Animation basics in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple plot animation using a loop.

MATLAB

for k = 1:10
    plot(k, k^2, 'o');
    [1];
end

Drag options to blanks, or click blank then click option'

Agrid on

Bhold off

Cclf

Dpause(0.5)

Attempts:

3 left

💡 Hint

Common Mistakes

Using hold off inside the loop clears the plot.

Using clf clears the figure and removes previous points.

The pause(0.5) command pauses the loop for half a second, allowing the plot to update and create an animation effect.

2fill in blank

medium

Complete the code to keep all points visible during the animation.

MATLAB

figure;
hold [1];
for k = 1:10
    plot(k, k^2, 'o');
    pause(0.3);
end

Drag options to blanks, or click blank then click option'

Aon

Btoggle

Coff

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using hold off erases previous points each iteration.

Using hold on keeps all plotted points visible, so the animation shows all points accumulating.

3fill in blank

hard

Fix the error in the code to update the plot correctly inside the loop.

MATLAB

x = 0:0.1:2*pi;
y = sin(x);
for k = 1:length(x)
    plot(x(1:k), y(1:k));
    [1];
end

Drag options to blanks, or click blank then click option'

Ahold off

Bclf

Cdrawnow

Dpause(0.1)

Attempts:

3 left

💡 Hint

Common Mistakes

Using pause alone may slow animation but not force immediate redraw.

Using clf clears the figure and removes the plot.

The drawnow command forces MATLAB to update the figure window immediately, which is necessary for smooth animation.

4fill in blank

hard

Fill both blanks to create an animated plot that updates y-values in a loop.

MATLAB

x = linspace(0, 2*pi, 100);
fig = figure;
for k = 1:50
    y = sin(x + [1]);
    plot(x, y);
    [2];
end

Drag options to blanks, or click blank then click option'

Ak*0.1

Bpause(0.05)

Cdrawnow

Dk+1

Attempts:

3 left

💡 Hint

Common Mistakes

Using pause without drawnow may delay but not update plot immediately.

Using k+1 in the first blank does not create a smooth phase shift.

Adding k*0.1 shifts the sine wave over time. drawnow updates the plot immediately for animation.

5fill in blank

hard

Fill all three blanks to create a smooth animation of a moving point.

MATLAB

t = linspace(0, 4*pi, 200);
fig = figure;
for i = 1:length(t)
    x = cos(t(i));
    y = sin(t(i));
    plot(x, y, 'ro', 'MarkerSize', [1]);
    axis([-1.5 1.5 -1.5 1.5]);
    [2];
    [3];
end

Drag options to blanks, or click blank then click option'

A10

Bdrawnow

Cpause(0.02)

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using too large marker size makes the point hard to see clearly.

Omitting drawnow causes the plot not to update smoothly.

Omitting pause makes animation too fast to see.

Setting marker size to 5 makes the point visible but not too big. drawnow updates the plot immediately, and pause(0.02) slows the loop for smooth animation.

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