Ranges and arrays

Overview

Teaching: 60 min
Exercises: 30 min

Questions

• What is Chapel and why is it useful?

Objectives

• Learn to define and use ranges and arrays.

Ranges and Arrays

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

Let’s examine what a range looks like (ranges.chpl in this example):

var example_range = 0..10;
writeln('Our example range was set to: ', example_range);

chpl ranges.chpl -o ranges.o
./ranges.o

Our example range was set to: 0..10

An array is a multidimensional sequence of values. Arrays can be any size, and are defined using ranges: Let’s define a 1-dimensional array of the size example_range and see what it looks like. Notice how the size of an array is included with its type.

var example_range = 0..10;
writeln('Our example range was set to: ', example_range);
var example_array: [example_range] real;
writeln('Our example array is now: ', example_array);

We can reassign the values in our example array the same way we would reassign a variable. An array can either be set all to a single value, or a sequence of values.

var example_range = 0..10;
writeln('Our example range was set to: ', example_range);
var example_array: [example_range] real;
writeln('Our example array is now: ', example_array);
example_array = 5;
writeln('When set to 5: ', example_array);
example_array = 1..11;
writeln('When set to a range: ', example_array);

chpl ranges.chpl -o ranges.o
./ranges.o

Our example range was set to: 0..10
Our example array is now: 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
When set to 5: 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
When set to a range: 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Notice how ranges are “right inclusive”, the last number of a range is included in the range. This is different from languages like Python where this does not happen.

Indexing elements

One final thing - we can retrieve and reset specific values of an array using [] notation. Let’s try retrieving and setting a specific value in our example so far:

var example_range = 0..10;
writeln('Our example range was set to: ', example_range);
var example_array: [example_range] real;
writeln('Our example array is now: ', example_array);
example_array = 5;
writeln('When set to 5: ', example_array);
example_array = 1..11;
writeln('When set to a range: ', example_array);
// retrieve the 5th index
writeln(example_array[5]);
// set index 5 to a new value
example_array[5] = 99999;
writeln(example_array);

chpl ranges.chpl -o ranges.o
./ranges.o

Our example range was set to: 0..10
Our example array is now: 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
When set to 5: 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
When set to a range: 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
6.0
1.0 2.0 3.0 4.0 5.0 99999.0 7.0 8.0 9.0 10.0 11.0

One very important thing to note - in this case, index 5 was actually the 6th element. This was caused by how we set up our array. When we defined our array using a range starting at 0, element 5 corresponds to the 6th element. Unlike most other programming languages, arrays in Chapel do not start at a fixed value - they can start at any number depending on how we define them! For instance, let’s redefine example_range to start at 5:

var example_range = 5..15;
writeln('Our example range was set to: ', example_range);
var example_array: [example_range] real;
writeln('Our example array is now: ', example_array);
example_array = 5;
writeln('When set to 5: ', example_array);
example_array = 1..11;
writeln('When set to a range: ', example_array);
// retrieve the 5th index
writeln(example_array[5]);
// set index 5 to a new value
example_array[5] = 99999;
writeln(example_array);

chpl ranges.chpl -o ranges.o
./ranges.o

Our example range was set to: 5..15
Our example array is now: 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
When set to 5: 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
When set to a range: 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
1.0
99999.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

Back to our simulation

Let’s define a two dimensional array for use in our simulation:

// this is our "plate"
var temp: [0..rows+1, 0..cols+1] real = 25;

This is a matrix (2D array) with (rows + 2) rows and (cols + 2) columns of real numbers, all initialised as 25.0. 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, temp[0,0] is the real variable located at the first row and first column of the array temp, while temp[3,7] is the one at the 4th row and 8th column; temp[2,3..15] access columns 4th to 16th of the 3th row of temp, and temp[0..3,4] corresponds to the first 4 rows on the 5th column of temp.

We must now be ready to start coding our simulations. Let’s print some information about the initial configuration, compile the code, and execute it to see if everything is working as expected.

const rows = 100;
const cols = 100;
const niter = 500;
const x = 50;                   // row number of the desired position
const y = 50;                   // column number of the desired position
const mindif = 0.0001;          // smallest difference in temperature that would be accepted before stopping

// this is our "plate"
var temp: [0..rows+1, 0..cols+1] real = 25;

writeln('This simulation will consider a matrix of ', rows, ' by ', cols, ' elements.');
writeln('Temperature at start is: ', temp[x, y]);

>> chpl base_solution.chpl -o base_solution
>> ./base_solution

This simulation will consider a matrix of 100 by 100 elements.
Temperature at start is: 25.0

Key Points

• A range is a sequence of integers.

• An array holds a sequence of values.

• Chapel arrays can start at any index, not just 0 or 1.

• You can index arrays with the [] brackets.