Ada Programming/Lexical elements

< Ada Programming

Ada Lovelace 1838.jpg

Character setEdit

The character set used in Ada programs is composed of:

  • Upper-case letters: A, ..., Z and lower-case letters: a, ..., z.
  • Digits: 0, ..., 9.
  • Special characters.

Take into account that in Ada 95 the letter range includes accented characters and other letters used in Western Europe languages, those belonging to the ISO Latin-1 character set, as ç, ñ, ð, etc.

In Ada 2005 the character set has been extended to the full Unicode set, so the identifiers and comments can be written in almost any language in the world.

Ada is a case-insensitive language, i. e. the upper-case set is equivalent to the lower-case set except in character string literals and character literals.

Lexical elementsEdit

In Ada we can find the following lexical elements:


Temperature_In_Room := 25;  -- Temperature to be preserved in the room.

This line contains 5 lexical elements:

  • The identifier Temperature_In_Room.
  • The compound delimiter :=.
  • The number 25.
  • The single delimiter ;.
  • The comment -- Temperature to be preserved in the room..


Definition in BNF:

identifier ::= letter { [ underscore ] letter | digit }
letter ::= A | ... | Z | a | ... | z
digit ::= 0 | ... | 9
underscore ::= _

From this definition we must exclude the keywords that are reserved words in the language and cannot be used as identifiers.


The following words are legal Ada identifiers:

Time_Of_Day  TimeOfDay  El_Niño_Forecast  Façade  counter ALARM

The following ones are NOT legal Ada identifiers:

_Time_Of_Day  2nd_turn  Start_  Access  Price_In_$  General__Alarm

Exercise: could you give the reason for not being legal for each one of them?


The numeric literals are composed of the following characters:

  • digits 0 .. 9
  • the decimal separator .,
  • the exponentiation sign e or E,
  • the negative sign - (in exponents only) and
  • the underscore _.

The underscore is used as separator for improving legibility for humans, but it is ignored by the compiler. You can separate numbers following any rationale, e.g. decimal integers in groups of three digits, or binary integers in groups of eight digits.

For example, the real number such as 98.4 can be represented as: 9.84E1, 98.4e0, 984.0e-1 or 0.984E+2, but not as 984e-1.

For integer numbers, for example 1900, it could be written as 1900, 19E2, 190e+1 or 1_900E+0.

A numeric literal could also be expressed in a base different to 10, by enclosing the number between # characters, and preceding it by the base, which can be a number between 2 and 16. For example, 2#101# is 1012, that is 510; a hexadecimal number with exponent is 16#B#E2, that is 11 × 16² = 2,816.

Note that there are no negative literals; e.g. -1 is not a literal, rather it is the literal 1 preceded by the unary minus operator.

Character literalsEdit

Their type is Standard.Character, Wide_Character or Wide_Wide_Character. They are delimited by an apostrophe (').


'A' 'n' '%'

String literalsEdit

String literals are of type Standard.String, Wide_String or Wide_Wide_String. They are delimited by the quotation mark (").


"This is a string literal"


Single delimiters are one of the following special characters:

&    '    (    )    *    +    ,    -    .    /    :    ;    <    =    >    

Compound delimiters are composed of two special characters, and they are the following ones:

=>    ..    **    :=    /=    >=    <=    <<    >>    <>

You can see a full reference of the delimiters in Ada Programming/Delimiters.


Comments in Ada start with two consecutive hyphens (--) and end in the end of line.

-- This is a comment in a full line
My_Savings := My_Savings * 10.0; -- This is a comment in a line after a sentence
My_Savings := My_Savings * -- This is a comment inserted inside a sentence

A comment can appear where an end of line can be inserted.

Reserved wordsEdit

Reserved words are equivalent in upper-case and lower-case letters, although the typical style is the one from the Reference Manual, that is to write them in all lower-case letters.

In Ada some keywords have a different meaning depending on context. You can refer to Ada Programming/Keywords and the following pages for each keyword.

Ada Keywords
abort else new return
abs elsif not reverse
abstract (Ada 95) end null
accept entry select
access exception of separate
aliased (Ada 95) exit or some (Ada 2012)
all others subtype
and for out synchronized (Ada 2005)
array function overriding (Ada 2005)
at tagged (Ada 95)
generic package task
begin goto pragma terminate
body private then
if procedure type
case in protected (Ada 95)
constant interface (Ada 2005) until (Ada 95)
is raise use
declare range
delay limited record when
delta loop rem while
digits renames with
do mod requeue (Ada 95) xor

See alsoEdit


Ada Reference ManualEdit

Read in another language