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Annotation of 42BSD/usr.lib/learn/C/L41.1a, revision 1.1.1.1

1.1       root        1: #print
                      2: The problem is to produce a function
                      3:        bitct(x)
                      4: which examines the bits in x, returning a count of
                      5: the number of 1-bits.  There are various ways of doing
                      6: this job: here are two.
                      7: (1) a sane way.  Shift the word x right 16 times (you are
                      8: on UNIX) and check the rightmost bit each time, counting
                      9: the number of times it is '1'.
                     10: (2) a machine-independent (sort of) way.  The logical
                     11: bitwise AND of x and x-1 contains one fewer one bit than x itself.
                     12: Loop anding x and x-1 until you get zero.
                     13: Program either algorithm.  Compile and test it.  Leave it on
                     14: a file bitct.c and type "ready".
                     15: #once #create tzaqc.c
                     16: main()
                     17: {
                     18:        int x;
                     19:        x=23069;
                     20:        if (bitct(x) != goodct(x)) 
                     21:                return(1);
                     22:        x=0;
                     23:        if (bitct(x) != goodct(x)) 
                     24:                return(1);
                     25:        x=16384;
                     26:        if (bitct(x) != goodct(x)) 
                     27:                return(1);
                     28:        x = -1;
                     29:        if (bitct(x) != goodct(x)) 
                     30:                return(1);
                     31:        x= -200;
                     32:        if (bitct(x) != goodct(x)) 
                     33:                return(1);
                     34:        return(0);
                     35: }
                     36: goodct(x)
                     37: {
                     38:        int k, i;
                     39:        for(k=i=0; i<16; i++)
                     40:                {
                     41:                k =+ (x&1);
                     42:                x= x>>1;
                     43:                }
                     44:        return(k);
                     45: }
                     46: #user
                     47: cc tzaqc.c bitct.o
                     48: a.out
                     49: #succeed
                     50: /*  a possible solution */
                     51: bitct(x)
                     52: {
                     53:        int k, i;
                     54: 
                     55:        for(i=k=0; i<16; i++) {
                     56:                if (x&1)
                     57:                        k++;
                     58:                x >>= 1;
                     59:        }
                     60:        return(k);
                     61: }
                     62: /* by the way, if you really care about
                     63: this problem a table lookup by whole bytes
                     64: is faster */
                     65: #log
                     66: #next
                     67: 42.1a 10

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