Information Technology Reference
In-Depth Information
Graphics
Plotting the solutions is performed as in the Fortran case; i.e., an external plotting
program is used to plot the data stored in the file
sol.dat
.
There exist plotting programs written in C or C
CC
that can be used as libraries
for the present program such that the
u
array can be sent directly to a plotting routine,
without using a file for intermediate data storage. However, there are no “standard”
plotting libraries for C
CC
.
6.4.5
Java
The Java program for solving an ODE by Heun's method is quite similar to the
corresponding C
CC
program, except that the stand-alone functions
heun
and
dump
must be methods of a class in Java.
The implementation of the
heun
function, as described in Algorithm
6.12
,is
performed via the
advance
method in class
Heun
:
class Heun {
public static double[] advance (Func f, double u0,
double dt, int n)
{
double u [] = new double[n+1];
u[0] = u0; // initial condition
// advance n steps:
for (int i = 0; i < n; i++) {
double v = f.f(u[i]);
u[i+1] = u[i] + 0.5
*
dt
*
(v + f.f( u[i] + dt
*
v ));
}
return u;
}
}
The right-hand side of the ODE is represented as function objects in a
Func
hier-
archy, as explained in Sect.
6.3.4
. Arrays in Java are created in the same manner as
in C
CC
. However, there is no need to delete the array, since Java will do this when
the array is no longer in use. This makes it possible to create our array wherever it
is convenient, e.g., in the
Heun.advance
function, and return it from the function
to the calling code, if desired. (In C
CC
this is also possible, but it is usually a good
habit, from a programming safety point of view, to perform both the
new
and
delete
statements in the calling code. Declaring a variable
u
of type
std::valarray
inside
the C
CC
heun
function can of course be performed, but C
CC
requires us to
copy
this array if we want to return it, whereas with plain C/C
CC
and Java arrays we
avoid this copy, and only references/pointers are returned.)
File writing in Java is typically illustrated by the following statements:
PrintStream f = new PrintStream(new FileOutputStream("sol.dat"));
f.println(time + " " + u[i]);
f.close()
