MAT file save and load in MATLAB - Step-by-Step Execution

Concept Flow - MAT file save and load

Create variables in workspace

↓

Use save('file.mat', 'var1', 'var2', ...)

↓

MAT file 'file.mat' created with variables

↓

Clear or close workspace

↓

Use load('file.mat')

↓

Variables restored in workspace

First, variables are saved into a MAT file. Later, loading the MAT file restores those variables back into the workspace.

Execution Sample

MATLAB

a = 10;
b = [1, 2, 3];
save('data.mat', 'a', 'b');
clear;
load('data.mat');

This code saves variables 'a' and 'b' into 'data.mat', clears the workspace, then loads them back.

Execution Table

Step	Action	Variables in Workspace	File Created/Loaded	Notes
1	Assign a=10	a=10	None	Variable 'a' created
2	Assign b=[1,2,3]	a=10, b=[1 2 3]	None	Variable 'b' created
3	save('data.mat', 'a', 'b')	a=10, b=[1 2 3]	'data.mat' created	Variables saved to file
4	clear	None	'data.mat' exists	Workspace cleared
5	load('data.mat')	a=10, b=[1 2 3]	'data.mat' loaded	Variables restored

💡 All variables saved and loaded successfully; workspace restored.

Variable Tracker

Variable	Start	After Save	After Clear	After Load
a	undefined	10	undefined	10
b	undefined	[1 2 3]	undefined	[1 2 3]

Key Moments - 2 Insights

Why do variables disappear after 'clear' but come back after 'load'?

The 'clear' command removes variables from the workspace (see step 4 in execution_table). The 'load' command reads the saved MAT file and restores those variables (step 5).

What happens if you save without specifying variable names?

If you use save('file.mat') without variable names, MATLAB saves all workspace variables at that moment.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What does the 'save' command do?

A Creates a MAT file with variables 'a' and 'b'

B Deletes variables 'a' and 'b'

C Loads variables from 'data.mat'

D Clears the workspace

💡 Hint

Check the 'File Created/Loaded' and 'Notes' columns at step 3 in execution_table.

At which step do variables 'a' and 'b' get removed from the workspace?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Variables in Workspace' column in execution_table for step 4.

If you skip the 'clear' command, what will be the workspace state after loading 'data.mat'?

A Variables 'a' and 'b' will be duplicated

B Variables 'a' and 'b' will be overwritten with saved values

C Loading will fail because variables already exist

D Workspace will be empty

💡 Hint

Consider how 'load' works when variables already exist in the workspace.

Concept Snapshot

MAT file save and load in MATLAB:
- Use save('file.mat', 'var1', 'var2') to save variables.
- Use load('file.mat') to restore variables.
- 'clear' removes variables from workspace.
- Loading restores saved variables.
- Saving without variable names saves all workspace variables.

Full Transcript

This visual execution shows how MATLAB saves variables into a MAT file and loads them back. First, variables 'a' and 'b' are created. Then, the save command writes them into 'data.mat'. After clearing the workspace, the load command restores these variables. The variable tracker shows how 'a' and 'b' exist before saving, disappear after clearing, and come back after loading. Key moments clarify why variables vanish after clear and how save/load work. The quiz tests understanding of each step in the process.

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