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.