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RE: RE: The Big Test
by japhy (Canon) on Oct 17, 2000 at 19:06 UTC
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It's even simpler than computer science. It's something a
human should know how to do! How do you figure out of a
number is even? It can be divided by two evenly! Then, all
the person needs to know is that mathemetics (and Perl) offer
the modulus operation (Perl uses the % character,
like C and C++) to return the remainder when a number is
divided by another. With these two pieces of information:
- X is even if it is evenly divisible by 2
- X % Y returns the remainder when X is divided
by Y
any human that can find patterns should be able to tell how
to determine whether a number is even or odd.
$_="goto+F.print+chop;\n=yhpaj";F1:eval | [reply] |
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Well if Perl were to do it like this human does, the test would be:
if ($number =~ /\D/) {
print "$number is not an integer\n";
}
elsif ($number =~ /[02468]$/) {
print "$number is even\n";
else {
print "$number is odd\n";
}
Honestly, that is *exactly* how I do it! :-) | [reply]
[d/l] |
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Actually, I prefer to test a number for even-ness using the same concept: Check the last digit, er... bit:
print "Odd" if $_ & 1;
print "Even" if not $_ & 1;
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[d/l] |
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Yes, a human tells if a number is even by checking the last
digit. But why does that work? Because we all (I think)
learned some really nifty tricks when we were little:
- A number is divisible by 2 if it ends in an even digit
(0, 2, 4, 6, or 8)
- A number is divisible by 3 if the sum of its digits is
3, 6, or 9 (and you keep summing the digits until you get
to a one-digit number)
- A number is divisible by 4 if its last two digits are
divisible by 4 (recursive, yes, but it takes a little extra
brain power to determine if a two-digit number is divisible
by 4)
- A number is divisible by 5 if its last digit is a 0 or
a 5
- A number is divisible by 6 if it's divisible by 2 and 3
- and so on...
So there's a connection between the last digit of a number
and its remainder modulo 2. That doesn't discount the fact
that even-ness is determined via the remainder of the number
modulo 2, it's just that the last digit modulo 2 is a shortcut.
$_="goto+F.print+chop;\n=yhpaj";F1:eval
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One I used to see regularly when in my second Basic
class (*sigh*) was (in perl idiom)
if ($num/2 == int($num/2)) {print "$num even!\n":}
That still gives me the willies, though it is techically
correct(ish) and is more generally applicable than terminal
digit inspection =) Most people these days never got
taught what the mod function really does. Basic math
logic skills are passed off till college.
One thing I remember from Basic was that true was -1
and false was 0, and that all logical ops returned one
of those two values. We used to do this (sorta) in graphics programs
to get smoothly, randomly, changing colors and positions:
my @color= (127,127,127); #256x256x256 color palette
my @location= (50,50); #200x200 screen
for(;;) {
$color[0] += (int(rand*3)==1) - (int(rand*3)==1);
$color[1] += (int(rand*2)==1) - (int(rand*4)==1);
$color[2] += (int(rand*4)==1) - (int(rand*2)==1);
$location[0] += (int(rand*5)) - 2;
$location[1] += (int(rand*5)) - 2;
@color = map {$_ % 256} @color;
@location = map {$_ % 200} @location;
DrawPixel(@location,@color);
}
We did megacomplex things with math logic because
it was blazingly fast vs. if (){}; =) Seems like a
forgotten era now that I do web stuff.
--
$you = new YOU;
honk() if $you->love(perl) | [reply]
[d/l]
[select] |
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Right. There are people who do not know how to find patterns,
and they should not be programming.
$_="goto+F.print+chop;\n=yhpaj";F1:eval
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