Ranges and arrays

A series of integers (1,2,3,4,5, for example), is called a range in Chapel. Ranges are generated with the .. operator, and are useful, among other things, to declare arrays of variables. For example, the following variables

var T: [0..rows+1,0..cols+1] real;      // current temperatures
var Tnew: [0..rows+1,0..cols+1] real;   // newly computed temperatures

are 2D arrays (matrices) with (rows + 2) rows and (cols + 2) columns of real numbers, all initialized as 0.0. The ranges 0..rows+1 and 0..cols+1 used here, not only define the size and shape of the array, they stand for the indices with which we could access particular elements of the array using the [ , ] notation. For example, T[0,0] is the real variable located at the first row and first column of the array T, while T[3,7] is the one at the 4th row and 8th column; T[2,3..15] access columns 4th to 16th of the 3th row of T, and T[0..3,4] corresponds to the first 4 rows on the 5th column of T. Similarly, with

T[1..rows,1..cols] = 25;     // set the initial temperature

we assign an initial temperature of 25 degrees across all points of our metal plate.

We must now be ready to start coding our simulations … here is what we are going to do:

• this simulation will consider a matrix of rows by cols elements
• it will run up to niter iterations, or until the largest difference in temperature between iterations is less than tolerance
• at each iteration print out the temperature at the position (iout,jout)

Using expressions to create arrays

In Chapel arrays can also be initialized with expressions (similarly to list comprehensions in Python):

writeln([i in 1..10] i**2);   // prints 1 4 9 16 25 36 49 64 81 100
var x = [i in 1..10] if (i%2 == 0) then i else 0;
writeln(x);                   // prints 0 2 0 4 0 6 0 8 0 10
writeln(x.type:string);       // 1D array of type int(64)