# Algorithms/Ada Implementation

Top,
Chapters:
1,
2,
3,
4,
5,
6,
7,
8,
9,
*A*

Welcome to the Ada implementations of the Algorithms Wikibook. For those who are new to Ada Programming a few notes:

- All examples are fully functional with all the needed input and output operations. However, only the code needed to outline the algorithms at hand is copied into the text - the full samples are available via the download links. (Note: It can take up to 48 hours until the cvs is updated).
- The algorithms in the book are written in a pseudolanguage. Every computer language has its own conventions how to write identifiers; some languages are case sensitive, Ada isn't; some write identifiers in CamelCase. Ada uses the convention of separating words by underscores and capitalizing the first character of each word. For numerical values, Ada uses the convention to separate digit groups by underscores for better readability - compare 10000000 to 10_000_000 or 5000001 to 50_000_01 (e.g. 50 thousand € and one ¢).
- We seldom use predefined types in the sample code but define special types suitable for the algorithms at hand.
- Ada allows for default function parameters; however, we always fill in and name all parameters, so the reader can see which options are available.
- We seldom use shortcuts - like using the attributes Image or Value for String <=> Integer conversions.

All these rules make the code more elaborate than perhaps needed. However, we also hope it makes the code easier to understand

## Chapter 1: Introduction[edit | edit source]

The following subprograms are implementations of the *Inventing an Algorithm* examples.

### To Lower[edit | edit source]

The Ada example code does not append to the array as the algorithms. Instead we create an empty array of the desired length and then replace the characters inside.

`function`

To_Lower (C : Character)`return`

Character`renames`

Ada.Characters.Handling.To_Lower; -- tolower - translates all alphabetic, uppercase characters -- in str to lowercase`function`

To_Lower (Str : String)`return`

String`is`

Result : String (Str'Range);`begin`

`for`

C`in`

Str'Range`loop`

Result (C) := To_Lower (Str (C));`end`

`loop`

;`return`

Result;`end`

To_Lower;

Would the append approach be impossible with Ada? No, but it would be significantly more complex and slower.

### Equal Ignore Case[edit | edit source]

-- equal-ignore-case -- returns true if s or t are equal, -- ignoring case`function`

Equal_Ignore_Case (S : String; T : String)`return`

Boolean`is`

O :`constant`

Integer := S'First - T'First;`begin`

`if`

T'Length /= S'Length`then`

`return`

False; -- if they aren't the same length, they -- aren't equal`else`

`for`

I`in`

S'Range`loop`

`if`

To_Lower (S (I)) /= To_Lower (T (I + O))`then`

`return`

False;`end`

`if`

;`end`

`loop`

;`end`

`if`

;`return`

True;`end`

Equal_Ignore_Case;

## Chapter 6: Dynamic Programming[edit | edit source]

### Fibonacci numbers[edit | edit source]

The following codes are implementations of the Fibonacci-Numbers examples.

#### Simple Implementation[edit | edit source]

...

To calculate Fibonacci numbers negative values are not needed so we define an integer type which starts at 0. With the integer type defined you can calculate up until `Fib (87)`

. `Fib (88)`

will result in an `Constraint_Error`

.

`type`

Integer_Type`is`

`range`

0 .. 999_999_999_999_999_999;

You might notice that there is not equivalence for the `assert (n >= 0)`

from the original example. Ada will test the correctness of the parameter *before* the function is called.

`function`

Fib (n : Integer_Type)`return`

Integer_Type`is`

`begin`

`if`

n = 0`then`

`return`

0;`elsif`

n = 1`then`

`return`

1;`else`

`return`

Fib (n - 1) + Fib (n - 2);`end`

`if`

;`end`

Fib; ...

#### Cached Implementation[edit | edit source]

...

For this implementation we need a special cache type can also store a -1 as "not calculated" marker

`type`

Cache_Type`is`

`range`

-1 .. 999_999_999_999_999_999;

The actual type for calculating the fibonacci numbers continues to start at 0. As it is a `subtype`

of the cache type Ada will automatically convert between the two. (the conversion is - of course - checked for validity)

`subtype`

Integer_Type`is`

Cache_Type`range`

0 .. Cache_Type'Last;

In order to know how large the cache need to be we first read the actual value from the command line.

` Value : ``constant`

Integer_Type :=
Integer_Type'Value (Ada.Command_Line.Argument (1));

The Cache array starts with element 2 since Fib (0) and Fib (1) are constants and ends with the value we want to calculate.

`type`

Cache_Array`is`

`array`

(Integer_Type`range`

2 .. Value)`of`

Cache_Type;

The Cache is initialized to the first valid value of the cache type — this is `-1`

.

` F : Cache_Array := (``others`

=> Cache_Type'First);

What follows is the actual algorithm.

`function`

Fib (N : Integer_Type)`return`

Integer_Type`is`

`begin`

`if`

N = 0`or`

`else`

N = 1`then`

`return`

N;`elsif`

F (N) /= Cache_Type'First`then`

`return`

F (N);`else`

F (N) := Fib (N - 1) + Fib (N - 2);`return`

F (N);`end`

`if`

;`end`

Fib; ...

This implementation is faithful to the original from the Algorithms book. However, in Ada you would normally do it a little different:

when you use a slightly larger array which also stores the elements 0 and 1 and initializes them to the correct values

`type`

Cache_Array`is`

`array`

(Integer_Type`range`

0 .. Value)`of`

Cache_Type; F : Cache_Array := (0 => 0, 1 => 1,`others`

=> Cache_Type'First);

and then you can remove the first `if`

path.

`if`

N = 0`or`

`else`

N = 1`then`

`return`

N; els`if`

F (N) /= Cache_Type'First`then`

This will save about 45% of the execution-time (measured on Linux i686) while needing only two more elements in the cache array.

#### Memory Optimized Implementation[edit | edit source]

This version looks just like the original in WikiCode.

`type`

Integer_Type`is`

`range`

0 .. 999_999_999_999_999_999;`function`

Fib (N : Integer_Type)`return`

Integer_Type`is`

U : Integer_Type := 0; V : Integer_Type := 1;`begin`

`for`

I`in`

2 .. N`loop`

Calculate_Next :`declare`

T :`constant`

Integer_Type := U + V;`begin`

U := V; V := T;`end`

Calculate_Next;`end`

`loop`

;`return`

V;`end`

Fib;

#### No 64 bit integers[edit | edit source]

Your Ada compiler does not support 64 bit integer numbers? Then you could try to use decimal numbers instead. Using decimal numbers results in a slower program (takes about three times as long) but the result will be the same.

The following example shows you how to define a suitable decimal type. Do experiment with the `digits`

and `range`

parameters until you get the optimum out of your Ada compiler.

`type`

Integer_Type`is`

`delta`

1.0`digits`

18`range`

0.0 .. 999_999_999_999_999_999.0;

You should know that floating point numbers are unsuitable for the calculation of fibonacci numbers. They will not report an error condition when the number calculated becomes too large — instead they will lose in precision which makes the result meaningless.