plot3 for 3D lines in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the plot3 function do in MATLAB?

The plot3 function draws lines or points in 3D space using x, y, and z coordinates.

beginner

How do you call plot3 to plot a line through points (1,2,3), (4,5,6), and (7,8,9)?

Use plot3([1 4 7], [2 5 8], [3 6 9]) to connect the points with a 3D line.

beginner

What is the role of the third input vector in plot3(x, y, z)?

The third vector z specifies the height or depth coordinate for each point in 3D space.

intermediate

How can you change the color and style of the 3D line in plot3?

Add a format string like 'r--' to plot3, e.g., plot3(x, y, z, 'r--') for a red dashed line.

beginner

What MATLAB command helps you add labels to the axes in a 3D plot?

Use xlabel, ylabel, and zlabel to add labels to the x, y, and z axes respectively.

Which of these commands plots a 3D line through points (1,2,3), (2,3,4), and (3,4,5)?

A plot3([1 2 3], [2 3 4], [3 4 5])

B plot([1 2 3], [2 3 4], [3 4 5])

C plot3([1 2 3], [3 4 5])

D plot([1 2 3], [2 3 4])

plot3 requires three vectors for x, y, and z coordinates. Option A correctly provides all three.

What does the command plot3(x, y, z, 'g:') do?

A Plots a green solid 2D line

B Plots a red dashed 3D line

C Plots a green dotted 3D line

D Plots a blue dotted 3D line

The format string 'g:' means green color with dotted line style in 3D plot.

Which function adds a label to the z-axis in a 3D plot?

A ylabel

B zlabel

C xlabel

D title

zlabel adds a label to the z-axis in MATLAB plots.

If vectors x, y, and z have different lengths, what happens when you call plot3(x, y, z)?

A MATLAB ignores extra points

B MATLAB plots with missing points

C MATLAB automatically adjusts vector sizes

D MATLAB gives an error

All input vectors must have the same length; otherwise, MATLAB throws an error.

What is the default line color when you use plot3 without specifying a color?

A Blue

B Red

C Green

D Black

MATLAB's default line color for plots is blue.

Explain how to use plot3 to draw a 3D line connecting points in space.

Think about how you provide points in 3D and how to customize the line.

Describe how to add labels to each axis in a 3D plot created with plot3.

Remember the functions that name each axis.

Practice

1. What does the MATLAB function plot3(X, Y, Z) do?

easy

A. Plots a 3D line or points using vectors X, Y, and Z as coordinates.

B. Creates a 2D plot using X and Y only.

C. Generates a histogram of the data in X, Y, and Z.

D. Plots a pie chart based on the values in X, Y, and Z.

Solution

1. Step 1: Understand the function purpose

   plot3 is designed to plot lines or points in three-dimensional space using three vectors: X, Y, and Z.

2. Step 2: Compare options with function behavior

   Only Plots a 3D line or points using vectors X, Y, and Z as coordinates. correctly describes plotting in 3D using X, Y, and Z coordinates. Other options describe 2D plots or unrelated chart types.

3. Final Answer:

   Plots a 3D line or points using vectors X, Y, and Z as coordinates. -> Option A

4. Quick Check:

   plot3 = 3D line plot [OK]

Hint: Remember plot3 needs three coordinate vectors X, Y, Z [OK]

Common Mistakes:

• Thinking plot3 creates 2D plots
• Confusing plot3 with histogram or pie chart functions
• Using only two vectors instead of three

2. Which of the following is the correct syntax to plot a red dashed 3D line using plot3?

easy

A. plot3(X, Y, Z, 'rd')

B. plot3(X, Y, Z, 'r--')

C. plot3(X, Y, Z, color='red', linestyle='dashed')

D. plot3(X, Y, Z, 'red-dash')

Solution

1. Step 1: Recall MATLAB LineSpec format

   MATLAB uses short codes like 'r' for red and '--' for dashed lines combined as 'r--' in plot3.

2. Step 2: Evaluate each option

   plot3(X, Y, Z, 'r--') uses correct LineSpec syntax. plot3(X, Y, Z, 'red-dash') uses invalid string. plot3(X, Y, Z, color='red', linestyle='dashed') uses Python-like syntax, invalid in MATLAB. plot3(X, Y, Z, 'rd') 'rd' means red diamond marker, not dashed line.

3. Final Answer:

   plot3(X, Y, Z, 'r--') -> Option B

4. Quick Check:

   LineSpec 'r--' = red dashed line [OK]

Hint: Use 'r--' for red dashed line in plot3 [OK]

Common Mistakes:

• Using full words like 'red' or 'dashed' instead of codes
• Confusing marker codes with line style codes
• Trying Python-style named arguments

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

X = 1:3; Y = [2 4 6]; Z = [3 6 9]; plot3(X, Y, Z, 'g-o'); grid on; xlabel('X-axis'); ylabel('Y-axis'); zlabel('Z-axis');

medium

A. An error because vectors X, Y, Z have different lengths.

B. A 2D green line plot with circle markers on X and Y only.

C. A 3D scatter plot without lines connecting points.

D. A green 3D line with circle markers connecting points (1,2,3), (2,4,6), (3,6,9) with grid and axis labels.

Solution

1. Step 1: Analyze vectors and plot3 call

   X, Y, Z are vectors of equal length 3. plot3 with 'g-o' plots green lines with circle markers connecting points in 3D.

2. Step 2: Understand grid and labels

   grid on adds grid lines. xlabel, ylabel, and zlabel add axis labels. So the plot is a labeled 3D line with markers and grid.

3. Final Answer:

   A green 3D line with circle markers connecting points (1,2,3), (2,4,6), (3,6,9) with grid and axis labels. -> Option D

4. Quick Check:

   plot3 with equal vectors and 'g-o' = green line with circles [OK]

Hint: Equal length vectors plot connected 3D lines with markers [OK]

Common Mistakes:

• Assuming 2D plot instead of 3D
• Thinking different vector lengths cause error here
• Confusing scatter plot with plot3 line plot

4. Identify the error in this MATLAB code snippet:

X = 1:4; Y = [2 3 4]; Z = [5 6 7 8]; plot3(X, Y, Z);

medium

A. Vectors X, Y, and Z must have the same length to plot3.

B. plot3 requires the vectors to be row vectors only.

C. The plot3 function cannot plot more than 3 points.

D. Missing line style argument causes syntax error.

Solution

1. Step 1: Check vector lengths

   X has length 4, Y has length 3, Z has length 4. plot3 requires all three vectors to have the same length.

2. Step 2: Understand consequences of length mismatch

   Because Y is shorter, MATLAB will throw an error about vector size mismatch when calling plot3.

3. Final Answer:

   Vectors X, Y, and Z must have the same length to plot3. -> Option A

4. Quick Check:

   Equal vector lengths required for plot3 [OK]

Hint: Check all coordinate vectors have same length before plot3 [OK]

Common Mistakes:

• Assuming plot3 accepts different length vectors
• Thinking row vs column vector matters here
• Believing line style argument is mandatory

5. You want to plot two 3D lines on the same figure: one blue solid line and one red dashed line. Which code correctly does this?

hard

A. plot3(X1, Y1, Z1, 'b-'); hold off; plot3(X2, Y2, Z2, 'r--');

B. plot3(X1, Y1, Z1, 'b-'); plot3(X2, Y2, Z2, 'r--');

C. plot3(X1, Y1, Z1, 'b-'); hold on; plot3(X2, Y2, Z2, 'r--'); hold off;

D. plot3(X1, Y1, Z1, 'b-'); hold on; plot3(X2, Y2, Z2, 'r--'); hold on;

Solution

1. Step 1: Understand hold on/off usage

   To plot multiple lines on the same figure, hold on keeps the current plot so new plots add to it. hold off releases it.

2. Step 2: Analyze each option's hold usage

   plot3(X1, Y1, Z1, 'b-'); hold on; plot3(X2, Y2, Z2, 'r--'); hold off; correctly uses hold on before second plot and hold off after to finish. plot3(X1, Y1, Z1, 'b-'); plot3(X2, Y2, Z2, 'r--'); misses hold so second plot overwrites first. plot3(X1, Y1, Z1, 'b-'); hold off; plot3(X2, Y2, Z2, 'r--'); calls hold off too early. plot3(X1, Y1, Z1, 'b-'); hold on; plot3(X2, Y2, Z2, 'r--'); hold on; calls hold on twice unnecessarily.

3. Final Answer:

   plot3(X1, Y1, Z1, 'b-'); hold on; plot3(X2, Y2, Z2, 'r--'); hold off; -> Option C

4. Quick Check:

   Use hold on/off to plot multiple lines [OK]

Hint: Use hold on before second plot, hold off after [OK]

Common Mistakes:

• Not using hold on causes plots to overwrite
• Calling hold off too early removes previous plots
• Calling hold on multiple times unnecessarily