Hi Andy --
Let me start with some meta stuff:
First, note that you're pointing to the 1.14 version of the documentation
but probably want to be looking at the current (1.17) version. I'm
guessing you got there through a Google search. We've been working to
teach Google to point people to our latest documents, but haven't yet been
successful at that... In the meantime, anytime you're on a Chapel
documentation page, try to glance at the color of the version menu (upper
left), and if it's orange, use that as a warning that you might want to
jump to the latest version (which you can often do by pulling down on the
version menu and choosing the latest version if the page hasn't changed
location between releases... otherwise, you'll have to search for it
within the documentation's search function directly).
Second, the page you pointed to is for associative arrays which support
dictionary or hash-table styles of arrays, associating arbitrary keys to
values. Coming from Fortran (and looking at the example on the
allocatable arrays page you pointed to), you're much more likely to want
to use Chapel's rectangular domains and arrays which are described here:
https://chapel-lang.org/docs/primers/arrays.html
Third, if you're a Stack Overflow user, questions like this are great to
post there so that they'll help others in the future as well. Gitter is
another place that short questions can be asked if you prefer a chat
format.
OK, now to your specific question about allocatable arrays: Chapel's
arrays are _very_ dynamically allocatable (and reallocatable). There are
a few ways you can approach this (and the primer above may be more useful
than my terse response here):
First, declarations and computations can be mixed in Chapel, so you could
do something like the following to intermix the setting of a size like
numParticles and the declaration of the arrays themselves:
// this will eventually say how large our arrays should be
var numParticles: int;
// ...do some arbitrary computations and stuff...
// read in a value for numParticles
read(numParticles);
// now declare our arrays over that size
var charge: [1..numParticles] real,
xyz: [1..numParticles, 1..3] real;
You can often just get the size right the first time by using a variable
initializer that does the computation:
var numParticles = read(int),
charge: [1..numParticles] real,
xyz: [1..numParticles, 1..3] real;
And this is often a good place to use a 'config' declaration of the sort I
was mailing you about earlier:
config const numParticles = read(int);
var charge: [1..numParticles] real,
xyz: [1..numParticles, 1..3] real;
Finally, you can always reallocate arrays over time by separating their
index set ("domain" in Chapel) from their declaration as follows:
// create a domain that's initially empty
var particleSpace = {1..0};
// create some initially empty arrays
var charge: [particleSpace] real,
xyz: [particleSpace] [1..3] real;
// read a size
const numParticles = read(int);
// re-assign the domain -- this will cause the arrays declared
// using particleSpace to be re-allocated to incorporate these new
// indices.
particleSpace = {1..numParticles};
// ...do some computations with charge and xyz here...
// double the size of particle space (for some reason).
// Now all the arrays will be twice as large
particleSpace = {1..2*numParticles};
Reallocating arrays is expensive by nature, so typically you don't want to
do this unless you have no other choice; but this gives you a sense of how
you can when the need arises.
Hope this helps,
-Brad
On Tue, 19 Jun 2018, Andrew Halper wrote:
Hello,
I was wondering if there is a recommended strategy in Chapel for imitating
Fortran90's "allocatable" arrays
<http://kea.princeton.edu/ChE422/arrays.htm>? (From what I read here
<https://chapel-lang.org/docs/1.14/primers/primers/associative.html#primers-associative>,
it seems like there might not be one, for good reasons, and that's fine.)
Thank you,
--
Andy Halper
USGS
Integrated Modeling and Prediction Division
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