plot3 for 3D lines in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple 3D line plot

What will be the output of the following MATLAB code snippet?

MATLAB

x = 1:5;
y = x.^2;
z = x + 1;
plot3(x, y, z, '-o');

A A 2D plot showing y vs x with circles

B Syntax error due to missing semicolon

C A 3D scatter plot without lines

D A 3D line plot with points marked by circles connecting (1,1,2) to (5,25,6)

💡 Hint

Remember plot3 plots lines in 3D space using x, y, and z coordinates.

The plot3 function connects points in 3D space with lines and markers. Here, x, y, and z are vectors of equal length, so it plots a 3D line with circle markers at each point.

🧠 Conceptual

intermediate

Understanding plot3 input vectors

Which of the following statements about the inputs to plot3(x,y,z) is TRUE?

A x, y, and z must be vectors of the same length to plot a 3D line.

B x, y, and z can be scalars or vectors of different lengths.

C Only x and y need to be vectors; z can be a scalar.

D plot3 only accepts matrices as inputs.

💡 Hint

Think about how points in 3D space are defined for a line.

plot3 connects points defined by corresponding elements in x, y, and z vectors. They must be the same length to form valid points.

🔧 Debug

advanced

Identify the error in 3D line plotting code

What error will the following MATLAB code produce?

MATLAB

x = 1:4;
y = [2 4 6];
z = 1:4;
plot3(x, y, z);

A Error: Vectors x, y, and z must be the same length.

B No error; plots a 3D line with missing points.

C Error: plot3 requires matrices, not vectors.

D Warning: y vector is shorter, MATLAB pads with zeros.

💡 Hint

Check the length of each vector used in plot3.

The y vector has length 3, while x and z have length 4. plot3 requires all input vectors to be the same length.

📝 Syntax

advanced

Correct syntax for plotting multiple 3D lines

Which option correctly plots two separate 3D lines in one figure?

MATLAB

x1 = 1:3; y1 = [2 3 4]; z1 = [1 0 1];
x2 = 3:5; y2 = [5 6 7]; z2 = [2 2 2];

A plot3(x1, y1); plot3(x2, y2, z2);

B plot3(x1, y1, z1); hold on; plot3(x2, y2, z2); hold off;

C plot3(x1, y1, z1, x2, y2, z2);

D plot3([x1 x2], [y1 y2], [z1 z2]);

💡 Hint

Use hold on to plot multiple lines in the same figure.

Option B plots the first line, then holds the plot to add the second line. Option B merges points into one line. Option B misses z for first line. Option B is invalid syntax.

🚀 Application

expert

Number of line segments in a 3D plot

Given vectors x, y, z each of length 10, how many line segments does plot3(x, y, z) draw?

A 11

B 10

C 9

D 8

💡 Hint

Each line segment connects two consecutive points.

With 10 points, plot3 connects point 1 to 2, 2 to 3, ..., 9 to 10, making 9 segments.

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