Ada Reference Manual (Ada 2022) — Legal Information
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2.4.2 Based Literals

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A based_literal is a numeric_literal expressed in a form that specifies the base explicitly. 

Syntax

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based_literal ::= 
   base # based_numeral [.based_numeral] # [exponent]
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base ::= numeral
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based_numeral ::= 
   extended_digit {[underline] extended_digit}
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extended_digit ::= digit | A | B | C | D | E | F

Legality Rules

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The base (the numeric value of the decimal numeral preceding the first #) shall be at least two and at most sixteen. The extended_digits A through F represent the digits ten through fifteen, respectively. The value of each extended_digit of a based_literal shall be less than the base. 

Static Semantics

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The conventional meaning of based notation is assumed. An exponent indicates the power of the base by which the value of the based_literal without the exponent is to be multiplied to obtain the value of the based_literal with the exponent. The base and the exponent, if any, are in decimal notation.
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The extended_digits A through F can be written either in lower case or in upper case, with the same meaning. 

Examples

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Examples of based literals: 
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2#1111_1111#  16#FF#      016#0ff#  -- integer literals of value 255
16#E#E1      2#1110_0000#           -- integer literals of value 224
16#F.FF#E+2  2#1.1111_1111_1110#E11 -- real literals of value 4095.0
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