How to Use Contour in MATLAB: Syntax and Examples

In MATLAB, use the contour function to create contour plots that show lines of constant values for a matrix. You provide X and Y coordinates along with a matrix Z of values, and MATLAB draws contour lines representing levels in Z.

Syntax

The basic syntax for the contour function is:

• contour(Z): Draws contour lines for matrix Z with default X and Y coordinates.
• contour(X, Y, Z): Uses X and Y matrices or vectors to specify coordinates for Z.
• contour(X, Y, Z, N): Draws N contour levels.
• contour(X, Y, Z, levels): Draws contour lines at specified levels (vector of values).

You can also customize colors and line styles by adding name-value pairs.

matlab

contour(Z)
contour(X, Y, Z)
contour(X, Y, Z, N)
contour(X, Y, Z, levels)

Example

This example creates a contour plot of a simple function Z = X^2 + Y^2 over a grid. It shows how to use contour with X, Y, and Z matrices and specify the number of contour levels.

matlab

x = -2:0.1:2;
y = -2:0.1:2;
[X, Y] = meshgrid(x, y);
Z = X.^2 + Y.^2;
contour(X, Y, Z, 10)
title('Contour plot of Z = X^2 + Y^2')
xlabel('X axis')
ylabel('Y axis')

Output

A contour plot window showing 10 contour lines forming concentric circles centered at (0,0).

Common Pitfalls

Common mistakes when using contour include:

• Not matching the sizes of X, Y, and Z matrices. They must be the same size if X and Y are matrices.
• Using vectors X and Y that do not match the dimensions of Z.
• Passing a scalar instead of a vector for contour levels.
• Forgetting to use element-wise operators like .^ when computing Z.

Always check matrix sizes with size() before plotting.

matlab

x = -2:0.1:2;
y = -2:0.1:2;
Z = x.^2 + y.^2; % Incorrect: Z is 1D vector
contour(x, y, Z) % This will error because sizes mismatch

% Correct way:
[X, Y] = meshgrid(x, y);
Z = X.^2 + Y.^2;
contour(X, Y, Z)

Quick Reference

Tips for using contour in MATLAB:

• Use meshgrid to create X and Y matrices for 2D functions.
• Specify contour levels as a vector for precise control.
• Use clabel to add labels to contour lines.
• Customize colors with colormap and line styles with additional arguments.

Key Takeaways

Use contour(X, Y, Z) with matching X, Y, Z sizes to plot contour lines.

Specify the number or exact values of contour levels to control plot detail.

Always create X and Y grids with meshgrid for 2D functions.

Check matrix dimensions to avoid size mismatch errors.

Use clabel to label contour lines for better readability.