MAT file save and load in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after saving and loading variables?

Consider the following MATLAB code that saves variables to a MAT file and then loads them back. What will be the value of result after running all commands?

MATLAB

a = 5;
b = 10;
save('data.mat', 'a', 'b');
clear a b
load('data.mat');
result = a + b;

A Error: Undefined variable 'a'.

B 0

C 15

D 5

💡 Hint

Remember that save stores variables to a file and load restores them into the workspace.

The variables a and b are saved to 'data.mat'. After clearing them, load brings them back. So, result = a + b = 5 + 10 = 15.

❓ Predict Output

intermediate

What happens when loading a MAT file with a missing variable?

Given the following MATLAB commands, what will be the output of whos after loading?

MATLAB

x = 42;
save('file.mat', 'x');
clear x
load('file.mat', 'y');
vars = whos;

A Error: Variable 'y' not found in file.mat

B vars is empty (no variables loaded)

C vars contains variable 'y' with value 42

D vars contains variable 'x' with value 42

💡 Hint

Loading a variable name that does not exist in the MAT file does not load anything.

The MAT file contains only variable 'x'. Loading 'y' which does not exist results in no variables loaded. So vars is empty.

🔧 Debug

advanced

Why does this MAT file load fail with an error?

Examine the code below. It tries to save and load a variable but causes an error on load. What is the cause?

MATLAB

data = rand(3);
save('mydata.mat', 'data');
clear data
load('mydata.mat', 'Data');

A No error, variable 'Data' is loaded correctly

B Error because file 'mydata.mat' does not exist

C Error because 'clear data' removes the file

D Error because variable name 'Data' does not match saved variable 'data' (case sensitive)

💡 Hint

MATLAB variable names are case sensitive when loading specific variables.

The saved variable is named 'data' (lowercase). Loading 'Data' (uppercase D) fails because it does not exist in the file.

📝 Syntax

advanced

Which option correctly saves multiple variables to a MAT file?

Select the correct MATLAB command to save variables a, b, and c into a file named 'vars.mat'.

A save('vars.mat', 'a', 'b', 'c');

B save('vars.mat', a, b, c);

C save('vars.mat', [a b c]);

D save('vars.mat', {'a', 'b', 'c'});

💡 Hint

Variable names must be passed as strings to the save function.

The save function requires variable names as strings to save them. Option A correctly passes the names as strings.

🚀 Application

expert

How many variables are loaded from this MAT file?

Suppose a MAT file 'mixed.mat' contains variables x, y, and z. What is the number of variables in the workspace after running this code?

MATLAB

load('mixed.mat', 'x', 'z');
vars = whos;

A 2

B 1

C 3

D 0

💡 Hint

Only variables specified in the load command are loaded.

Only variables 'x' and 'z' are loaded, so vars contains 2 variables.

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