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Modulo Function returns incorrect results

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Matthew Boehm

22 years ago

Found some weird behavior in the built in mod function (%):

$a = 4;
$b = 3;
print ( $a % $b );  (returns 1 as it should)

Now swap $a and $b:

$a = 3;
$b = 4;
print ( $a % $b );

This returns 3. 3?! 3/4 is 0.75. Shouldn't this return 0?

Try this one:

$a = 20;
$b = 10;
print ( $a % $b );

Shouldn't that return 2? It returns 0;

$a = 11;
$b = 21;
print ( $a % $b );

Returns 11.  What is going on?  Was my C.S. Professor wrong in telling us
that the % function returns the left side of the decimal in a division?

This is PHP 4.3.4

Any help would be appreciated.

Thanks,
Matthew

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Matthew Boehm
dr_mac@mail.utexas.edu
The University of Texas at Austin, Department of Geography

"Why did the prison use Windows2K as a guard? Because it always locks up!"
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George Schlossnagle

22 years ago

Hi.

I think you don't understand what modulo does.  The examples you give
all suggest you think it is simply integer divide.  a % b returns the
remainder when the integer a is divided by the integer b.  All your
'examples' illustrate correct behavior of the modulo operator.

George


On Aug 5, 2004, at 9:02 PM, Matthew Boehm wrote:

Thomas Goyne

22 years ago

On Thu, 05 Aug 2004 20:02:04 -0500, Matthew Boehm <dr_mac@mail.utexas.edu>
wrote:

> Returns 11.  What is going on?  Was my C.S. Professor wrong in telling us
> that the % function returns the left side of the decimal in a division?

I suggest you demand a refund for all courses you paid for with that
professor.

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Jason Garber

22 years ago

Hello Matthew,

3/4 = (0*4)+3     -->  3%4 = 3
9/4 = (2*4)+1     -->  9%4 = 1
20/10 = (2*10)+0  -->  20%10 = 0

4 goes into 3 a total of ZERO times with a remainder of 3.
10 goes into 20 a total of TWO times with a remainder of 0.

Take a look at a standard (integer) long division problem with
remainders.

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Best regards,
Jason                            mailto:jason@ionzoft.com

Thursday, August 5, 2004, 9:02:04 PM, you wrote:

MB> Found some weird behavior in the built in mod function (%):

MB> $a = 4;
MB> $b = 3;
MB> print ( $a % $b );  (returns 1 as it should)

MB> Now swap $a and $b:

MB> $a = 3;
MB> $b = 4;
MB> print ( $a % $b );

MB> This returns 3. 3?! 3/4 is 0.75. Shouldn't this return 0?

MB> Try this one:

MB> $a = 20;
MB> $b = 10;
MB> print ( $a % $b );

MB> Shouldn't that return 2? It returns 0;

MB> $a = 11;
MB> $b = 21;
MB> print ( $a % $b );

MB> Returns 11.  What is going on?  Was my C.S. Professor wrong in telling us
MB> that the % function returns the left side of the decimal in a division?

MB> This is PHP 4.3.4

MB> Any help would be appreciated.

MB> Thanks,
MB> Matthew
MB> --
MB> ----------------------------------------------------------------------------
MB> Matthew Boehm
MB> dr_mac@mail.utexas.edu
MB> The University of Texas at Austin, Department of Geography

MB> "Why did the prison use Windows2K as a guard? Because it always locks up!"
MB> <?PHP echo "PHP kicks ASP!"; ?>
MB> ----------------------------------------------------------------------------

Lester Caine

22 years ago

Matthew Boehm wrote:

> Returns 11.  What is going on?  Was my C.S. Professor wrong in telling us
> that the % function returns the left side of the decimal in a division?

Try RIGHT side, but as an integer remainder ( fraction * divisor )

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Lester Caine
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