How to find remainder of a division in Ruby?

Question

I'm trying to get the remainder of a division using Ruby.

Let's say we're trying to divide 208 by 11.

The final should be "18 with a remainder of 10"...what I ultimately need is that 10.

Here's what I've got so far, but it chokes in this use case (saying the remainder is 0).

division = 208.to_f / 11
rounded = (division*10).ceil/10.0
remainder = rounded.round(1).to_s.last.to_i

Solution 1

The modulo operator:

> 208 % 11
=> 10

Solution 2

If you need just the integer portion, use integers with the / operator, or the Numeric#div method:

quotient = 208 / 11
#=> 18

quotient = 208.0.div 11
#=> 18

If you need just the remainder, use the % operator or the Numeric#modulo method:

modulus = 208 % 11
#=> 10

modulus = 208.0.modulo 11
#=> 10.0

If you need both, use the Numeric#divmod method. This even works if either the receiver or argument is a float:

quotient, modulus = 208.divmod(11)
#=> [18, 10]

208.0.divmod(11)
#=> [18, 10.0]

208.divmod(11.0)
#=> [18, 10.0]

Also of interest is the Numeric#remainder method. The differences between all of these can be seen in the documentation for divmod.

Solution 3

please use Numeric#remainder because mod is not remainder

Modulo:

5.modulo(3)
#=> 2
5.modulo(-3)
#=> -1

Remainder:

5.remainder(3)
#=> 2
5.remainder(-3)
#=> 2

here is the link discussing the problem https://rob.conery.io/2018/08/21/mod-and-remainder-are-not-the-same/

Comments

[Shpigford]

How the HECK did I make it so complicated? Thank you.

[Michael Kohl]

I don't want to nitpick, but it's not an operator, but a method of the Fixnum class: >> 208.%(11) #=> 10

[fbonetti]

@MichaelKohl Technically all ruby "operators" are methods. For example, 5 + 5 is really just shorthand for 5.+(5).

[Ben Aubin]

@fbonetti not all... &, |, and, not, and a few other operators are not methods.

[fbonetti]

@penne12 That's not entirely true. &, | are methods on Fixnum; they are NOT operators: ruby-doc.org/core-2.2.0/Fixnum.html#method-i-26.

[fbonetti]

@penne12 here's a complete list of all ruby keywords: java2s.com/Code/Ruby/Language-Basics/Rubysreservedwords.htm

[Ben Aubin]

@fbonetti sorry - I meant && and ||

[noraj]

The divmod method was exactly what I was looking for, needing both remainder and integer portion.

[Zane]

I wholeheartedly disagree with the claim that there is a uniqe definition for remainder and modulo. There are implementations in programming languages - a modulo in algebra is related, but not the same. Note that 11 = 3 mod 4 in algebra, and not 11 mod 4 = 3.

[Adam Sibik]

I must disagree with @Zane's above claim. In algebra, the expression 11 = 3 mod 4 does not contain an operator;mod is not an operator here. It's merely a description of the number system for 3. To avoid confusion, it'd even be typically written 11 = 3 (mod 4).

[Zane]

@AdamSibik. I don't know why you think you "must disagree". From my point of your view, your remark supports the claim: there is no unique definition.