Lesson 3

Last modified:January 1, 1970 GMT
An Introduction to Matlab(tm): Lesson 3
New commands in this lesson:
plot(x,y) creates a Cartesian plot of the vectors x & y
plot(y) creates a plot of y vs. the numerical values of the
elements in the y-vector.
semilogx(x,y) plots log(x) vs y
semilogy(x,y) plots x vs log(y)
loglog(x,y)
grid

plots log(x) vs log(y)

creates a grid on the graphics plot

title('text') places a title at top of graphics plot
xlabel('text') writes 'text' beneath the x-axis of a plot
ylabel('text') writes 'text' beside the y-axis of a plot
text(x,y,'text')

writes 'text' at the location (x,y)

text(x,y,'text','sc') writes 'text' at point x,y assuming
lower left corner is (0,0) and upper
right corner is (1,1).
polar(theta,r) creates a polar plot of the vectors r & theta
where theta is in radians.
bar(x)

creates a bar graph of the vector x. (Note also
the command stairs(x).)

bar(x,y) creates a bar-graph of the elements of the vector y,
locating the bars according to the vector elements
of 'x'. (Note also the command stairs(x,y).)

CARTESIAN OR X-Y PLOTS
One of 'matlab' most powerful features is the ability to

create graphic plots. Here we introduce the elementary ideas for
simply presenting a graphic plot of two vectors. More complicated
and powerful ideas with graphics can be found in the 'matlab'
documentation.
A Cartesian or orthogonal x,y plot is based on plotting the
x,y data pairs from the specified vectors. Clearly, the vectors x
and y must have the same number of elements. Imagine that you wish
to plot the function ex for values of x from 0 to 2.
x = 0:.1:2;
y = exp(x);
plot(x,y)
NOTE: The use of selected functions such as exp() and sin() will be
used in these tutorials without explanation if they take on an
unambiguous meaning consistent with past experience. Here it is
observed that operating on a matrix with these functions simply
creates a matrix in which the elements are based on applying the
function to each corresponding element of the argument.
Of course the symbols x,y are arbitrary. If we wanted to plot
temperature on the ordinate and time on the abscissa, and vectors
for temperature and time were loaded in 'matlab', the command would
be
plot(time,temperature)
Notice that the command plot(x,y) opens a graphics window. If you
now execute the command 'grid', the graphics window is redrawn.
(Note you move the cursor to the command window before typing new
commands.) To avoid redrawing the window, you may use the line
continuation ellipsis. Consider
plot(x,y),...
grid,...
title('Exponential Function'),...
xlabel('x'),...
ylabel('exp(x)'),
text(.6,.4,' y = exp(x)','sc')
Note that if you make a mistake in typing a series of lines of
code, such as the above, the use of line continuation can be
frustrating. In a future lesson, we will learn how to create batch
files (called '.m' files) for executing a series of 'matlab'
commands. This approach gives you better opportunity to return to

the command string and edit errors that have been created.
Having defined the vectors x and y, construct semilog and loglog plots using these or other vectors you may wish to create. Note
in particular what occurs when a logarithmic scale is selected for
a vector that has negative or zero elements. Here, the vector x has
a zero element (x(1)=0). The logarithm of zero or any negative
number is undefined and the x,y data pair for which a zero or
negative number occurs is discarded and a warning message provided.
Try the commands plot(x) and plot(y) to ensure you understand
how they differ from plot(x,y).
Notice that 'matlab' draws a straight line between the datapairs. If you desire to see a relatively smooth curve drawn for a
rapidly changing function, you must include a number of data
points. For example, consider the trigonometric function, sin(x1)
(x1 is selected as the argument here to distinguish it from the
vector 'x' that was defined earlier. )
Plot sin(x1) over the limits 0 <= x1 <= pi. First define x1
with 5 elements,
x1 = 0 : pi/4 : pi
y1 = sin(x1)
plot(x1,y1)
Now, repeat this after defining a new vector for x1 and y1 with 21
elements. (Note the use of the semicolon here to prevent the
printing and scrolling on the monitor screen of a vector or matrix
with a large number of elements!)
x1 = 0 : .05*pi : pi ;
y1 =sin(x1);
plot(x1,y1)
POLAR PLOTS
Polar plots are constructed much the same way as are Cartesian
x-y plots; however, the arguments are the angle 'theta' in radians
measured CCW from the horizontal (positive-x) axis, and the length
of the radius vector extended from the origin along this angle.
Polar plots do not allow the labeling of axes; however, notice that
the scale for the radius vector is presented along the vertical and
when the 'grid' command is used the angles-grid is presented in 15-

degree segments. The 'title' command and the 'text' commands are
functional with polar plots.
angle = 0:.1*pi:3*pi;
radius = exp(angle/20);
polar(angle,radius),...
title('An Example Polar Plot'),...
grid
Note that the angles may exceed one revolution of 2*pi.
BAR GRAPHS
To observe how 'matlab' creates a bargraph, return to the
vectors x,y that were defined earlier. Create bar and stair graphs
using these or other vectors you may define. Of course the 'title'
and 'text' commands can be used with bar and stair graphs.
bar(x,y) and bar(y)
stair (x,y) and stair(y)

MULTIPLE PLOTS
More than a single graph can be presented on one graphic plot.
One common way to accomplish this is hold the graphics window open
with the 'hold' command and execute a subsequent plotting command.
x1=0:.05*pi:pi;
y1=sin(x1);
plot(x1,y1)
hold
y2=cos(x1);
plot(x1,y2)
The "hold" command will remain active until you turn it off with
the command 'hold off'.
You can create multiple graphs by using multiple arguments.
In addition to the vectors x,y created earlier, create the vectors
a,b and plot both vector sets simultaneously as follows.
a = 1 : .1 : 3;
b = 10*exp(-a);

plot(x,y,a,b)
Multiple plots can be accomplished also by using matrices
rather than simple vectors in the argument. If the arguments of
the 'plot' command are matrices, the COLUMNS of y are plotted on
the ordinate against the COLUMNS of x on the abscissa. Note that x
and y must be of the same order! If y is a matrix and x is a
vector, the rows or columns of y are plotted against the elements
of x. In this instance, the number of rows OR columns in the matrix
must correspond to the number of elements in 'x'. The matrix 'x'
can be a row or a column vector!
Recall the row vectors 'x' and 'y' defined earlier. Augment
the row vector 'y' to create the 2-row matrix, yy.
yy=[y;exp(1.2*x)];
plot(x,yy)

PLOTTING DATA POINTS & OTHER FANCY STUFF
Matlab connects a straight line between the data pairs
described by the vectors used in the 'print' command. You may wish
to present data points and omit any connecting lines between these
points. Data points can be described by a variety of characters
( . , + , * , o and x .) The following command plots the x,y data
as a "curve" of connected straight lines and in addition places an
'o' character at each of the x1,y1 data pairs.
plot(x,y,x1,y1,'o')
Lines can be colored and they can be broken to make
distinctions among more than one line. Colored lines are effective
on the color monitor and color printers or plotters. Colors are
useless on the common printers used on this network. Colors are
denoted in 'matlab' by the symbols r(red), g(green), b(blue),
w(white) and i(invisible). The following command plots the x,y
data in red solid line and the r,s data in broken green line.
plot(x,y,'r',r,s,'--g')
PRINTING GRAPHIC PLOTS
Executing the 'print' command will send the contents of the

current graphics widow to the local printer.
You may wish to save in a graphics file, a plot you have
created. To do so, simply append to the 'print' command the name
of the file. The command
print filename
will store the contents of the graphics window in the file titled
'filename.ps' in a format called postscript. You need not include
the .ps suffix, as matlab will do this. When you list the files
in your matlab directory, it is convenient to identify any graphics
files by simply looking for the files with the .ps suffix. If
you desire to print one of these postscript files, you can
conveniently "drag and drop" the file from the file-manager window
into the printer icon.
Back to Matlab In-House Tutorials

Lesson 3

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