MAT file save and load in MATLAB - Mini Project: Build & Apply

MAT file save and load

📖 Scenario: You are working with MATLAB to save and load data for a small project. You want to store some variables in a MAT file and then load them back later to use.

🎯 Goal: Learn how to save variables to a MAT file and load them back in MATLAB.

📋 What You'll Learn

Create variables with exact values

Save variables to a MAT file named data.mat

Load variables from the MAT file

Display the loaded variables

💡 Why This Matters

🌍 Real World

Saving and loading data is common when working on projects that need to store results or share data between sessions.

💼 Career

MATLAB users in engineering, data science, and research often save and load data to manage experiments and analyses efficiently.

1

Create variables

Create a variable called age with the value 25 and a variable called name with the value 'Alice'.

MATLAB

% Your code here

Hint

Use = to assign values. Strings use single quotes in MATLAB.

2

Save variables to MAT file

Save the variables age and name to a MAT file named data.mat using the save command.

MATLAB

age = 25;
name = 'Alice';
% Your code here

Hint

Use save('filename', 'var1', 'var2') to save variables.

3

Clear variables and load from MAT file

Clear the variables age and name from the workspace using clear. Then load the variables back from data.mat using the load command.

MATLAB

age = 25;
name = 'Alice';
save('data.mat', 'age', 'name');
% Your code here

Hint

Use clear var1 var2 to remove variables. Use load('filename') to load variables.

4

Display loaded variables

Display the values of age and name using disp commands.

MATLAB

age = 25;
name = 'Alice';
save('data.mat', 'age', 'name');
clear age name;
load('data.mat');
% Your code here

Hint

Use disp(variable) to show the value of a variable.

Practice

1. What does the MATLAB command save('data.mat', 'x') do?

easy

A. Saves the variable x to a file named data.mat.

B. Loads the variable x from the file data.mat.

C. Deletes the variable x from the workspace.

D. Creates a new variable x with the name data.mat.

Solution

1. Step 1: Understand the save command

   The save command stores variables from the workspace into a MAT file.

2. Step 2: Analyze the given command

   save('data.mat', 'x') saves the variable x into the file named data.mat.

3. Final Answer:

   Saves the variable x to a file named data.mat. -> Option A

4. Quick Check:

   save stores variables [OK]

Hint: Remember: save stores, load retrieves variables [OK]

Common Mistakes:

• Confusing save with load command
• Thinking save deletes variables
• Assuming save creates variables

2. Which of the following is the correct syntax to load variables from a MAT file named results.mat?

easy

A. load('results.mat'); save;

B. load results.mat;

C. load = 'results.mat';

D. load('results.mat');

Solution

1. Step 1: Recall the syntax for loading MAT files

   The correct syntax to load variables from a MAT file is load('filename.mat').

2. Step 2: Check each option

   load('results.mat'); uses parentheses and quotes correctly: load('results.mat');. load results.mat; misses quotes, which causes an error. load = 'results.mat'; assigns a string to load, which is invalid. load('results.mat'); save; mixes load and save incorrectly.

3. Final Answer:

   load('results.mat'); -> Option D

4. Quick Check:

   Use parentheses and quotes with load [OK]

Hint: Use load('filename.mat') with quotes and parentheses [OK]

Common Mistakes:

• Omitting quotes around filename
• Using assignment instead of function call
• Mixing save and load commands

3. Consider the following MATLAB code:

x = 10; y = 20; save('mydata.mat', 'x'); clear; load('mydata.mat'); disp(y);

What will be the output?

medium

A. Error: Undefined function or variable 'y'.

B. 20

C. 10

D. No output

Solution

1. Step 1: Understand the save and clear commands

   The command save('mydata.mat', 'x') saves only variable x. The clear command removes all variables from workspace.

2. Step 2: Analyze the load and disp commands

   After load('mydata.mat'), only x is restored. Variable y is not loaded. So, disp(y) causes an error because y is undefined.

3. Final Answer:

   Error: Undefined function or variable 'y'. -> Option A

4. Quick Check:

   Only saved variables load back [OK]

Hint: Only saved variables are loaded back, others cause errors [OK]

Common Mistakes:

• Assuming all variables are saved by default
• Expecting cleared variables to still exist
• Confusing variable names in load

4. You run this MATLAB code:

save('data.mat', 'a', 'b')
clear a b
load('data.mat')

But after loading, variables a and b are missing. What is the likely problem?

medium

A. The clear command syntax is incorrect.

B. Variables a and b were never created before saving.

C. The load command is missing output arguments.

D. The file data.mat was not saved properly.

Solution

1. Step 1: Check if variables exist before saving

   If variables a and b do not exist before save, nothing is saved.

2. Step 2: Understand clear and load behavior

   clear a b removes variables a and b. Loading data.mat restores only saved variables. If none were saved, nothing loads.

3. Final Answer:

   Variables a and b were never created before saving. -> Option B

4. Quick Check:

   Save only stores existing variables [OK]

Hint: Create variables before saving to store them [OK]

Common Mistakes:

• Trying to save variables that don't exist
• Misusing clear command syntax
• Expecting load to create variables

5. You want to save multiple variables a, b, and c into a MAT file, but only if a is greater than 10. Which MATLAB code correctly does this?

hard

A. if a > 10
save('file.mat', 'a', 'b', 'c');
end

B. save('file.mat', 'a', 'b', 'c') if a > 10;

C. if (a > 10) save('file.mat', 'a', 'b', 'c'); end

D. save('file.mat', 'a', 'b', 'c') && (a > 10);

Solution

1. Step 1: Understand MATLAB if statement syntax

   MATLAB uses if condition ... end blocks. The condition must be on its own line or followed by a newline.

2. Step 2: Check each option's syntax

   if (a > 10) save('file.mat', 'a', 'b', 'c'); end uses correct one-line if syntax with parentheses and semicolon. save('file.mat', 'a', 'b', 'c') if a > 10; uses invalid inline syntax. if a > 10 save('file.mat', 'a', 'b', 'c'); end uses multiline if syntax but with newline characters which may not be valid in all contexts. save('file.mat', 'a', 'b', 'c') && (a > 10); uses invalid logical operator syntax.

3. Final Answer:

   if (a > 10) save('file.mat', 'a', 'b', 'c'); end -> Option C

4. Quick Check:

   Use proper if-end blocks or one-line if syntax in MATLAB [OK]

Hint: Use if ... end blocks or one-line if syntax for conditional save [OK]

Common Mistakes:

• Using invalid inline if syntax
• Mixing logical operators with commands
• Omitting end for if block