Scope and display blocks in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested function variable scope

What is the output of the following MATLAB code?

MATLAB

function main
    x = 5;
    function nested()
        x = 10;
        disp(x)
    end
    nested();
    disp(x)
end

main

A 10\n5

B 5\n10

C 10\n10

D 5\n5

💡 Hint

Remember that nested functions share variables with their enclosing functions by default.

The nested function has its own local variable x which shadows the outer x. Assigning x = 10 inside the nested function does not modify the outer x. Therefore, the first disp(x) shows 10, and the second shows 5.

❓ Predict Output

intermediate

Display block output with loop variable

What will be displayed when running this MATLAB script?

MATLAB

for i = 1:3
    disp(i)
end

A Error: disp cannot display multiple values

B 3

C 1 2 3

D 1\n2\n3

💡 Hint

The disp function prints each value on a new line.

The loop runs from 1 to 3, and disp(i) prints each number on its own line.

❓ Predict Output

advanced

Effect of clear inside a function on variable scope

What is the output of this MATLAB code?

MATLAB

function testClear
    a = 7;
    disp(a)
    clear a
    disp(a)
end

testClear

A Error: clear cannot be used inside functions

B 7\n7

C 7\nError: Undefined function or variable 'a'.

D 7\n0

💡 Hint

Think about what happens when you clear a variable inside a function.

After clearing a, it no longer exists in the function workspace, so the second disp(a) causes an error.

❓ Predict Output

advanced

Output of nested function modifying outer variable with 'global'

What will be the output of this MATLAB code?

MATLAB

global x
x = 3;
function outer()
    global x
    x = 5;
    function inner()
        global x
        x = 10;
        disp(x)
    end
    inner();
    disp(x)
end
outer();
disp(x)

A 10\n5\n3

B 10\n10\n10

C 5\n10\n3

D Error: Nested functions cannot access global variables

💡 Hint

Global variables are shared across functions and nested functions.

All functions access the same global x. The inner function sets it to 10, which is displayed three times.

🧠 Conceptual

expert

Understanding scope and display blocks in MATLAB scripts

Consider a MATLAB script that defines a variable a = 1 in the base workspace, then calls a function that also defines a = 2 and displays it. After the function call, the script displays a. Which statement is true?

A The function's <code>a</code> is local and does not affect the base workspace; the script displays 1 after the function call.

B The function's <code>a</code> overwrites the base workspace variable; the script displays 2 after the function call.

C The script will error because <code>a</code> is redefined inside the function.

D Both variables <code>a</code> are shared and display 2 everywhere after the function call.

💡 Hint

Think about how MATLAB handles variable scope between base workspace and functions.

Variables inside functions have their own workspace and do not affect variables in the base workspace unless explicitly returned or declared global.

Practice

1. What is the main purpose of a Scope block in Simulink?

easy

A. To show numeric values in the workspace

B. To display signal graphs during simulation

C. To save simulation data to a file

D. To generate code from the model

Solution

1. Step 1: Understand the function of Scope blocks

   Scope blocks are used to visualize signals as graphs during simulation.

2. Step 2: Compare with other options

   Other options describe different functionalities not related to Scope blocks.

3. Final Answer:

   To display signal graphs during simulation -> Option B

4. Quick Check:

   Scope = signal graphs [OK]

Hint: Scope shows graphs, Display shows numbers [OK]

Common Mistakes:

• Confusing Scope with Display block
• Thinking Scope saves data
• Assuming Scope shows numeric values

2. Which of the following is the correct way to add a Display block in Simulink?

easy

A. Drag the Display block from the Simulink library to the model

B. Write display() in the MATLAB command window

C. Use the Scope block and rename it to Display

D. Add a To Workspace block and rename it Display

Solution

1. Step 1: Identify how to add blocks in Simulink

   Blocks are added by dragging them from the Simulink library browser into the model.

2. Step 2: Check other options

   Options A, B, and D describe incorrect or unrelated methods.

3. Final Answer:

   Drag the Display block from the Simulink library to the model -> Option A

4. Quick Check:

   Add blocks by dragging from library [OK]

Hint: Add blocks by dragging from library browser [OK]

Common Mistakes:

• Trying to add blocks via MATLAB commands
• Renaming blocks instead of adding correct ones
• Confusing Display with To Workspace block

3. Consider this Simulink model snippet: a sine wave connected to both a Scope and a Display block. What will you see during simulation?

medium

A. Both Scope and Display show numeric values

B. The Scope shows numeric values; the Display shows a graph

C. The Scope shows a graph of the sine wave; the Display shows the current numeric value

D. Neither block shows any output

Solution

1. Step 1: Understand Scope and Display outputs

   Scope shows signal graphs; Display shows current numeric values during simulation.

2. Step 2: Apply to sine wave input

   The sine wave signal will appear as a graph on Scope and as changing numbers on Display.

3. Final Answer:

   The Scope shows a graph of the sine wave; the Display shows the current numeric value -> Option C

4. Quick Check:

   Scope = graph, Display = number [OK]

Hint: Scope graphs signals; Display shows live numbers [OK]

Common Mistakes:

• Mixing up graph and numeric display
• Expecting Display to show full signal history
• Thinking Scope shows only numbers

4. You added a Scope block but it shows no signal during simulation. What is the most likely cause?

medium

A. The model has no input blocks

B. The Display block is missing

C. The simulation is paused

D. The Scope block is not connected to any signal

Solution

1. Step 1: Check Scope connection

   Scope must be connected to a signal line to display data.

2. Step 2: Consider other options

   Display block absence does not affect Scope; simulation paused would freeze output; no input blocks means no signals.

3. Final Answer:

   The Scope block is not connected to any signal -> Option D

4. Quick Check:

   Scope needs signal connection [OK]

Hint: Scope must connect to signal to show output [OK]

Common Mistakes:

• Assuming Display block affects Scope output
• Not checking signal connections
• Ignoring simulation state

5. You want to monitor multiple signals simultaneously during simulation and see both their graphs and numeric values updated live. Which combination of blocks should you use?

hard

A. Multiple Scope blocks and multiple Display blocks, each connected to one signal

B. One Scope block connected to all signals and one Display block connected to all signals

C. One Display block connected to all signals only

D. Multiple Display blocks connected to one signal each, no Scope blocks

Solution

1. Step 1: Understand monitoring multiple signals

   Each Scope or Display block connects to one signal to show its graph or numeric value.

2. Step 2: Evaluate options

   One Scope or Display cannot connect to multiple signals directly; multiple blocks are needed.

3. Final Answer:

   Multiple Scope blocks and multiple Display blocks, each connected to one signal -> Option A

4. Quick Check:

   One block per signal for graph and number [OK]

Hint: Use one block per signal to monitor multiple signals [OK]

Common Mistakes:

• Trying to connect one block to many signals
• Ignoring need for multiple displays
• Confusing Scope and Display capabilities