# The Modulo function (%)

**URL:** <https://forum.arduino.cc/t/the-modulo-function/41676>\
**Category:** Syntax & Programs\
**Created:** [March 10, 2009, 9:18am UTC](https://forum.arduino.cc/t/the-modulo-function/41676 "2009-03-10T09:18:52Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![calli](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@calli](https://forum.arduino.cc/u/calli)\
**Post date:** [March 10, 2009, 9:18am UTC](https://forum.arduino.cc/t/the-modulo-function/41676/1 "2009-03-10T09:18:52Z")

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Hi,

I just discovered that the modulo (%) function in the arduino ide is a mathematical, not a symmetrical like in most(?) other programming languages.

Is there a reason using this implementation? Is there somewhere a hidden modulo using the symetrical function?

Thanks,  
Carsten

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**Author:** ![calli](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@calli](https://forum.arduino.cc/u/calli)\
**Post date:** [March 10, 2009, 9:42am UTC](https://forum.arduino.cc/t/the-modulo-function/41676/2 "2009-03-10T09:42:55Z")

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Strange...

Python:

> > > z1 = -2  
> > > z2 = 15  
> > > print z1 - ((z1 / z2) \* z2)  
> > > 13

But in C (Arduino IDE) its still giving negative numbers...

int smod(int z1, int z2) {  
return(z1 - ((z1 / z2) \* z2));  
}

Any suggestions?  
Carsten

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**Author:** ![skumlerud](https://avatars.discourse-cdn.com/v4/letter/s/e8c25b/32.png) [@skumlerud](https://forum.arduino.cc/u/skumlerud)\
**Post date:** [March 10, 2009, 10:01am UTC](https://forum.arduino.cc/t/the-modulo-function/41676/3 "2009-03-10T10:01:16Z")

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> But in C (Arduino IDE) its still giving negative numbers...

Exactly what is the result in C? I would expect it to be -2, but I haven't tested this. -2 - (0 \* 15) = -2

I'm not familiar with Python, but from the result it looks like the equation is not equal to the C-example. It looks like -2 - (-1 \* 15) = 13.

So in C the division -2/15 = 0 with a remainder of -2. I'm not familiar at all with Python so I can't say why -1/15 = -1

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**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\
**Post date:** [March 10, 2009, 10:08am UTC](https://forum.arduino.cc/t/the-modulo-function/41676/4 "2009-03-10T10:08:00Z")

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return(z1 - ((z1 / z2) \* z2));

This is essentially the same as 0, but because of the truncation it might result in something other than 0.

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**Author:** ![skumlerud](https://avatars.discourse-cdn.com/v4/letter/s/e8c25b/32.png) [@skumlerud](https://forum.arduino.cc/u/skumlerud)\
**Post date:** [March 10, 2009, 11:04am UTC](https://forum.arduino.cc/t/the-modulo-function/41676/5 "2009-03-10T11:04:35Z")

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> I just discovered that the modulo (%) function in the arduino ide is a mathematical, not a symmetrical like in most(?) other programming languages.

I don't know what you mean by "mathematical" and "symmetrical", but modulo in C (and the "Arduino language" is plain C/C++ with some supporting libraries) is the remainder of an integer division.

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**Author:** ![calli](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@calli](https://forum.arduino.cc/u/calli)\
**Post date:** [March 10, 2009, 12:16pm UTC](https://forum.arduino.cc/t/the-modulo-function/41676/6 "2009-03-10T12:16:40Z")

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> **[Modulo](https://en.wikipedia.org/wiki/Modulo_function)**
>
> In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the modulus of the operation).
> Given two positive numbers a and n, a modulo n (often abbreviated as a mod n) is the remainder of the Euclidean division of a by n, where a is the dividend and n is the divisor.
> For example, the expression "5 mod 2" would evaluate to 1, because 5 divided by 2 has a quotient of 2 and a remainder of 1, while "9 mod 3" would evalu...

I think what I was talking about as symmetrical is the "Euclidian Division".

I use the modulo in Python regularly for clipping values to a range.

For now I used this:

```arduino
int smod(int z1, int z2) {
  int ze;
  ze=z1 % z2;
  if (ze>=0)  {
    return(ze);
  }
  else{
    return(z2+ze);
  }
}

```

For my special case (modulo 2 and 16, all modulo power of 2) I can also use

x & 1 and x & 15

Carsten

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**Author:** ![mem](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/mem/32/213477_2.png) [@mem](https://forum.arduino.cc/u/mem)\
**Post date:** [March 10, 2009, 12:32pm UTC](https://forum.arduino.cc/t/the-modulo-function/41676/7 "2009-03-10T12:32:23Z")

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> I use the modulo for clipping values to a range.

would the constrain function do what you want?  
[http://www.arduino.cc/en/Reference/Constrain](http://www.arduino.cc/en/Reference/Constrain)

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**Author:** ![calli](https://avatars.discourse-cdn.com/v4/letter/c/977dab/32.png) [@calli](https://forum.arduino.cc/u/calli)\
**Post date:** [March 10, 2009, 1:06pm UTC](https://forum.arduino.cc/t/the-modulo-function/41676/8 "2009-03-10T13:06:40Z")

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Not in this case, I want the values to wrap over.

See: [http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1236549392/6#6](http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1236549392/6#6)

Carsten

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<div class="post-metadata">

**Author:** ![system](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/system/32/1140315_2.png) [@system](https://forum.arduino.cc/u/system)\
**Post date:** [May 6, 2021, 8:04pm UTC](https://forum.arduino.cc/t/the-modulo-function/41676/9 "2021-05-06T20:04:02Z")

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