Animation basics in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB animation code?

Consider the following MATLAB code snippet that animates a moving point on a plot. What will be the final position of the point after the loop finishes?

MATLAB

figure;
h = plot(0,0,'ro','MarkerSize',10,'MarkerFaceColor','r');
xlim([0 10]); ylim([0 10]);
for k = 1:5
    set(h, 'XData', k, 'YData', k);
    pause(0.1);
end
finalX = get(h, 'XData');
finalY = get(h, 'YData');

A [5, 5]

B [1, 1]

C [10, 10]

D [0, 0]

💡 Hint

The loop moves the point from (1,1) to (5,5) step by step.

The loop updates the point's X and Y coordinates from 1 to 5. After the loop ends, the point is at (5,5).

🧠 Conceptual

intermediate

Which MATLAB function is essential to update graphics in an animation loop?

In MATLAB animations, which function forces the figure window to refresh and show updated graphics immediately inside a loop?

A clf

B pause

C refreshdata

D drawnow

💡 Hint

This function processes any pending callbacks and updates the figure window.

The drawnow function forces MATLAB to update the figure window immediately, which is essential for smooth animations.

🔧 Debug

advanced

Why does this MATLAB animation code freeze and not update the plot?

Examine the code below. It is supposed to animate a point moving along the x-axis, but the plot does not update during the loop. What is the cause?

MATLAB

figure;
h = plot(0,0,'bo','MarkerSize',8);
xlim([0 10]); ylim([0 1]);
for x = 1:10
    set(h, 'XData', x);
end

A The pause duration is too short to see updates

B The plot handle h is not valid

C Missing drawnow command inside the loop

D xlim and ylim are set incorrectly

💡 Hint

MATLAB needs a command to refresh the figure window during loops.

Without drawnow, MATLAB delays updating the figure until the loop ends, causing the animation to freeze.

📝 Syntax

advanced

Which option correctly creates a MATLAB animated line and adds points in a loop?

Select the code snippet that correctly creates an animated line and adds points to it inside a loop.

A

h = animatedline;
for k = 1:5
    addpoints(h, k, sin(k));
    pause(0.1);
end

B

h = animatedline();
for k = 1:5
    addpoints(h, k, sin(k));
    drawnow;
end

C

h = animatedline();
for k = 1:5
    addpoints(h, k, sin(k));
end
drawnow;

D

h = animatedline();
for k = 1:5
    addpoints(h, k, sin(k));
    refreshdata;
end

💡 Hint

The animated line needs drawnow inside the loop to update the plot.

Option B correctly creates the animated line, adds points, and calls drawnow inside the loop for smooth animation.

🚀 Application

expert

How many frames will this MATLAB animation produce?

Given the code below, how many frames will be displayed in the animation?

MATLAB

figure;
h = plot(0,0,'ko','MarkerSize',12);
xlim([0 20]); ylim([0 20]);
for i = 1:10
    for j = 1:5
        set(h, 'XData', i*j, 'YData', i+j);
        drawnow;
        pause(0.05);
    end
end

A 50

B 15

C 10

D 5

💡 Hint

Count how many times the inner loop runs and how many times the outer loop runs.

The outer loop runs 10 times and the inner loop runs 5 times each, so total frames = 10 * 5 = 50.

Practice

1. What is the main purpose of using drawnow in a MATLAB animation loop?

easy

A. To update the figure window and show the latest plot changes immediately

B. To pause the animation for a fixed time

C. To clear the figure before plotting new data

D. To set the axis limits for the plot

Solution

1. Step 1: Understand the role of drawnow

   drawnow forces MATLAB to update the figure window immediately with any changes made to the plot.

2. Step 2: Differentiate from other commands

   Unlike pause which delays execution, drawnow just refreshes the display without stopping the code.

3. Final Answer:

   To update the figure window and show the latest plot changes immediately -> Option A

4. Quick Check:

   drawnow updates display immediately [OK]

Hint: Remember: drawnow refreshes the plot instantly [OK]

Common Mistakes:

• Confusing drawnow with pause
• Thinking drawnow clears the plot
• Using drawnow to set axis limits

2. Which of the following is the correct syntax to create a simple animation loop in MATLAB?

easy

A. loop i in 1:10 { plot(i, i^2); pause(0.5); drawnow; }

B. for i = 1 to 10; plot(i, i^2); pause(0.5); drawnow; end

C. for i = 1:10; plot(i, i^2); pause(0.5); drawnow; end

D. while i <= 10; plot(i, i^2); pause(0.5); drawnow; i++; end

Solution

1. Step 1: Check MATLAB loop syntax

   MATLAB uses for variable = range syntax, so for i = 1:10 is correct.

2. Step 2: Verify loop body and commands

   The commands plot, pause, and drawnow are correctly placed and separated by semicolons.

3. Final Answer:

   for i = 1:10; plot(i, i^2); pause(0.5); drawnow; end -> Option C

4. Quick Check:

   MATLAB for-loop uses '=' and ':' [OK]

Hint: MATLAB loops use 'for var = range' syntax [OK]

Common Mistakes:

• Using 'for i = 1 to 10' which is invalid
• Using curly braces {} for loops
• Using i++ which is not MATLAB syntax

3. What will be the visible effect of running this MATLAB code?

figure;
axis([0 10 0 100]);
for x = 1:10
    y = x^2;
    plot(x, y, 'ro');
    drawnow;
    pause(0.3);
end

medium

A. Only the last red dot at (10,100) is visible

B. All red dots appear at once after the loop finishes

C. The plot window stays blank with no dots

D. Red dots appear one by one at points (1,1), (2,4), ..., (10,100)

Solution

1. Step 1: Understand the loop plotting behavior

   Each iteration plots a red circle at (x, x^2) and calls drawnow to update the figure immediately.

2. Step 2: Effect of axis limits and pause

   Axis limits prevent resizing, and pause(0.3) delays so dots appear one by one.

3. Final Answer:

   Red dots appear one by one at points (1,1), (2,4), ..., (10,100) -> Option D

4. Quick Check:

   drawnow + pause shows points sequentially [OK]

Hint: drawnow + pause shows animation step-by-step [OK]

Common Mistakes:

• Thinking all points appear at once
• Assuming plot clears previous points
• Ignoring axis limits effect

4. Identify the error in this MATLAB animation code:

for t = 1:5
    plot(t, t^2, 'bo');
    pause(0.5);
end

medium

A. Missing drawnow to update the plot window

B. Incorrect plot marker symbol 'bo'

C. Loop variable t should start from 0

D. Pause duration is too short for animation

Solution

1. Step 1: Check for plot update command

   The code plots points but does not call drawnow, so the figure may not update during the loop.

2. Step 2: Understand effect of missing drawnow

   Without drawnow, MATLAB delays updating the figure until the loop ends, so animation won't show step-by-step.

3. Final Answer:

   Missing drawnow to update the plot window -> Option A

4. Quick Check:

   Always use drawnow to refresh animation frames [OK]

Hint: Use drawnow inside loops to refresh plots [OK]

Common Mistakes:

• Thinking pause alone updates the plot
• Changing loop start without reason
• Confusing marker symbols with errors

5. You want to animate a moving sine wave in MATLAB without flickering or resizing. Which approach is best?

hard

A. Clear figure inside loop, plot new sine wave, use pause only

B. Set fixed axis limits before the loop, update plot data inside loop, use drawnow and pause

C. Plot sine wave once outside loop, then update axis limits inside loop

D. Use pause inside loop without drawnow or axis limits

Solution

1. Step 1: Prevent flickering by fixing axis limits

   Setting axis limits before the loop stops MATLAB from resizing the plot each frame, avoiding flicker.

2. Step 2: Update plot data and refresh display properly

   Update the sine wave data inside the loop, then call drawnow to refresh and pause to control speed.

3. Final Answer:

   Set fixed axis limits before the loop, update plot data inside loop, use drawnow and pause -> Option B

4. Quick Check:

   Fixed axis + drawnow + pause = smooth animation [OK]

Hint: Fix axis limits before loop to avoid flicker [OK]

Common Mistakes:

• Clearing figure each loop causing flicker
• Not fixing axis limits causing resizing
• Using pause without drawnow