Graphics Programs Reference
In-Depth Information
produce a grid withspacing 0
.
1 in bothdirections. We have also used
axis
square
to force the same scale on both axes.
You can specify particular level sets by including an additiona
l
vector a
r-
gument to
contour
. For example, to plot the circles of radii 1,
√
2
,
and
√
3,
type
>> contour(X, Y, X.ˆ2 + Y.ˆ2, [1 2 3])
The vector argument must contain at least two elements, so if you want
to plot a single level set, you must specify the same level twice. This is quite
useful for implicit plotting of a curve given by an equation in
x
and
y
.For
example, to plot the circle of radius 1 about the origin, type
>> contour(X, Y, X.ˆ2 + Y.ˆ2, [1 1])
Or to plot the lemniscate
x
2
−
y
2
=
(
x
2
+
y
2
)
2
, rewrite the equation as
(
x
2
+
y
2
)
2
−
x
2
+
y
2
=
0
and type
>> [X Y] = meshgrid(-1.1:0.01:1.1, -1.1:0.01:1.1);
>> contour(X, Y, (X.ˆ2 + Y.ˆ2).ˆ2 - X.ˆ2 + Y.ˆ2, [0 0])
>> axis square
>> title('The lemniscate xˆ2-yˆ2=(xˆ2+yˆ2)ˆ2')
The command
title
labels the plot with the indicated string. (In the default
string interpreter,
ˆ
is used for inserting an exponent and is used for sub-
scripts.) The result is shown in Figure 5-3.
If you have the Symbolic Math Toolbox, contour plotting can also be
donewiththecommand
ezcontour
,andimplicitplottingofacurve
f
(
x
,
y
)
=
0
can also be done with
ezplot
. One can obtain almost the same picture as
Figure 5-2 withthe command
>> ezcontour('xˆ2 + yˆ2', [-3, 3], [-3, 3]); axis square
and almost the same picture as Figure 5-3 with the command
>> ezplot('(xˆ2 + yˆ2)ˆ2 - xˆ2 + yˆ2', ...
[-1.1, 1.1], [-1.1, 1.1]); axis square
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