Workspace and variable management in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB code snippet?

Consider the following MATLAB commands executed in order:

clear all
x = 5;
y = 10;
clear x;
z = exist('x', 'var');

What is the value of z after running this code?

MATLAB

clear all
x = 5;
y = 10;
clear x;
z = exist('x', 'var');

A 3

B 0

C 2

D 1

💡 Hint

Recall that clear x removes variable x from the workspace.

After clear x, variable x no longer exists in the workspace. The exist function returns 0 when the variable is not found.

❓ Predict Output

intermediate

How many variables remain after this code runs?

Given the following MATLAB commands:

a = 1;
b = 2;
c = 3;
clearvars -except a c;

How many variables are in the workspace after these commands?

MATLAB

a = 1;
b = 2;
c = 3;
clearvars -except a c;

A 2

B 1

C 3

D 0

💡 Hint

The clearvars -except command keeps only specified variables.

Only variables a and c remain after clearing all except these two.

🔧 Debug

advanced

Identify the error in this workspace management code

What error will this MATLAB code produce?

clear all
x = 10;
if exist('x')
    disp('x exists');
end

MATLAB

clear all
x = 10;
if exist('x')
    disp('x exists');
end

A No error, prints 'x exists'

B Syntax error: missing second argument in exist

C Runtime error: undefined variable 'x'

D Logical error: exist returns 0 even if variable exists

💡 Hint

exist('x') without a second argument is valid and returns 1 for variables in the workspace.

No error occurs. exist('x') returns 1 because x is a variable in the workspace, so 'x exists' is printed.

❓ Predict Output

advanced

What is the output of this variable clearing sequence?

Consider this MATLAB code:

a = 1;
b = 2;
c = 3;
clear a b
vars = who;

What does vars contain after running this code?

MATLAB

a = 1;
b = 2;
c = 3;
clear a b
vars = who;

A {}

B {'a', 'b', 'c'}

C {'a', 'b'}

D {'c'}

💡 Hint

The clear command removes specified variables from the workspace.

After clearing a and b, only c remains. The who function lists current variables.

🧠 Conceptual

expert

Which command will save only variables starting with 'data' to a file?

You have variables data1, data2, temp in your workspace. Which MATLAB command saves only variables starting with 'data' to myfile.mat?

A save('myfile.mat', '-except', 'temp')

B save('myfile.mat', 'data1', 'data2')

C save('myfile.mat', '-regexp', '^data')

D save('myfile.mat', 'data*')

💡 Hint

Use the -regexp option to save variables matching a pattern.

Option C uses -regexp '^data' to save all variables starting with 'data'. Option C is invalid syntax. Option C requires listing all variables explicitly. Option C is invalid because -except is not a valid save option.

Practice

1. What does the clear command do in MATLAB workspace?

easy

A. Removes variables from the workspace

B. Displays all variables in the workspace

C. Saves variables to a file

D. Loads variables from a file

Solution

1. Step 1: Understand the purpose of clear

   The clear command is used to remove variables from the workspace, freeing memory.

2. Step 2: Differentiate from other commands

   who and whos display variables, save saves variables, and load loads variables.

3. Final Answer:

   Removes variables from the workspace -> Option A

4. Quick Check:

   clear removes variables [OK]

Hint: Clear removes variables, not shows or saves [OK]

Common Mistakes:

• Confusing clear with who or whos
• Thinking clear saves variables
• Assuming clear loads variables

2. Which of the following is the correct syntax to list all variables in the workspace with details?

easy

A. listvars

B. whos

C. showvars()

D. vars()

Solution

1. Step 1: Recall commands to list variables

   who lists variable names; whos lists variables with details like size and type.

2. Step 2: Identify correct syntax

   whos is the correct command; others are invalid or do not exist in MATLAB.

3. Final Answer:

   whos -> Option B

4. Quick Check:

   whos lists variables with details [OK]

Hint: Use whos to see detailed variable info [OK]

Common Mistakes:

• Using non-existent commands like listvars
• Confusing who with whos for details
• Adding parentheses to whos (it's not a function)

3. Consider the following MATLAB commands:

a = 5;
b = 10;
clear a;
whos

What variables will whos display?

medium

A. No variables

B. Only a

C. a and b

D. Only b

Solution

1. Step 1: Analyze the commands

   Variables a and b are created. Then clear a removes a from workspace.

2. Step 2: Determine remaining variables

   After clearing a, only b remains. whos will show b only.

3. Final Answer:

   Only b -> Option D

4. Quick Check:

   clear a removes a, so only b remains [OK]

Hint: Clear specific variable removes only that variable [OK]

Common Mistakes:

• Assuming clear removes all variables
• Thinking clear a clears variable b too
• Expecting whos to show cleared variables

4. What is wrong with this MATLAB code snippet?

clear x y
x = 10;
load('data.mat')

Assuming data.mat contains variable y with value 5.

medium

A. clear cannot take multiple variables

B. No error, code works as expected

C. load overwrites variable y after clear

D. Variable x is cleared after load

Solution

1. Step 1: Understand clear and load behavior

   clear x y removes variables x and y. Then x is assigned 10.

2. Step 2: Effect of load on variables

   load('data.mat') loads variable y from file, overwriting any existing y. Since y was cleared, it is now restored from file.

3. Step 3: Identify the issue

   The variable y is overwritten by load, which may be unexpected if user thought clear removed it permanently.

4. Final Answer:

   load overwrites variable y after clear -> Option C

5. Quick Check:

   load restores variables from file, overwriting workspace [OK]

Hint: Load restores variables, overwriting existing ones [OK]

Common Mistakes:

• Thinking clear cannot clear multiple variables
• Assuming load does not overwrite variables
• Believing x is cleared after load

5. You want to save only variables a and b from your workspace to a file named myvars.mat. Which command should you use?

hard

A. save myvars.mat a b

B. save('myvars.mat', 'a', b)

C. save('myvars.mat')

D. save('myvars.mat', a, b)

Solution

1. Step 1: Recall save syntax for specific variables

   To save specific variables, list their names without quotes after the filename without parentheses.

2. Step 2: Analyze options

   save myvars.mat a b uses correct syntax: save myvars.mat a b. save('myvars.mat', 'a', b) uses quotes but parentheses, which is invalid syntax in MATLAB. save('myvars.mat') saves all variables. save('myvars.mat', a, b) passes variables as values, which is incorrect.

3. Final Answer:

   save myvars.mat a b -> Option A

4. Quick Check:

   Use save filename var1 var2 to save specific variables [OK]

Hint: Use save filename var1 var2 without parentheses [OK]

Common Mistakes:

• Using parentheses with save command
• Putting variable names in quotes inside parentheses
• Saving all variables unintentionally