Help:Modulo and round

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The MediaWiki extension ParserFunctions enables users to perform simple mathematical computations.

#expr and #ifexpr allow mod and round.

Operator Operation Example
mod "Modulo" (PHP operator %): if a is nonnegative, a mod b is the remainder by division after truncating both operands to an integer, while (-a) mod b = - ( a mod b).

  • mod is different from all programming languages, see bugzilla:6068 (marked as won't fix). For compatibility it seems wise to apply it only with integer arguments.
  • It has been observed that on Windows a number, before mod is applied, is reduced to php data-type "signed INT", i.e. a signed 32-bits integer [1], i.e. mod gives wrong results for integers outside the range -2,147,483,648 through 2,147,483,647, see below.
Template:Evaldemo (should be 2.6)
Template:Evaldemo (should be 1.6)
Template:Evaldemo (should be 2.9)
round (PHP function round): rounds the number on the left to the nearest multiple of 1/10 raised to the power given on the right; if two are equally near, rounding is away from zero. Template:Evaldemo

Problem with mod in Windows:

See also Template:Tim, and operator fmod in mw:Extension:ParserFunctions (extended), which both do not have these problems. Also, in file expr.php (the standard one or the extended one) operation "$left % $right" can be replaced by "fmod($left,$right)".

Spaces around mod and round are good for readability but not needed for working properly:


(first additions, then round)

(mod and multiplication have equal precedence, evaluation from left to right)

To remind the reader of the precedence, one might write:

When using spaces where there is precedence, the layout of the expression may be confusing:
Instead one can write:
or simply use parentheses:

[edit] Mod

To get a positive mod even for a negative number, use e.g. (700000 + x) mod7 instead of x mod7. The range of the result is now 0-6, provided that x > -700000.

Alternatively, use

  • 6 - ( 700006 - x ) mod7


  • (x - 700006) mod7 + 6.

The range of the result is 0-6, provided that x < 700006.

Working for all x is:

  • (x mod7 + 7) mod7

[edit] Round

To round an integer plus one half for x > -100000 toward plus infinity, use:

  • (x + 100000 round 0) - 100000

and to round an integer plus one half for x < 100000 toward minus infinity, use:

  • (x - 100000 round 0) + 100000

To round x toward minus infinity, use:

  • x + ( x != x round 0 ) * ( ( ( x - .5 ) round 0 ) - x )

and toward plus infinity

  • x + ( x != x round 0 ) * ( ( ( x + .5 ) round 0 ) - x )

If x is a long expression this multiplies the length by 5! Under conditions for x there are alternatives:

To round x > -100000 toward minus infinity, use:

  • (x - 100000.5 round 0) + 100000

and to round x < 100000 toward plus infinity, use:

  • (x + 100000.5 round 0) - 100000

If x is a multiple of 1/n with n<1000 we can round toward minus infinity with:

  • x - .499 round 0

For arbitrary n > 1 we can choose instead of -.499 any number between -.5 and -.5 + 1/n.

To find the largest multiple of 7 not larger than x (i.e. to round toward minus infinity to a multiple of 7) we can do:

  • ((x-3)/7 round 0) * 7

[edit] See also


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