Why reading and writing data is fundamental in MATLAB - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB code reading a file?

Consider the following MATLAB code that reads data from a text file named data.txt containing numbers:

fid = fopen('data.txt', 'r');
data = fscanf(fid, '%d');
fclose(fid);
disp(data);

If data.txt contains the numbers 10 20 30 40 separated by spaces, what will be displayed?

MATLAB

fid = fopen('data.txt', 'r');
data = fscanf(fid, '%d');
fclose(fid);
disp(data);

A [10 20 30 40]

B [10 20 30 40 0]

C Error: File not found

D [10; 20; 30; 40]

💡 Hint

Remember that fscanf reads data column-wise and returns a column vector by default.

The fscanf function reads the numbers as integers and returns a column vector, so the output is a 4x1 vector displayed as:

10
20
30
40

❓ Predict Output

intermediate

What happens when writing data to a file in MATLAB?

Given this MATLAB code snippet:

fid = fopen('output.txt', 'w');
fprintf(fid, '%0.2f\n', [1.234, 5.678, 9.1011]);
fclose(fid);

What will be the content of output.txt after running this code?

MATLAB

fid = fopen('output.txt', 'w');
fprintf(fid, '%0.2f\n', [1.234, 5.678, 9.1011]);
fclose(fid);

A 1.23\n5.68\n9.10\n

B 1.234 5.678 9.1011

C 1.23 5.68 9.10

D Error: Invalid format specifier

💡 Hint

Look at the format string and how fprintf writes each number.

The format %0.2f\n writes each number with two decimals followed by a newline, so the file will have three lines:

1.23
5.68
9.10

❓ Predict Output

advanced

What is the output of this MATLAB code reading mixed data?

Analyze this MATLAB code snippet:

fid = fopen('mixed.txt', 'r');
data = textscan(fid, '%s %f');
fclose(fid);
disp(data{2});

If mixed.txt contains:

apple 10
banana 20
cherry 30

What will be displayed?

MATLAB

fid = fopen('mixed.txt', 'r');
data = textscan(fid, '%s %f');
fclose(fid);
disp(data{2});

A ['apple'; 'banana'; 'cherry']

B [10; 20; 30]

C ["apple", "banana", "cherry"]

D Error: Invalid format specifier

💡 Hint

textscan returns a cell array; data{2} accesses the numeric column.

The second element of the cell array data{2} contains the numeric values as a column vector, so the output is:

10
20
30

❓ Predict Output

advanced

What error does this MATLAB code produce when reading a non-existent file?

Consider this MATLAB code:

fid = fopen('nofile.txt', 'r');
data = fscanf(fid, '%d');
fclose(fid);

What error or output will occur?

MATLAB

fid = fopen('nofile.txt', 'r');
data = fscanf(fid, '%d');
fclose(fid);

A fid equals -1, fscanf returns empty, fclose runs without error

B fid equals -1, fscanf returns empty, fclose errors

C fid equals -1, fscanf errors, fclose errors

D Error: File not found

💡 Hint

Check what fopen returns when the file does not exist and how fscanf and fclose behave.

fopen returns -1 if the file does not exist. fscanf returns empty without error. fclose(-1) returns -1 but does not throw an error. So no error is thrown, but fid is -1 and data is empty.

🧠 Conceptual

expert

Why is reading and writing data fundamental in programming?

Which of the following best explains why reading and writing data is fundamental in programming?

A Because data reading and writing automatically optimizes program speed.

B Because reading and writing data is the only way to create variables in memory.

C Because programs need to interact with external information sources to be useful and persistent.

D Because all programming languages require manual file handling to run any code.

💡 Hint

Think about why programs need input and output beyond just running code.

Reading and writing data allows programs to get input from users or other systems and save results for later use, making programs practical and meaningful.

Practice

1. Why is reading and writing data important in MATLAB programming?

easy

A. It automatically fixes syntax errors.

B. It allows programs to save and load information from files.

C. It changes the color of the MATLAB interface.

D. It speeds up the execution of loops.

Solution

1. Step 1: Understand the role of file operations

   Reading and writing data lets MATLAB programs interact with files to save or load data.

2. Step 2: Identify the correct purpose

   Only It allows programs to save and load information from files. correctly states this purpose; others describe unrelated features.

3. Final Answer:

   It allows programs to save and load information from files. -> Option B

4. Quick Check:

   Reading and writing data = save/load files [OK]

Hint: Remember: reading/writing connects program to files [OK]

Common Mistakes:

• Confusing file operations with interface changes
• Thinking reading/writing speeds up loops
• Assuming it fixes code errors automatically

2. Which of the following is the correct way to open a file for reading in MATLAB?

easy

A. fileID = fopen('data.txt', 'w');

B. fileID = open('data.txt', 'read');

C. fileID = fread('data.txt');

D. fileID = fopen('data.txt', 'r');

Solution

1. Step 1: Recall fopen syntax for reading

   In MATLAB, fopen with mode 'r' opens a file for reading.

2. Step 2: Check each option

   fileID = fopen('data.txt', 'r'); uses correct syntax; B uses a non-existent function; C uses fread which reads data but does not open; D opens for writing, not reading.

3. Final Answer:

   fileID = fopen('data.txt', 'r'); -> Option D

4. Quick Check:

   fopen with 'r' = open for reading [OK]

Hint: Use fopen with 'r' to open files for reading [OK]

Common Mistakes:

• Using fopen with wrong mode like 'w' for reading
• Confusing fopen with fread
• Using non-existent open function

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

fileID = fopen('testfile.txt', 'w');
fwrite(fileID, 'Hello');
fclose(fileID);
fileID = fopen('testfile.txt', 'r');
data = fread(fileID, '*char')';
fclose(fileID);
disp(data);

medium

A. Error: file not found

B. testfile.txt

C. Hello

D. Empty output

Solution

1. Step 1: Write 'Hello' to file

   The code opens 'testfile.txt' for writing and writes the string 'Hello'.

2. Step 2: Read the file content and display

   It reopens the file for reading, reads the characters, and displays them.

3. Final Answer:

   Hello -> Option C

4. Quick Check:

   Write 'Hello' then read and display = Hello [OK]

Hint: Write then read file to see saved content [OK]

Common Mistakes:

• Forgetting to close file before reading
• Reading without specifying '*char' for characters
• Assuming file is empty after writing

4. Identify the error in this MATLAB code snippet for reading a file:

fileID = fopen('data.txt', 'r');
data = fread(fileID);
fclose(fileID);
disp(data);

medium

A. fread missing data type specifier

B. Missing file open mode

C. fclose called before fread

D. disp cannot display data

Solution

1. Step 1: Check fread usage

   fread without specifying data type returns numeric bytes, not readable characters.

2. Step 2: Identify correct usage

   To read text, fread should include '*char' to read characters properly.

3. Final Answer:

   fread missing data type specifier -> Option A

4. Quick Check:

   fread needs '*char' for text reading [OK]

Hint: Always specify data type in fread for correct reading [OK]

Common Mistakes:

• Not specifying '*char' in fread for text files
• Closing file before reading
• Assuming disp can't show numeric arrays

5. You want to save a matrix to a file and then load it back in MATLAB. Which sequence of commands correctly achieves this?

hard

A. fileID = fopen('matrix.dat', 'w'); fwrite(fileID, matrix, 'double'); fclose(fileID); fileID = fopen('matrix.dat', 'r'); matrix2 = fread(fileID, [size(matrix,1), size(matrix,2)], 'double'); fclose(fileID);

B. fopen('matrix.dat', 'w'); fwrite(fileID, matrix); fclose(fileID); fileID = fopen('matrix.dat', 'r'); matrix2 = fread(fileID, [size(matrix,1), size(matrix,2)], 'double'); fclose(fileID);

C. fileID = fopen('matrix.dat', 'r'); fwrite(fileID, matrix); fclose(fileID); fileID = fopen('matrix.dat', 'w'); matrix2 = fread(fileID); fclose(fileID);

D. fileID = fopen('matrix.dat', 'w'); fwrite(matrix); fclose(fileID); fileID = fopen('matrix.dat', 'r'); matrix2 = fread(fileID); fclose(fileID);

Solution

1. Step 1: Open file for writing and write matrix with type

   Open with fileID = fopen('matrix.dat', 'w'); then fwrite(fileID, matrix, 'double'); fclose(fileID). Specify data type 'double'.

2. Step 2: Open file for reading and read matrix with size and type

   Open with fileID = fopen('matrix.dat', 'r'); then matrix2 = fread(fileID, [size(matrix,1), size(matrix,2)], 'double'); fclose(fileID).

3. Final Answer:

   fileID = fopen('matrix.dat', 'w'); fwrite(fileID, matrix, 'double'); fclose(fileID); fileID = fopen('matrix.dat', 'r'); matrix2 = fread(fileID, [size(matrix,1), size(matrix,2)], 'double'); fclose(fileID); -> Option A

4. Quick Check:

   Open-write-close, then open-read-close with types = correct [OK]

Hint: Always specify fileID and data type when reading/writing matrices [OK]

Common Mistakes:

• Not assigning fopen result to fileID
• Using wrong file modes for reading/writing
• Omitting data type in fwrite or fread
• Calling fwrite without fileID