Ruby, modulus operator and POLS

Since I started my Ruby learning process, one of the most basic and important things I consider, is the POLS (Principle of the Least Surprise).

This week, a new question about “string pattern formatting”, came up on the forum.

Considering this code:

'%0.2f' % ((f.to_f - 32) / 9 * 5)

Which is the meaning of the second “%” ?

After some research, I found two options on that “formatting” subject.

1. You can format your data immediately, using the printf method.

f = -123.12345

puts sprintf('%0.3f', f)

2. You can create a template, and interpolate data with it later.

template = '%s, has always been in %s with %s.'

puts template % ['Oceania', 'war', 'Eurasia']

puts template % ['Luke Skywalker', 'love', 'Leia Organa']

OK, there is a nice functionality, but about POLS ?

For sure I have little experience with Ruby, but the fact, you can use the modulus operator over strings, really surprises-me.

I deal with a great assort of programming languages over 2 decades now, and never ever, applied the modulus operator over strings.

Think about this code:

puts 'abc' % 'def' # <= 'abc'

puts 'abc' % 10 # <= 'abc'

And there is another point. Negative numbers:

Witch should be the -5 % 3 result ? 2 ? -2 ?

In fact in Ruby, -5 % 3 = 1

Why ?

Some programming languages adopt the C89 standard about that subject. That is the way Ruby does:

remainder = a % n

is the same of:

remainder = a - n * floor_funtion(a / n)

where floor_function from a decimal value, returns the highest integer, less than or equal to it.

So, in Ruby this is the formula for modulus remainder result:

r = ((a) - ((n) * ((a) / (n))))

* Extra parenthesis, to put negative values easily and clear.

Ahh, take a look at those SuperRubyBoy's operators !