Back to some basic stuff (enum range).
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Thread: Back to some basic stuff (enum range).

  1. #1
    Join Date
    Dec 2010
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    588

    Back to some basic stuff (enum range).

    In the book The C++ Programming Language

    It reads
    "An enumerator can be initialized by a constant expression of integral type. The range
    of an enumeration holds all the enumeration's enumerator values rounded up to the nearest binary power minus 1"
    Code:
    enum e1 { dark, light };  // range 0:1
    enum e2 { a = 3, b = 9 }; // range: 0:15
    enum e3 { min = -10, max = 1000000 }; // range -1048576:1048575
    I don't understand how you get the ranges. Could anyone explain a little bit?
    Thanks
    Jack

  2. #2
    Join Date
    Jul 2002
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    Portsmouth. United Kingdom
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    Re: Back to some basic stuff (enum range).

    enum e1 { dark, light }; // range 0:1
    Depends on the values of dark & light.
    If dark = 0 & light = 1 then the values can be represented in one bit.
    The range of an unsigned 1 bit number is 0 to 1

    enum e2 { a = 3, b = 9 }; // range: 0:15
    In binary 3 = 11 & 9 = 1001
    You need a minimum of 4 bits to cover this range.
    The range of an unsigned 4 bit number is 0000 to 1111 = 0 to 15

    enum e3 { min = -10, max = 1000000 }; // range -1048576:1048575
    In binary, 1000000 = 11110100001001000000, which needs 20 bits.
    As the other number is negative and assuming we are using 2's complement to represent negative numbers,
    we need 21 bits.
    1000000 = 011110100001001000000
    -10 = 111111111111111110110
    The range of a 2's complement 21 bit number is -1048576 to 1048575
    "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
    Richard P. Feynman

  3. #3
    Join Date
    Dec 2010
    Posts
    588

    Re: Back to some basic stuff (enum range).

    Thanks John for your detailed explanation, I get it now

  4. #4
    Join Date
    Jul 2002
    Location
    Portsmouth. United Kingdom
    Posts
    2,699

    Re: Back to some basic stuff (enum range).

    Sorry, being a bit dumb there.
    dark & light by definition will have the values of 0 & 1
    "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
    Richard P. Feynman

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