Ruby modulus. Why are these two sums printing different answers?

Question

I'm learning ruby and having problems with the %Modulus sums.

puts "example #{101 % 4}"

The above prints 1 in the terminal which is what I expected.

Why does the below print 101 in the terminal? Surely it's the same as above?

puts "example #{100 + 1 % 4}"

I understand that % is just another way of saying 'X divided by Y with J remaining". Therefore surely the 2nd example should also return 1?

Any help will be greatly appreciated.

Answer 1

As pointed out it's to do with operator precedence, which you can control using parentheses () for example. The rules for Ruby are outlined in the official Ruby documentation as follows:

From highest to lowest, this is the precedence table for ruby. High precedence operations happen before low precedence operations.

!, ~, unary +

**

unary -

*, /, %

+, -

<<, >>

&

|, ^

>, >=, <, <=

<=>, ==, ===, !=, =~, !~

&&

||

.., ...

?, :

modifier-rescue

=, +=, -=, etc.

defined?

not

or, and

modifier-if, modifier-unless, modifier-while, modifier-until

{ } blocks

For a more general overview of order or precedence (outside the scope of programming) see the wikipedia entry here.

What happens if there's a tie?

It's worth noting that if there's a tie (i.e. two operators of the same precedence are in your calculation) then the operations are carried out from left to right.

Consider:

10 % 3 * 4
#=> 4 
10 * 3 % 4
#=> 2 

or

10 * 10 / 2 * 4
#=> 200 
10 / 10 * 2 * 4
#=> 8

Answer 2

Parentheses are important. Because of operator precedence rules, the second example is seen by ruby as

100 + (1 % 4)

Which gives

100 + 1

which equals 101

You probably meant

(100 + 1) % 4

Answer 3

Because % has an higher precedence than +. So you could do something like

puts "example #{(100 + 1) % 4}"