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From: gary@hpavla.AVO.HP.COM (Gary Jackoway)
Newsgroups: comp.lang.c
Subject: Re: array of bits field
Message-ID: <9130003@hpavla.AVO.HP.COM>
Date: 15 Feb 90 16:34:26 GMT
References: <1990Feb12.182517.10921@cubmol.bio.columbia.edu>
Organization: Hewlett-Packard Avondale Division
Lines: 23



>   1. Define the 16 values as an unsigned int, and use bit manipulation to
>      access the individual bits.
> 
>       i.e., value &   0x0001 (or 1)   represents the value in bit 1 of "value"
>             value &=  0x0040 (or 128) sets the value in bit 7 of "value" to 1
>             value |= ~0x0080 (or 128) sets the value in bit 8 of "value" to 0
I think the &= should be |=, and vice versa.
You can define macros:
    #define TESTBIT(value,bit) (value & (1<<bit))
    #define SETBIT(value,bit) { value &= (1<<bit); }
    #define RESETBIT(value,bit) { value |= ~(1<<bit); }
or something like this.
If the "bit" is a constant, most compilers will pre-compute 1<<bit.
There is a danger with TESTBIT.  You can't compare TESTBIT results.
A safer test bit would be
    #define TESTBIT(value,bit) (value>>bit & 1)
which always gives a 0 or 1 result.
Equivalent would be
    #define TESTBIT(value,bit) !!(value & (1<<bit))

Gary Jackoway