contour plots in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a contour plot of matrix Z.

MATLAB

contour([1]);

Drag options to blanks, or click blank then click option'

Amesh

Bplot

CZ

Dsurf

Attempts:

3 left

💡 Hint

Common Mistakes

Using plot instead of contour.

Using mesh or surf which create 3D plots.

The contour function creates contour plots of the matrix Z.

2fill in blank

medium

Complete the code to generate X and Y grid matrices for contour plotting.

MATLAB

[X, Y] = meshgrid([1], [2]);

Drag options to blanks, or click blank then click option'

Acontour

BZ

CY

Dx

Attempts:

3 left

💡 Hint

Common Mistakes

Passing matrix Z instead of vector x.

Using contour or Y as input which are not vectors.

The meshgrid function creates grid matrices from vector x for X and Y.

3fill in blank

hard

Fix the error in the code to plot contour lines with 10 levels.

MATLAB

contour(X, Y, Z, [1]);

Drag options to blanks, or click blank then click option'

A10

B5

C'10'

D[10]

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around 10 making it a string.

Using brackets which create an array.

The number of contour levels should be a numeric scalar, so use 10 without quotes or brackets.

4fill in blank

hard

Fill both blanks to create a contour plot with labeled contour lines.

MATLAB

C = contour(X, Y, Z, [1]);
clabel([2]);

Drag options to blanks, or click blank then click option'

A20

BZ

CC

DX

Attempts:

3 left

💡 Hint

Common Mistakes

Using the data matrix Z instead of the contour matrix C for labeling.

Not specifying the number of contour levels.

Use 20 contour levels and label the contour matrix C returned by contour.

5fill in blank

hard

Fill all three blanks to create a filled contour plot with a colorbar and title.

MATLAB

contourf(X, Y, Z, [1]);
colorbar([2]);
title([3]);

Drag options to blanks, or click blank then click option'

A15

B'eastoutside'

C'My Contour Plot'

D'north'

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect colorbar location strings.

Not quoting the title string.

Use 15 contour levels, place the colorbar outside on the east, and add a descriptive title.

Practice

1. What does a contour plot in MATLAB primarily represent?

easy

A. A 3D surface plot

B. Lines connecting points with the same value

C. A bar chart of values

D. A scatter plot of points

Solution

1. Step 1: Understand contour plot purpose

   A contour plot shows curves that connect points with equal values on a grid.

2. Step 2: Compare with other plot types

   3D surface plots show height as a surface, bar charts show bars, scatter plots show points, so they differ.

3. Final Answer:

   Lines connecting points with the same value -> Option B

4. Quick Check:

   Contour plot = lines of equal value [OK]

Hint: Contour plots connect equal values with lines [OK]

Common Mistakes:

• Confusing contour plots with 3D surface plots
• Thinking contour plots show individual points
• Mixing contour plots with bar charts

2. Which of the following is the correct syntax to create a basic contour plot in MATLAB using matrices X, Y, and Z?

easy

A. contour(X, Y, Z)

B. plot(X, Y, Z)

C. contourplot(X, Y, Z)

D. surface(X, Y, Z)

Solution

1. Step 1: Recall MATLAB contour syntax

   The function to create contour plots is contour(X, Y, Z).

2. Step 2: Eliminate incorrect functions

   plot() is for 2D plots, contourplot() is not a MATLAB function, surface() creates 3D surfaces.

3. Final Answer:

   contour(X, Y, Z) -> Option A

4. Quick Check:

   Use contour() for contour plots [OK]

Hint: Use contour(X,Y,Z) to plot contours [OK]

Common Mistakes:

• Using plot() instead of contour()
• Typing contourplot() which is invalid
• Using surface() which makes 3D plots

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

X = 1:3; Y = 1:3; [Xg,Yg] = meshgrid(X,Y); Z = Xg.^2 + Yg.^2; contour(Xg, Yg, Z, 3);

medium

A. An error because contour needs vectors, not matrices

B. A 3D surface plot of Z values

C. A scatter plot of points (Xg, Yg, Z)

D. A contour plot with 3 contour lines representing levels of Z

Solution

1. Step 1: Understand meshgrid and Z calculation

   meshgrid creates grids Xg and Yg; Z is sum of squares, a smooth surface.

2. Step 2: Interpret contour call with 3 levels

   contour with 3 as last argument draws 3 contour lines at different Z levels.

3. Final Answer:

   A contour plot with 3 contour lines representing levels of Z -> Option D

4. Quick Check:

   contour with level count = contour lines count [OK]

Hint: Last argument in contour sets number of lines [OK]

Common Mistakes:

• Thinking contour creates 3D surface plots
• Believing contour requires vectors, not matrices
• Confusing contour with scatter plots

4. Identify the error in this MATLAB code snippet for contour plotting:

X = 1:5; Y = 1:5; Z = X + Y; contour(X, Y, Z);

medium

A. contour requires only two arguments

B. X and Y must be matrices, not vectors

C. Z must be a matrix matching grid size, not a vector

D. No error, code runs fine

Solution

1. Step 1: Check dimensions of X, Y, Z

   X and Y are vectors; Z is vector sum, not a matrix grid.

2. Step 2: Understand contour input requirements

   contour needs Z as matrix matching X, Y grid sizes; here Z is vector, causing error.

3. Final Answer:

   Z must be a matrix matching grid size, not a vector -> Option C

4. Quick Check:

   Z size mismatch causes error [OK]

Hint: Z must be matrix matching X,Y grid for contour [OK]

Common Mistakes:

• Passing vector Z instead of matrix
• Assuming contour accepts vector inputs
• Ignoring dimension mismatch errors

5. You want to plot contour lines only at specific Z values: 5, 10, and 15. Which MATLAB command correctly achieves this given matrices X, Y, and Z?

hard

A. contour(X, Y, Z, [5 10 15])

B. contour(X, Y, Z, 3)

C. contour(X, Y, Z, 'Levels', [5 10 15])

D. contour(X, Y, Z, 'LineLevels', [5 10 15])

Solution

1. Step 1: Recall contour level specification syntax

   To specify exact contour levels, pass a vector of levels as the fourth argument.

2. Step 2: Evaluate options for specifying levels

   contour(X, Y, Z, [5 10 15]) uses vector [5 10 15] correctly; contour(X, Y, Z, 3) sets number of lines, not levels; options C and D use invalid syntax.

3. Final Answer:

   contour(X, Y, Z, [5 10 15]) -> Option A

4. Quick Check:

   Use vector of levels as 4th argument [OK]

Hint: Pass vector of levels as 4th argument to contour [OK]

Common Mistakes:

• Using number instead of vector for levels
• Trying unsupported name-value pairs
• Confusing level count with specific levels