MAT file save and load in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a MAT file in MATLAB?

A MAT file is a binary file format used by MATLAB to save variables and data so they can be loaded later. It stores workspace variables in a file with a .mat extension.

beginner

How do you save variables to a MAT file in MATLAB?

Use the save command. For example, save('filename.mat', 'var1', 'var2') saves variables var1 and var2 to the file named filename.mat.

beginner

How do you load variables from a MAT file in MATLAB?

Use the load command. For example, load('filename.mat') loads all variables saved in filename.mat into the workspace.

intermediate

What happens if you use load with an output variable like data = load('file.mat')?

The loaded variables are stored as fields inside the structure data. You can access variables with data.varname.

beginner

How can you save all workspace variables to a MAT file?

Use save('filename.mat') without specifying variable names. This saves all current workspace variables to the file.

Which command saves variables 'a' and 'b' to a MAT file named 'data.mat'?

A save('a', 'b', 'data.mat')

B load('data.mat', 'a', 'b')

C save('data.mat', 'a', 'b')

D load('a', 'b', 'data.mat')

The save command with the filename first and then variable names saves those variables to the MAT file.

What does load('file.mat') do in MATLAB?

A Loads variables from file.mat into the workspace

B Saves variables to file.mat

C Deletes file.mat

D Creates a new MAT file

The load command reads variables from the MAT file and puts them into the workspace.

If you want to load variables from a MAT file into a structure variable, which syntax is correct?

A save('file.mat', data)

B data = load('file.mat')

C load('file.mat', data)

D load data = 'file.mat'

Assigning the output of load to a variable stores the loaded variables as fields in that structure.

How do you save all current workspace variables to a MAT file named 'allvars.mat'?

A save('allvars.mat')

B save('allvars.mat', '*')

C load('allvars.mat')

D save allvars.mat

Calling save with only the filename saves all workspace variables to that MAT file.

What file extension do MATLAB MAT files use?

A .txt

B .matlab

C .m

D .mat

MAT files use the .mat extension to store MATLAB variables.

Explain how to save specific variables to a MAT file and then load them back into MATLAB workspace.

Think about the save and load commands and how you name variables.

Describe what happens when you load a MAT file into a structure variable and how to access the saved variables.

Consider the difference between loading directly and loading into 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