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MA/CS 375

Fall 2002

Lecture 4

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Plotting in Matlab

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Example 5 (building and plotting a function of a vector)

1) create a vector x, with 50 equally spaced entries from 0 to 1.0

2) create a vector f, with entries of cosine evaluated at entries of 2*pi*x

3) plot f as a function of x, using + symbols

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Example 5(figure created by Matlab)

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Example 5 (building and plotting a function of a vector)

1) create a vector x, with 50 equally spaced entries from 0 to 1.0

2) create a vector f, with entries of cos evaluated at entries of x

3) plot f as a function of x, using a blue line

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Example 5(figure created by Matlab)

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Example 6 (building and plotting two

functions of a vector)1) create a vector x, with

50 entries from 0.0 to 1.0

2) create a vector f, with cos(2*pi*x).

3) create a vector g with entries sin(2*pi*x)

4) plot f as a function of x, using a blue line

5) on same graph plot g as a function of x, using red + symbols.

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Example 6

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Close Up of Example 6 code

Instruct Matlab to keep the current graph window and its contents for the next plot.

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Close Up of Example 6 code

Instruct Matlab that you have finished plotting in the current graph window.

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Quick Reference For Plot(Taken from Matlab: > help plot)

Various line types, plot symbols and colors may be obtained with PLOT(X,Y,S) where S is a character string made from one element from any or all the following 3 colunms:

y yellow . point - solid

m magenta o circle : dotted

c cyan x x-mark -. dashdot

r red + plus -- dashed

g green * star

b blue s square

w white d diamond

k black v triangle (down)

^ triangle (up)

< triangle (left)

> triangle (right)

p pentagram

h hexagram

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Adding a Title, Labeling Axes and Legends

• Ok – so we have made a cool graph

• But it isn’t much use to anybody else unless you label everything in sight!

• The following functions will modify the current figure: title, xlabel, ylabel, legend

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Example 7

The label commands

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Details:

• Set up the legend box with descriptions of each curve:>> legend('f=cos(2*pi*x)', 'g=sin(2*pi*x)');

• Label the axis>> xlabel('x');

• Label the y-axis>> ylabel('f,g');

• Give the graph a title>> title('The very boring graphs');

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Modifying the Figure Properties

So I am not so pleased by the default fontsused by Matlab.

• It is easy to change the font:

• In the figure window click on the text object you wish to change

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Click on “Edit/Current Object Properties”

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I prefer a larger Arial font

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The Final Product

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Alternative Ways To Change Fonts..

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Individual Exercise1) Create two curves on the same graph:

F = 1/(1+x^2) G = exp(x^3)

2) Plot the points at 33 points equally spaced between 0 and 1

3) Use black *’s for F and green o’s for G4) Label the horizontal and vertical axes5) Create a title (including your name)6) Create a legend7) Make sure the font’s are legible for my old eyes

(also figure out how to change the axis fonts)8) Print out your graph and hand it in.

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Plotting Two-Dimensional Data

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Example 7(Plotting Two-dimensional Data)

1) create two vectors x1d and y1d, with 20 entries from 0.0 to 1.0

2) create matrices of values using the x1d and y1d to distribute points in the unit square with 20 points in the x and y directions.

3) plot results in a graph.

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Results From Example 7

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Where are we going with this?

• We can evaluate a function at the two dimensional points (x2d,y2d)

• We can plot contours of this function where: a contour is a curve in the domain of the function where the value of the function is constant.

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Example 8(plotting contours of a two-dimensional function)

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Example 9(plotting two-dimensional function as a surface)

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Example 9 cont.(plotting two-dimensional function as a smooth surface)

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Summary of Lecture 4

• We have learnt how to:– plot a graph – plot multiple graphs in the same window– improve the fonts– label everything in sight….– plot contours of a two-dimensional function– plot a two-dimensional function as a surface