.. default-domain:: chpl

.. _primers-hello3-datapar:

Data-parallel hello world
=========================

`View hello3-datapar.chpl on GitHub <https://github.com/chapel-lang/chapel/blob/master/test/release/examples/hello3-datapar.chpl>`_



This program uses Chapel's data parallel features to create a
parallel hello world program that utilizes multiple cores on a
single `locale` (compute node).




The following `configuration constant` indicates the number of
messages to print out.  The default can be overridden on the
command-line (e.g., ``./hello --numMessages=1000000``).


.. code-block:: chapel

    config const numMessages = 100;



Next, we use a data-parallel `forall-loop` to iterate over a
`range` representing the number of messages to print.  By default,
forall-loops will typically be executed cooperatively by a number
of tasks proportional to the hardware parallelism on which the loop
is running. Ranges like ``1..numMessages`` are always local to the
current task's locale, so this forall-loop will execute using the
number of local processing units or cores.

Because the messages are printed within a parallel loop, they may
be displayed in any order.  The `writeln()` procedure protects
against finer-grained interleaving of the messages themselves.


.. code-block:: chapel

    forall msg in 1..numMessages do
      writeln("Hello, world! (from iteration ", msg, " of ", numMessages, ")");



For further examples of using data parallelism, refer to the data
parallel :ref:`primer examples <primers>`.