Why reading and writing data is fundamental in MATLAB - See It in Action

Why reading and writing data is fundamental

📖 Scenario: You are working on a simple project to manage student scores. You need to read scores from a file, process them, and save the results back to a file. This is a common task in many real-world projects where data comes from external sources and results need to be stored.

🎯 Goal: Build a MATLAB program that reads student scores from a file, calculates the average score, and writes the average to a new file.

📋 What You'll Learn

Read data from a text file called scores.txt

Calculate the average of the scores

Write the average score to a file called average.txt

💡 Why This Matters

🌍 Real World

Many real-world programs need to read data from files, process it, and save results. For example, analyzing sales data, processing sensor readings, or managing student grades.

💼 Career

Understanding file input/output is essential for data analysis, software development, and automation tasks in many jobs.

1

Create the data file scores.txt

Create a text file called scores.txt with these exact scores on separate lines: 85, 90, 78, 92, 88.

MATLAB

% Create the file 'scores.txt' with the given scores
% Your code here

Hint

Use fopen to open the file for writing, fprintf to write each score on a new line, and fclose to close the file.

2

Read the scores from scores.txt

Open the file scores.txt for reading and read all the scores into a variable called scores using fscanf.

MATLAB

fileID = fopen('scores.txt','w');
fprintf(fileID, '85\n90\n78\n92\n88\n');
fclose(fileID);

% Open 'scores.txt' for reading and read scores into 'scores'
% Your code here

Hint

Use fopen with 'r' to read, fscanf with '%d' to read integers, and fclose to close the file.

3

Calculate the average score

Calculate the average of the scores stored in the variable scores and save it in a variable called averageScore.

MATLAB

fileID = fopen('scores.txt','w');
fprintf(fileID, '85\n90\n78\n92\n88\n');
fclose(fileID);

fileID = fopen('scores.txt','r');
scores = fscanf(fileID, '%d');
fclose(fileID);

% Calculate the average score and store in 'averageScore'
% Your code here

Hint

Use the mean function to calculate the average of the vector scores.

4

Write the average score to average.txt

Open a new file called average.txt for writing and write the text 'Average score: ' followed by the value of averageScore using fprintf. Then close the file. Finally, print the exact text Average score: 86.6 to the MATLAB console.

MATLAB

fileID = fopen('scores.txt','w');
fprintf(fileID, '85\n90\n78\n92\n88\n');
fclose(fileID);

fileID = fopen('scores.txt','r');
scores = fscanf(fileID, '%d');
fclose(fileID);

averageScore = mean(scores);

% Write the average score to 'average.txt' and print it
% Your code here

Hint

Use fprintf with format 'Average score: %.1f' to write and print the average with one decimal place.

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