Ranges and arrays

Last updated on 2024-11-18 | Edit this page

Estimated time: 90 minutes

Overview

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. Let’s examine what a range looks like; we store the following code as ranges.chpl. Here we introduce a very simple loop, cycling through all elements of the range and printing their values (we will study for loops in a separate section):

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

BASH

chpl ranges.chpl
./ranges

OUTPUT

Our example range was set to: 0..10
0
1
...
9
10

Among other uses, ranges can be used to declare arrays of variables. An array is a multidimensional collection of values of the same type. Arrays can be of any size. 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 to 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);

BASH

chpl ranges.chpl
./ranges

OUTPUT

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

We can retrieve and reset specific values of an array using [] notation. Note that we use the same square bracket notation in two different contexts: (1) to declare an array, with the square brackets containing the array’s full index range [example_range], and (2) to access specific array elements, as we will see below. 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);

BASH

chpl ranges.chpl
./ranges

OUTPUT

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);

BASH

chpl ranges.chpl
./ranges

OUTPUT

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 and set its initial values:

// this is our "plate"
var temp: [0..rows+1, 0..cols+1] real;
temp[1..rows,1..cols] = 25;     // set the initial temperature on the internal grid

This is a matrix (2D array) with (rows + 2) rows and (cols + 2) columns of real numbers. 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 divide our “plate” into two parts: (1) the internal grid 1..rows,1..cols on which we set the initial temperature at 25.0, and (2) the surrounding layer of ghost points with row indices equal to 0 or rows+1 and column indices equal to 0 or cols+1. The temperature in the ghost layer is equal to 0.0 by default, as we do not assign a value there.

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 tolerance = 0.0001;       // smallest difference in temperature that would be accepted before stopping
const outputFrequency: int = 20;   // the temperature will be printed every outputFrequency iterations

// this is our "plate"
var temp: [0..rows+1, 0..cols+1] real;
temp[1..rows,1..cols] = 25;     // set the initial temperature on the internal grid

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

BASH

chpl base_solution.chpl
./base_solution

OUTPUT

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 non-negative number of values of the same type.”
“Chapel arrays can start at any index, not just 0 or 1.”
“You can index arrays with the [] brackets.”