contour plots in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a contour plot in MATLAB?

A contour plot in MATLAB is a graphical representation that shows curves connecting points of equal value in a 3D surface, displayed in 2D. It helps visualize how a function changes over two variables.

beginner

Which MATLAB function is used to create a basic contour plot?

The contour function is used to create a basic contour plot in MATLAB. For example, contour(X, Y, Z) plots contour lines for matrix Z over grid defined by X and Y.

intermediate

How do you add labels to contour lines in MATLAB?

Use the clabel function after creating a contour plot. It adds numeric labels to contour lines to show their values, making the plot easier to understand.

intermediate

What is the difference between contour and contourf in MATLAB?

contour draws contour lines only, while contourf creates filled contour plots where the areas between lines are colored, giving a clearer visual of value ranges.

intermediate

How can you specify the number of contour levels in a MATLAB contour plot?

You can specify the number of contour levels by passing a number as the fourth argument to contour, like contour(X, Y, Z, 10) to draw 10 contour levels.

Which MATLAB function creates a filled contour plot?

A mesh

B contour

C plot

D contourf

The contourf function creates filled contour plots, coloring the areas between contour lines.

What does the clabel function do in contour plots?

A Adds labels to contour lines

B Changes contour colors

C Creates contour lines

D Removes contour lines

clabel adds numeric labels to contour lines to show their values.

How do you specify the grid points for contour plots in MATLAB?

A No need to specify grid points

B Using a single vector

C Using X and Y matrices

D Using a scalar value

You specify grid points with X and Y matrices that define the coordinates for Z values.

What does a contour line represent in a contour plot?

A Points with the same function value

B Random points

C Maximum values only

D Minimum values only

Contour lines connect points where the function has the same value.

Which command creates 20 contour levels in a plot?

A contour(X, Y, Z)

B contour(X, Y, Z, 20)

C contourf(X, Y, Z, 5)

D contourf(X, Y, Z)

Passing 20 as the fourth argument to contour sets 20 contour levels.

Explain how to create a basic contour plot in MATLAB and how to add labels to it.

Think about the functions used and what each input represents.

Describe the difference between contour and filled contour plots and when you might use each.

Consider how the plot looks and what information it shows.

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