CSV file handling in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading CSV with readmatrix

What is the output of this MATLAB code when reading a CSV file with numeric data only?

MATLAB

data = readmatrix('data.csv');
disp(data);

A [1 2 3; 4 5 6; 7 8 9]

B Error: File not found

C A cell array with numeric and text data

D A table with variable names

💡 Hint

readmatrix reads numeric data into a numeric array.

readmatrix reads numeric CSV data into a numeric matrix. If the file contains only numbers, the output is a numeric array.

❓ Predict Output

intermediate

Writing CSV with writematrix

What will be the content of 'output.csv' after running this MATLAB code?

MATLAB

M = [10 20 30; 40 50 60];
writematrix(M, 'output.csv');

A A CSV file with text headers

B A CSV file with one row: '10 20 30 40 50 60'

C An empty file

D A CSV file with two rows: '10,20,30' and '40,50,60'

💡 Hint

writematrix writes numeric arrays as CSV rows.

writematrix writes each row of the matrix as a line in the CSV file, with commas separating values.

🔧 Debug

advanced

Reading mixed data CSV with readmatrix

What is the result of this MATLAB code when reading a CSV file with mixed text and numbers using readmatrix?

MATLAB

data = readmatrix('mixed.csv');

A Returns a table with mixed data

B Error: Unable to convert text to numeric

C No error, returns numeric matrix with NaN for text

D Error: File not found

💡 Hint

readmatrix converts non-numeric data to NaN.

readmatrix reads the file into a numeric matrix, replacing text or non-numeric data with NaN. No error occurs.

❓ Predict Output

advanced

Reading CSV with readtable and accessing data

Given a CSV file 'people.csv' with columns 'Name,Age,Height', what is the output of this code?

MATLAB

T = readtable('people.csv');
disp(T.Age(2));

A The entire Age column

B The age value of the second person in the table

C Error: Age is not a field

D The name of the second person

💡 Hint

readtable creates a table with column names as fields.

readtable reads CSV into a table. Accessing T.Age(2) returns the second row's Age value.

🧠 Conceptual

expert

Choosing the best function for mixed CSV data

Which MATLAB function is best suited to read a CSV file containing both numeric and text data with headers?

A readtable

B readmatrix

C writematrix

D csvread

💡 Hint

Consider which function handles mixed data and headers well.

readtable reads CSV files with mixed data types and headers into a table, preserving data types and column names.

Practice

1. Which MATLAB function is best suited to read numeric data from a CSV file into a matrix?

easy

A. csvread

B. readtable

C. readmatrix

D. load

Solution

1. Step 1: Understand the data type

   Numeric data is best handled as a matrix in MATLAB.

2. Step 2: Choose the appropriate function

   readmatrix is designed to read numeric CSV data directly into a matrix.

3. Final Answer:

   readmatrix -> Option C

4. Quick Check:

   Numeric CSV data = readmatrix [OK]

Hint: Use readmatrix for numeric CSV files quickly [OK]

Common Mistakes:

• Using readtable for purely numeric data
• Using load which is for MAT files
• Confusing csvread with readmatrix (csvread is older)

2. Which of the following is the correct syntax to write a matrix A to a CSV file named data.csv in MATLAB?

easy

A. writetable('data.csv', A);

B. writecsv(A, 'data.csv');

C. csvwrite(A, 'data.csv');

D. writematrix(A, 'data.csv');

Solution

1. Step 1: Identify the correct function for matrix writing

   writematrix is the modern function to write numeric matrices to CSV files.

2. Step 2: Check syntax correctness

   The syntax writematrix(A, 'data.csv'); correctly specifies the matrix and filename.

3. Final Answer:

   writematrix(A, 'data.csv'); -> Option D

4. Quick Check:

   Write matrix to CSV = writematrix [OK]

Hint: Use writematrix(matrix, filename) to save numeric CSV [OK]

Common Mistakes:

• Using non-existent function writecsv
• Using csvwrite which is outdated
• Using writetable for numeric matrix without conversion

3. What will be the output of the following MATLAB code?

A = [1, 2; 3, 4];
writematrix(A, 'test.csv');
B = readmatrix('test.csv');
disp(B);

medium

A. [1 2; 3 4]

B. [1 2 3 4]

C. [1; 2; 3; 4]

D. Error: File not found

Solution

1. Step 1: Write matrix to CSV

   The matrix A is saved as a 2x2 CSV file with rows [1,2] and [3,4].

2. Step 2: Read matrix from CSV

   readmatrix reads the CSV back into the same 2x2 numeric matrix.

3. Final Answer:

   [1 2; 3 4] -> Option A

4. Quick Check:

   Write and read matrix preserves shape [OK]

Hint: writematrix and readmatrix keep matrix shape intact [OK]

Common Mistakes:

• Expecting a single row vector output
• Confusing disp output format with array shape
• Assuming file read error without saving first

4. Consider this MATLAB code snippet:

T = readtable('data.csv');
writetable(T, 'output.csv');

If data.csv contains mixed numeric and text data, what is a common error to watch for?

medium

A. Using csvwrite to write tables causes incorrect output

B. Using writetable on a numeric matrix causes syntax error

C. Forgetting to close the file after reading causes runtime error

D. Using readmatrix instead of readtable causes syntax error

Solution

1. Step 1: Understand data type in CSV

   Mixed numeric and text data require table format to preserve all data correctly.

2. Step 2: Identify correct write function

   csvwrite only supports numeric data and will fail or produce incorrect output for tables.

3. Final Answer:

   Using csvwrite to write tables causes incorrect output -> Option A

4. Quick Check:

   csvwrite not for tables = error [OK]

Hint: Use writetable for mixed data, not csvwrite [OK]

Common Mistakes:

• Using readmatrix for mixed data losing text
• Assuming file must be closed manually in MATLAB
• Using writetable on numeric matrix without conversion

5. You have a CSV file with columns: Name (text), Age (numeric), and Score (numeric). You want to read it, increase all Score values by 10%, and save the updated data back to a CSV file. Which MATLAB code snippet correctly does this?

hard

A. data = csvread('file.csv');
data(:,3) = data(:,3) * 1.1;
csvwrite('file.csv', data);

B. T = readtable('file.csv');
T.Score = T.Score * 1.1;
writetable(T, 'file.csv');

C. T = readtable('file.csv');
T(:,3) = T(:,3) * 1.1;
writetable(T, 'file.csv');

D. A = readmatrix('file.csv');
A(:,3) = A(:,3) * 1.1;
writematrix(A, 'file.csv');

Solution

1. Step 1: Read mixed data correctly

   Use readtable to read CSV with text and numeric columns preserving all data types.

2. Step 2: Update the numeric Score column

   Access the Score column by name and multiply by 1.1 to increase by 10%.

3. Step 3: Save updated table back to CSV

   Use writetable to save the modified table preserving text and numeric data.

4. Final Answer:

   T = readtable('file.csv'); T.Score = T.Score * 1.1; writetable(T, 'file.csv'); -> Option B

5. Quick Check:

   Mixed data update = readtable + modify column + writetable [OK]

Hint: Use readtable and writetable for mixed data updates [OK]

Common Mistakes:

• Using readmatrix loses text column
• Trying to multiply table columns by index incorrectly
• Using csvread/csvwrite which don't support text