# A quick look at DAPPLE programming

To give you a quick look at what DAPPLE programs look like, let's look
at a program for computing Pascal's triangle. First of all, here is
Pascal's triangle:

1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 28 56 70 56 28 8 1
1 9 36 84 126 126 84 36 9 1
1 10 45 120 210 252 210 120 45 10 1
Each row is computed from the previous row; each entry in a new row
is found by adding the number above it to the number above and to the
left.

Here is the beginning of a DAPPLE program to compute Pascal's triangle:

const int N = 11;
intVector arow(N);
This defines an integer vector called "arow", with N elements
numbered 0, 1, ..., N-1. This vector will soon contain one row of the
triangle, but for now the elements are uninitialized. Vectors may be
initialized as part of their definition, to a scalar, an array,
another vector, or a function of the index. Now we see an example of
the latter:
// defined by DAPPLE; returns i
extern int Identity(int i);
// which virtual processor am I?
const intVector VP(N, Identity);
This defines an N-element integer vector called "VP", initialized so
that element i has value i.
ifp (VP == 0) {
arow = 1;
cout << arow << endl;
} else
arow = 0;
This uses the * parallel if * statement to initialize "arow".
The "then" clause executes only for those virtual processors where the
condition (VP == 0) is true, in this case, only virtual processor 0.
Thus, it assigns and prints arow[0]. This one element is of course
the entire first row of Pascal's triangle. The "else" clause executes
for the remaining virtual processors.
// to each element, add the element to the left
for (int i = 1; i <= N-1; i++) {
arow += shift(arow, 1);
ifp (VP <= i)
cout << arow << endl;
}
This for loop computes and prints N-1 more rows. Each time through
the loop we compute a new row of the triangle, in parallel, by adding
the current row to itself, shifted one to the right (a zero is shifted
in at the left side). Then, we print out the vector, but only
elements 0 through i, i.e., the non-zero elements of "arow". The
output looks like that above.
You can see whole program, if you
like.

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