What’s the difference between these two operations?

a) .* (element-wise multiplication)
b) * (matrix multiplication)

Problem 37: Given vec1 = [2, 4, 6] and vec2 = [1, 2, 3], calculate:

a) vec1 + vec2
b) vec1 .* vec2
c) vec1 ./ vec2
d) vec1 .^ vec2

Problem 38: Given A = [1 2; 3 4] and B = [2 0; 1 2], calculate by hand:

a) A .* B
b) A ./ B
c) A .^ 2

Matrix Multiplication#

Problem 39: Given A = [1 2; 3 4] and B = [2 0; 1 2], calculate by hand:

a) A * B
b) B * A
c) Are the results the same? Why or why not?

Problem 40: Can you perform matrix multiplication A * B for the following cases? If yes, what are the dimensions of the result?

a) A is 3×4, B is 4×2
b) A is 2×3, B is 2×3
c) A is 3×1, B is 1×4
d) A is 2×2, B is 2×2

Problem 41: Given A = [1 2 3] (1×3) and B = [4; 5; 6] (3×1), calculate:

a) A * B
b) B * A
c) What are the dimensions of each result?

Logical Operations and Indexing#

Problem 42: Given vec = [5, 12, 8, 3, 15, 9], what would be the result of:

a) vec < 10 (write the logical array)
b) vec > 7 (write the logical array)
c) vec == 8 (write the logical array)

Problem 43: Given vec = [5, 12, 8, 3, 15, 9] and the logical arrays from Problem 42a, what would be returned by:

a) vec(vec < 10)
b) vec(vec > 7)

Problem 44: Given this matrix:

mat = [2  5  8
       3  1  6
       7  4  9]

Write the logical array that would result from:

a) mat > 5
b) mat < 4

Statistical Functions#

Problem 45: Given vec = [3, 7, 2, 9, 5, 8, 1], calculate by hand:

a) min(vec)
b) max(vec)
c) sum(vec)
d) mean(vec)

Problem 46: Given the matrix:

mat = [2 5 8
       3 1 6
       7 4 9]

Calculate by hand:

a) sum(mat) (sum of each column)
b) max(mat) (max of each column)
c) min(mat, 2) (min of each row)
d) mean(mat, 2) (mean of each row)
e) sum(sum(mat)) (overall sum)

Problem 47: Given vec = [10 20 30 40 50], calculate:

a) diff(vec)
b) cumsum(vec)
c) prod(vec)

Problem 48: Given vec = [8, 2, 15, 4, 11, 3, 9], what would be returned by:

a) sort(vec)
b) median(vec)

Matrix Manipulation Functions#

Problem 49: Given mat = [1 2 3; 4 5 6] (a 2×3 matrix), what would be the result of:

a) mat' (transpose)
b) fliplr(mat) (flip left-right)
c) flipud(mat) (flip up-down)
d) rot90(mat) (rotate 90 degrees)

Problem 50: Given vec = [2, 5, 8, 11], what would be the result of:

a) fliplr(vec)
b) flip(vec)

Problem 51: Given mymat = [1 2; 3 4], what would be the result of:

a) repmat(mymat, 2, 1) (replicate 2 rows, 1 column)
b) repmat(mymat, 1, 3) (replicate 1 row, 3 columns)
c) repelem(mymat, 2, 2) (replicate each element 2×2)

Square Matrix Functions#

Problem 52: Given this matrix:

mat = [1 0 0
       0 5 0
       0 0 3]

a) What would diag(mat) return?
b) What would trace(mat) return?
c) What would isdiag(mat) return?
d) Is this matrix symmetric? What would issymmetric(mat) return?

Problem 53: Given vec = [2, 4, 6], what would diag(vec) create?

Problem 54: What does eye(5) create? Write out the matrix.

Week 13A: File I/O and Scripts#

Script Basics#

Problem 55: What’s the difference between running these two lines in a script?

x = 5 * 3
y = 5 * 3;

Problem 56: Write the fprintf statement to display “Temperature: 72.5 degrees” where 72.5 is stored in a variable called temp.

Problem 57: You have Fahrenheit temperature stored in variable F. Write the code to:

a) Convert to Celsius: C = (F - 32) × 5/9
b) Convert to Kelvin: K = C + 273.15
c) Display all three with labels using fprintf

Directory Operations#

Problem 58: Write MATLAB commands to:

a) Display your current working directory
b) Check if a folder called “Data” exists
c) Create the folder “Data” if it doesn’t exist

Problem 59: Rewrite this Windows-specific path using fullfile:

path = 'Results\Figures\plot1.png';

Problem 60: What command would you use to check if a file called data.csv exists?

Reading Data#

Problem 61: You have a CSV file scores.csv with student names and test scores. Write the command to read it into a table variable called studentData.

Problem 62: After reading an image with img = imread('photo.png'), how would you:

a) Display the image
b) Extract just the red channel
c) Save the red channel as a new image

Writing Data#

Problem 63: You have variables time and temperature. Write the code to save both variables to a MAT file called experiment_data.mat.

Problem 64: Write code using fopen, fprintf, and fclose to create a text file output.txt that contains:

Experiment Results
Average: 45.6
Maximum: 89.2

where 45.6 is stored in variable avg and 89.2 is in variable maxVal.

Problem 65: Write code to save a figure to a file called results_plot.png in a folder called Figures.

Comprehensive Review Problems (All Readings)#

Problem 66: Given A = [2 5; 1 3] and B = [4 1; 2 6]:

a) Calculate A + B
b) Calculate A .* B
c) Calculate A * B
d) Calculate A .^ 2
e) Calculate A'

Problem 67: Write MATLAB code that would:

a) Create a row vector from 1 to 20
b) Extract every other element (1, 3, 5, …)
c) Compute the square of each extracted element
d) Calculate the sum of all squared values

Problem 68: Given vec = [8, 3, 15, 6, 12, 9]:

a) Create a logical array showing which values are greater than 7
b) Extract only the values greater than 7
c) Replace all values less than 10 with 10
d) Calculate the mean of the modified vector

Problem 69: Create the following in MATLAB code:

a) A 3×3 matrix with 1s on the diagonal and 0s elsewhere
b) Multiply this matrix by the vector [5; 10; 15]
c) What is special about multiplying by an identity matrix?

Problem 70: Write complete MATLAB code to:

a) Create x values from 0 to 10 with 100 points
b) Calculate y = sin(x) and z = cos(x)
c) Create a figure with two subplots (one above the other)
d) Plot y vs x in the first subplot with a red solid line
e) Plot z vs x in the second subplot with a blue dashed line
f) Add appropriate titles and labels to both subplots
g) Add a grid to both plots

Problem 71: Debug this code that’s supposed to count how many numbers in an array are positive:

numbers = [-5, 3, -2, 8, 0, -1, 6];
count = 0;

for i = 1:length(numbers)
    if numbers(i) > 0
        count = count;
    end
end

disp(count)

Problem 72: This code is supposed to find the position of the maximum value in an array. Find and fix the bug:

data = [10, 25, 15, 30, 20];
maxPos = 0;
maxVal = 0;

for i = 1:length(data)
    if data(i) > maxVal
        maxVal = data(i);
        maxPos = i;
    end
end

disp(['Maximum value is at position: ', num2str(maxPos)])

Problem 73: Which version would run faster for large arrays, and why?