Data and block data |
The data statement
The data statement is another way to input data that are
known at the time when the program is written. It is similar to the assignment
statement. The syntax is:
A D V E R T I S E M E N T
data list-of-variables/ list-of-values/,
...
where the three dots means that this pattern can be
repeated. Here is an example:
data m/10/, n/20/, x/2.5/, y/2.5/
We could also have written this
data m,n/10,20/, x,y/2*2.5/
We could have accomplished the same thing by the
assignments
m = 10
n = 20
x = 2.5
y = 2.5
The data statement is more compact and therefore often more
convenient. Notice especially the shorthand notation for assigning identical
values repeatedly.
The data statement is performed only once, right before the
execution of the program starts. For this reason, the data statement is mainly
used in the main program and not in subroutines.
The data statement can also be used to initialize arrays
(vectors, matrices). This example shows how to make sure a matrix is all zeros
when the program starts:
real A(10,20)
data A/ 200 * 0.0/
Some compilers will automatically initialize arrays like
this but not all, so if you rely on array elements to be zero it is a good idea
to follow this example. Of course you can initialize arrays to other values than
zero. You may even initialize individual elements:
data A(1,1)/ 12.5/, A(2,1)/ -33.3/, A(2,2)/ 1.0/
Or you can list all the elements for small arrays like
this:
integer v(5)
real B(2,2)
data v/10,20,30,40,50/, B/1.0,-3.7,4.3,0.0/
The values for two-dimensional arrays will be assigned in
column-first order as usual.
The block data statement
The data statement cannot be
used for variables contained in a common block. There is a special "subroutine"
for this purpose, called block data. It is not really a subroutine, but it
looks a bit similar because it is given as a separate program unit. Here is an
example:
block data
integer nmax
parameter (nmax=20)
real v(nmax), alpha, beta
common /vector/v,alpha,beta
data v/20*100.0/, alpha/3.14/, beta/2.71/
end
Just as the data statement, block data is executed once
before the execution of the main program starts. The position of the block data
"subroutine" in the source code is irrelevant (as long as it is not nested
inside the main program or a subprogram).
