A Tangent into Ada

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The following instructions are for installing and programming Ada on Arch Linux.

Prerequisites

Install the Ada compiler GNAT with the following command:

$ pacman -S gcc-ada

After pacman has finished the install the gnat command will be available to compile Ada programs.

Hello, World!

The first thing to do in any new language is the classic "Hello, World!" program. This will introduce us to the feel of Ada and give us an idea of what we can expect.

Lets get started. First we will create a new directory for this program.

$ mkdir ~/src/hello_world
$ cd ~/src/hello_world

Now get started writing the program in a new file called hello_world.adb.

with Ada.Text_IO;

procedure Hello_World is
begin
   Ada.Text_IO.Put_Line("Hello, World!");
end Hello_World;

Compile and run:

$ gnat make hello_world.adb
$ ./hello_world
Hello, World!

Great, we finsihed our first Ada program! Next up is the first problem from Project Euler.

Find the sum of all the multiples of 3 or 5 below 1000.

Open a new file called euler1.adb and type the following text inside:

with Ada.Integer_Text_IO;

procedure Euler1 is
    package IO renames Ada.Integer_Text_IO;
    Sum : Integer;
begin

    Sum := 0;

    for I in Integer range 1..999 loop
        if I mod 3 = 0 then
            Sum := Sum + I;
        elsif I mod 5 = 0 then
            Sum := Sum + I;
        end if;
    end loop;

    IO.Put(Sum); 
end Euler1;

Again, compile and run:

$ gnat make euler1.adb
$ ./euler1
     233168

Great! 233168 is the correct answer. Now on to problem number 2! This problem is a little harder, but it is still just counting numbers.

Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed four million.

Start another file titled euler2.adb and type the following text:

with Ada.Integer_Text_IO;
use Ada;

procedure Euler2 is

    Fib1 : Natural;
    Fib2 : Natural;
    Sum : Natural;

begin
    Sum := 0;
    Fib1 := 1;
    Fib2 := 2;

    while Fib1 < 4_000_000 and Fib2 < 4_000_000 loop

        if Fib1 > Fib2 then
            if Fib1 mod 2 = 0 then
                Sum := Sum + Fib1;
            end if;
            Fib2 := Fib1 + Fib2;
        else
            if Fib2 mod 2 = 0 then
                Sum := Sum + Fib2;
            end if;
            Fib1 := Fib1 + Fib2;
        end if;
    end loop;

    Integer_Text_IO.Put(Sum);

end Euler2;

Compile and run to get the result:

$ gnat make euler2.adb
$ ./euler2
    4613732

And this concludes the short tangent into Ada. The compile is up and running and we are solving some problems. Go ahead and do a few more at Project Euler.

Update

Problem number 2 of Project Euler actually has a more efficient solution.

The Fibonacci sequence actually has even integers every third number.

Fibonacci   1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610...
Even / Odd  O, O, E, O, O, E,  O,  O,  E,  O,  O,   E,   O,   O,   E...

With this knowlege, the only calculations that need to be done are for every third number in the sequence. The sequence in terms of x and y is:

x, y, x + y, x + 2y, 2x + 3y, 3x + 5y...

Now for the source text:

with Ada.Integer_Text_IO;
use Ada;

procedure Euler2 is

    X : Natural;
    Y : Natural;
    Sum : Natural;
    Old_X : Natural;

begin

    Sum := 0;
    X := 1;
    Y := 1;

    while Y < 4_000_000 loop
        Sum := Sum + X + Y;
        Old_X := X;
        X := X + 2 * Y;
        Y := 2 * Old_X + 3 * Y;
    end loop;

    Integer_Text_IO.Put(Sum);

end Euler2;

Compile and run. The result should be the same.

$ gnat make euler2.adb
$ ./euler2
    4613732