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Using the mod() function with negative numbers 

Using the mod() function with negative numbers  
 
There are different ways of thinking about remainders when you deal 
with negative numbers, and he is probably confusing two of them. The 
mod function is defined as the amount by which a number exceeds the 
largest integer multiple of the divisor that is not greater than that 
number. In this case, -340 lies between -360 and -300, so -360 is the 
greatest multiple LESS than -340; we subtract 60 * -6 = -360 from -340 
and get 20: 
 
 -420 -360 -300 -240 -180 -120  -60   0   60   120  180  240  300  360
--+----+----+----+----+----+----+----+----+----+----+----+----+----+--
       | |                                                    |  |
   -360| |-340                                             300|  |340
       |=|                                                    |==|
        20                                                     40


Working with a positive number like 340, the multiple we subtract is 
smaller in absolute value, giving us 40; but with negative numbers, we 
subtract a number with a LARGER absolute value, so that the mod 
function returns a positive value. This is not always what people 
expect, but it is consistent.

If you want the remainder, ignoring the sign, you have to take the 
absolute value before using the mod function.
 
Source: http://www.mathforum.org
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