Subplot for multiple panels in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this MATLAB subplot code?

Consider the following MATLAB code that creates subplots and plots simple lines. What will be the output displayed?

MATLAB

x = 1:5;
y = x.^2;
subplot(2,1,1);
plot(x,y);
title('Top plot');
subplot(2,1,2);
plot(y,x);
title('Bottom plot');

A Two plots stacked vertically: top plot shows y = x^2, bottom plot shows x versus y

B Two plots stacked vertically: top plot shows y = x^2, bottom plot shows x vs y (same as top plot)

C One plot only showing y = x^2, bottom plot is empty

D Error: subplot indices exceed matrix dimensions

💡 Hint

Remember that subplot(m,n,p) divides the figure into an m-by-n grid and activates the p-th subplot.

The code creates a 2-row, 1-column grid of subplots. The first subplot (top) plots y = x^2. The second subplot (bottom) plots x versus y, which is the inverse relationship. Both plots appear stacked vertically.

❓ Predict Output

intermediate

What happens if subplot indices overlap?

What will be the result of running this MATLAB code?

MATLAB

x = 0:0.1:2*pi;
y1 = sin(x);
y2 = cos(x);
subplot(2,2,1);
plot(x,y1);
title('Sine');
subplot(2,2,1);
plot(x,y2);
title('Cosine');

A Two separate subplots appear side by side

B Both sine and cosine plots appear in the first subplot layered

C Error: subplot index repeated

D Only the cosine plot is visible in the first subplot; sine plot is overwritten

💡 Hint

Calling subplot with the same index activates the same panel, so new plots overwrite previous ones.

The second subplot(2,2,1) call activates the same subplot as the first. The cosine plot overwrites the sine plot, so only cosine is visible.

🔧 Debug

advanced

Why does this subplot code produce an error?

Examine the MATLAB code below. It is intended to create a 3x1 subplot and plot data in each panel. Why does it produce an error?

MATLAB

x = 1:10;
for i = 1:4
  subplot(3,1,i);
  plot(x, x*i);
  title(['Plot ' num2str(i)]);
end

A Error because subplot index 4 exceeds the 3x1 grid size

B Error because loop variable i is not valid for subplot

C No error; plots appear in first three subplots, fourth plot ignored

D Error because plot command syntax is incorrect

💡 Hint

Check the maximum subplot index allowed for a 3x1 grid.

The subplot grid has 3 panels (indices 1 to 3). Using index 4 causes an error because it is outside the grid.

🚀 Application

advanced

How to create a 2x2 grid with one large subplot?

You want to create a 2x2 subplot grid in MATLAB where the first subplot spans the entire top row, and the other two subplots are below it side by side. Which code achieves this layout?

A subplot(2,2,1); plot(...); subplot(2,2,2); plot(...); subplot(2,2,3); plot(...);

B subplot(2,2,[1 2]); plot(...); subplot(2,2,3); plot(...); subplot(2,2,4); plot(...);

C subplot(3,2,1); plot(...); subplot(3,2,3); plot(...); subplot(3,2,4); plot(...);

D subplot(1,3,1); plot(...); subplot(1,3,2); plot(...); subplot(1,3,3); plot(...);

💡 Hint

Use vector indexing in subplot to span multiple grid cells.

Using subplot(2,2,[1 2]) merges the first and second subplot positions into one large subplot spanning the top row. The other two subplots fill the bottom row.

🧠 Conceptual

expert

How does MATLAB handle subplot handles when reusing panels?

In MATLAB, when you create subplots and save their handles, what happens if you call subplot again with an existing panel index? How does this affect the handle and the plot?

A A new axes object is created and the handle changes; old plot remains unchanged

B MATLAB throws an error because subplot index is reused

C The handle points to the existing axes; new plots overwrite the old content in that panel

D The subplot command ignores the call and keeps the old plot unchanged

💡 Hint

Think about how MATLAB manages axes objects and figure panels.

Calling subplot with an existing panel index activates that axes. The handle points to the same axes, and new plots overwrite the previous content.

Practice

1. What does the MATLAB function subplot(2, 3, 4) do?

easy

A. Plots data in the 2nd row and 3rd column of a figure.

B. Creates a 2x3 matrix and fills it with the number 4.

C. Clears the 4th subplot in a 2 by 3 grid.

D. Divides the figure into 2 rows and 3 columns, then selects the 4th panel for plotting.

Solution

1. Step 1: Understand subplot grid parameters

   The first two numbers (2, 3) define the grid size: 2 rows and 3 columns.

2. Step 2: Identify the panel number

   The third number (4) selects the 4th panel in row-wise order for plotting.

3. Final Answer:

   Divides the figure into 2 rows and 3 columns, then selects the 4th panel for plotting. -> Option D

4. Quick Check:

   subplot(m, n, p) = grid m x n, panel p [OK]

Hint: Remember subplot(m,n,p) means m rows, n columns, p panel [OK]

Common Mistakes:

• Confusing the order of rows and columns
• Thinking subplot creates a matrix of numbers
• Assuming subplot clears panels automatically

2. Which of the following is the correct syntax to create a 3x1 subplot and plot in the second panel?

easy

A. subplot(3, 1, 2); plot(x, y);

B. subplot(1, 3, 2); plot(x, y);

C. subplot(3, 2, 1); plot(x, y);

D. subplot(2, 3, 2); plot(x, y);

Solution

1. Step 1: Identify the grid layout

   A 3x1 subplot means 3 rows and 1 column, so subplot(3, 1, p) is correct.

2. Step 2: Select the second panel

   The third argument '2' selects the second panel in the vertical stack.

3. Final Answer:

   subplot(3, 1, 2); plot(x, y); -> Option A

4. Quick Check:

   3 rows, 1 column, panel 2 = subplot(3,1,2) [OK]

Hint: Rows first, then columns, then panel number [OK]

Common Mistakes:

• Swapping rows and columns in subplot arguments
• Using wrong panel number for desired position
• Confusing subplot grid with plot commands

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

subplot(1,2,1); plot(1:3, [2 4 6]);
subplot(1,2,2); plot(1:3, [1 3 5]);

medium

A. Two plots side by side: first with points (1,2),(2,4),(3,6), second with (1,1),(2,3),(3,5).

B. One plot with both data sets overlaid.

C. Error because subplot indices are incorrect.

D. Two plots stacked vertically.

Solution

1. Step 1: Understand subplot grid

   subplot(1,2,1) creates a grid with 1 row and 2 columns, selecting the first panel.

2. Step 2: Plot data in each panel

   The first plot shows points (1,2), (2,4), (3,6). The second subplot(1,2,2) plots points (1,1), (2,3), (3,5).

3. Final Answer:

   Two plots side by side: first with points (1,2),(2,4),(3,6), second with (1,1),(2,3),(3,5). -> Option A

4. Quick Check:

   subplot(1,2,p) = two side-by-side plots [OK]

Hint: subplot(1,2,p) means two plots side by side [OK]

Common Mistakes:

• Thinking plots overlay in one panel
• Assuming vertical stacking instead of horizontal
• Misreading subplot indices

4. Identify the error in this MATLAB code snippet:

subplot(2,2,5);
plot(rand(5));

medium

A. plot function cannot be used inside subplot.

B. Panel index 5 is invalid for a 2x2 grid.

C. Missing semicolon after subplot command.

D. rand(5) is incorrect syntax.

Solution

1. Step 1: Check subplot grid size

   A 2x2 grid has only 4 panels numbered 1 to 4.

2. Step 2: Validate panel index

   Using 5 as the panel index is invalid and causes an error.

3. Final Answer:

   Panel index 5 is invalid for a 2x2 grid. -> Option B

4. Quick Check:

   Panel number must be ≤ rows*columns [OK]

Hint: Panel number must not exceed total grid cells [OK]

Common Mistakes:

• Using panel number larger than grid size
• Thinking semicolon affects subplot indexing
• Believing plot can't be inside subplot

5. You want to create a 2x2 grid of plots showing sine waves with different frequencies. Which code correctly sets up the subplots and plots the waves?

hard

A. subplot(2,2);
for k = 1:4
plot(sin(k*x));
end

B. for k = 1:4
subplot(4,1,k);
x = linspace(0,2*pi,100);
plot(x, sin(k*x));
end

C. for k = 1:4
subplot(2,2,k);
x = linspace(0,2*pi,100);
plot(x, sin(k*x));
end

D. for k = 1:4
subplot(2,2,k+1);
x = linspace(0,2*pi,100);
plot(x, sin(k*x));
end

Solution

1. Step 1: Set up correct subplot grid

   subplot(2,2,k) correctly creates a 2x2 grid and selects each panel from 1 to 4.

2. Step 2: Plot sine waves with increasing frequency

   Inside the loop, x is defined and sin(k*x) is plotted in each subplot panel.

3. Step 3: Check other options for errors

   for k = 1:4 subplot(4,1,k); x = linspace(0,2*pi,100); plot(x, sin(k*x)); end uses 4x1 grid, not 2x2. subplot(2,2); for k = 1:4 plot(sin(k*x)); end calls subplot once without panel index. for k = 1:4 subplot(2,2,k+1); x = linspace(0,2*pi,100); plot(x, sin(k*x)); end uses k+1 which causes invalid panel 5.

4. Final Answer:

   for k = 1:4 subplot(2,2,k); x = linspace(0,2*pi,100); plot(x, sin(k*x)); end -> Option C

5. Quick Check:

   Loop with subplot(2,2,k) plots 4 panels correctly [OK]

Hint: Use subplot(2,2,k) inside loop k=1 to 4 [OK]

Common Mistakes:

• Using wrong subplot grid size
• Not specifying panel index in subplot
• Using invalid panel numbers