in reply to Re: help with algorithm
in thread help with algorithm

Ok, maybe I'm a little dense: if I understand you correctly, I should be able to determine "monday" from the 1 in the first position of the binary number "1010". However, how does this work for other values, like say, Tuesday, which would show up as 4, which is 100 in binary??

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Re: Re: Re: help with algorithm
by sauoq (Abbot) on Nov 17, 2002 at 01:24 UTC

    Maybe this will make things clearer... 

     
my $Sun = 2**0;    # That's 1  or 00000001 in binary. 
my $Mon = 2**1;    # That's 2  or 00000010 in binary. 
my $Tue = 2**2;    # That's 4  or 00000100 in binary. 
my $Wed = 2**3;    # That's 8  or 00001000 in binary. 
my $Thu = 2**4;    # That's 16 or 00010000 in binary. 
my $Fri = 2**5;    # That's 32 or 00100000 in binary. 
my $Sat = 2**6;    # That's 64 or 01000000 in binary.

my $mask = 19;    # For example.  00010011 in binary. 

print "Sunday\n"   if ($mask & $Sun); # True
print "Monday\n"   if ($mask & $Mon); # True
print "Tuesday\n"  if ($mask & $Tue); 
print "Wednesay\n" if ($mask & $Wed); 
print "Thursday\n" if ($mask & $Thu); # True
print "Friday\n"   if ($mask & $Fri); 
print "Saturday\n" if ($mask & $Sat);

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    -sauoq
"My two cents aren't worth a dime.";
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Re: Re: Re: help with algorithm
by larsen (Parson) on Nov 17, 2002 at 02:03 UTC
    The first example is 1010, that is (I'm considering the binary digits from right to left):

    0 * 20 + 1 * 21 + 0 * 22 + 1 * 23

    Your second example is 100. As usual, from right to left...

    0 * 20 + 0 * 21 + 1 * 22

    Got the pattern? Now I'm going to check what powers of 2 are on (i.e. multiplied by 1). In the first example they are 21 (Monday) and 23 (Wednesday). In the second example only 22, Tuesday.

    I hope this will clarify my node.
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