Multiple plots (hold on) in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB code with hold on?

Consider the following MATLAB code that plots two lines using hold on. What will be the final plot display?

MATLAB

x = 0:0.1:2*pi;
y1 = sin(x);
y2 = cos(x);
plot(x, y1, '-r');
hold on;
plot(x, y2, '-b');
hold off;

A Only the red sine wave is shown because hold on does not work without hold off.

B Only the blue cosine wave is shown because the second plot overwrites the first.

C A single plot showing both a red sine wave and a blue cosine wave.

D An error occurs because hold on and hold off cannot be used together.

💡 Hint

Think about what hold on does in MATLAB plotting.

The hold on command allows multiple plots to be drawn on the same figure without erasing the previous plots. So both sine and cosine waves appear together.

❓ Predict Output

intermediate

What happens if you omit hold on when plotting multiple lines?

Given this MATLAB code, what will the plot show?

MATLAB

x = 1:5;
y1 = x;
y2 = x.^2;
plot(x, y1, '-g');
plot(x, y2, '-m');

A Both green linear and magenta quadratic curves are shown on the same plot.

B Only the magenta quadratic curve is shown because the second plot overwrites the first.

C An error occurs because hold on is missing.

D The plot shows a green line and a magenta line overlapping exactly.

💡 Hint

Remember what happens when you call plot twice without hold on.

Without hold on, the second plot command clears the first plot, so only the second line appears.

🔧 Debug

advanced

Why does this MATLAB code plot only one line despite using hold on?

Examine the code below. It is intended to plot two lines on the same figure, but only one line appears. What is the cause?

MATLAB

x = 0:0.5:5;
plot(x, x, '-k');
hold on
plot(x, x.^2, '-r');
hold off;

A The code runs fine and both lines appear; the problem is elsewhere.

B Missing semicolon after <code>hold on</code> causes the second plot to overwrite the first.

C The first plot command is overwritten because <code>hold on</code> was not called before it.

D The <code>hold on</code> command is missing parentheses, so it is not executed properly.

💡 Hint

Check the syntax of the hold on command.

In MATLAB, hold on is a command and does not require parentheses, but missing a semicolon does not affect its execution. However, if hold on is typed without a space or misspelled, it won't work. Here, the code is correct and both lines should appear. The problem is that hold on is missing a semicolon but that does not cause the issue. The actual cause is that hold on missing parentheses is incorrect because hold on does not use parentheses. So the only plausible cause is that the user forgot to call hold on before the first plot, but here it is after the first plot, which is correct. So the problem is a trick: the code is correct and both lines appear. So the correct answer is D.

📝 Syntax

advanced

Which option causes a syntax error in MATLAB when using hold on?

Identify the code snippet that will cause a syntax error in MATLAB.

A plot(x, y1) hold on plot(x, y2) hold off

B plot(x, y1); hold on; plot(x, y2);

C hold on; plot(x, y1); plot(x, y2); hold off;

D plot(x, y1); hold on; plot(x, y2); hold off;

💡 Hint

Look for missing semicolons or missing command separators.

Option A lacks semicolons or commas between commands, causing MATLAB to raise a syntax error.

🚀 Application

expert

How many lines are visible after running this MATLAB code?

Consider this MATLAB code snippet:

x = 1:4;
plot(x, x, '-o');
hold on;
plot(x, x.^2, '-x');
hold off;
plot(x, x.^3, '-s');

How many lines will be visible on the final figure?

A Two lines: linear and quadratic only.

B Two lines: quadratic and cubic only.

C Three lines: linear, quadratic, and cubic.

D One line: cubic only.

💡 Hint

Remember what hold off does after plotting.

The first plot draws the linear line. hold on allows the quadratic line to be added. Then hold off disables holding, so the next plot command clears the figure and draws only the cubic line.

Practice

1. What does the hold on command do in MATLAB plotting?

easy

A. Clears the current figure before plotting new data.

B. Allows multiple plots to be drawn on the same figure without erasing previous plots.

C. Saves the current plot to a file.

D. Changes the color of the plot lines automatically.

Solution

1. Step 1: Understand the purpose of hold on

   The hold on command tells MATLAB to keep the current plot and add new plots on top of it.

2. Step 2: Compare with other options

   Other commands like hold off or default plotting clear the figure before new plots, so they do not allow multiple plots together.

3. Final Answer:

   Allows multiple plots to be drawn on the same figure without erasing previous plots. -> Option B

4. Quick Check:

   hold on = multiple plots on one figure [OK]

Hint: Remember: hold on means keep adding plots [OK]

Common Mistakes:

• Thinking hold on clears the figure
• Confusing hold on with saving plots
• Assuming hold on changes plot colors

2. Which of the following is the correct way to plot two lines on the same figure in MATLAB?

easy

A. plot(x1, y1); hold on; plot(x2, y2); hold off;

B. hold on; plot(x1, y1); hold off; plot(x2, y2);

C. plot(x1, y1); plot(x2, y2);

D. plot(x1, y1); hold off; plot(x2, y2);

Solution

1. Step 1: Identify the correct sequence to keep multiple plots

   Using hold on after the first plot keeps it visible for the second plot.

2. Step 2: Confirm proper use of hold off

   Calling hold off after plotting both lines resets plotting behavior for future plots.

3. Final Answer:

   plot(x1, y1); hold on; plot(x2, y2); hold off; -> Option A

4. Quick Check:

   Use hold on between plots to combine them [OK]

Hint: Use hold on right after first plot to add more [OK]

Common Mistakes:

• Not using hold on before second plot
• Using hold off too early
• Plotting without hold on clears previous plot

3. What will be the output of the following MATLAB code?

t = 0:0.1:1;
plot(t, t);
hold on;
plot(t, t.^2);

medium

A. Two plots will appear: a straight line and a curve on the same figure.

B. Only the plot of t vs t^2 will be shown.

C. An error will occur because hold on is used incorrectly.

D. Only the plot of t vs t will be shown.

Solution

1. Step 1: Analyze the first plot command

   The first plot draws a straight line y = t on the figure.

2. Step 2: Understand the effect of hold on and second plot

   Using hold on keeps the first plot visible, so the second plot y = t^2 is drawn on the same figure as a curve.

3. Final Answer:

   Two plots will appear: a straight line and a curve on the same figure. -> Option A

4. Quick Check:

   hold on keeps previous plot, so both appear [OK]

Hint: hold on keeps old plots visible for new ones [OK]

Common Mistakes:

• Thinking only last plot shows
• Assuming hold on causes error
• Confusing plot commands order

4. Identify the error in this MATLAB code that tries to plot two lines on the same figure:

plot(x, y1)
hold off
plot(x, y2)

medium

A. There is no error; the code works correctly.

B. The variable x is not defined.

C. hold off should be called before the first plot.

D. hold off should be replaced with hold on to keep both plots visible.

Solution

1. Step 1: Understand the effect of hold off

   Calling hold off resets plotting to clear the figure before the next plot.

2. Step 2: Identify correct command to keep plots

   To keep both plots visible, hold on should be used instead of hold off.

3. Final Answer:

   hold off should be replaced with hold on to keep both plots visible. -> Option D

4. Quick Check:

   Use hold on to combine plots, not hold off [OK]

Hint: hold off clears plots; use hold on to keep them [OK]

Common Mistakes:

• Using hold off instead of hold on
• Thinking hold off adds plots
• Ignoring hold commands altogether

5. You want to plot three different data sets on the same figure with different colors and then reset plotting to default for future plots. Which sequence of commands is correct?

hard

A. plot(x1, y1, 'r'); plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold on; hold off;

B. hold on; plot(x1, y1, 'r'); plot(x2, y2, 'g'); plot(x3, y3, 'b');

C. plot(x1, y1, 'r'); hold on; plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold off;

D. plot(x1, y1, 'r'); hold off; plot(x2, y2, 'g'); hold on; plot(x3, y3, 'b');

Solution

1. Step 1: Use hold on after first plot to combine multiple plots

   After plotting the first data set, hold on keeps the figure active for additional plots.

2. Step 2: Plot all data sets with specified colors and then reset with hold off

   Plotting the second and third data sets adds them to the same figure. Calling hold off resets plotting behavior for future plots.

3. Final Answer:

   plot(x1, y1, 'r'); hold on; plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold off; -> Option C

4. Quick Check:

   hold on after first plot, hold off after last plot [OK]

Hint: hold on after first plot, hold off after last [OK]

Common Mistakes:

• Not calling hold off after multiple plots
• Calling hold on before first plot
• Mixing hold on and hold off incorrectly