Are there more elegant ways to prevent negative numbers in Ruby? - ruby

Question

Given that I'd like to do the following calculation:
total = subtotal - discount
Because discount might be greater than subtotal, there is code like the following:
class Calculator
def initialize(subtotal: subtotal, discount: discount)
#subtotal = subtotal
#discount = discount
end
def total
[subtotal - discount, 0].max
end
private
def subtotal
#subtotal
end
def discount
#discount
end
end
When seeing the [subtotal - discount, 0].max part or any similar code, I often have to pause and think.
Are there more elegant ways to handle this kind of calculation?

Answer 1

I think your solution is essentially correct, and probably the most readable besides a small refactor. I might change it slightly like so:
def total
final_total = subtotal - discount
[final_total, 0].max
end
The ruby expression [final_total, 0].max is essentially the traditional solution in mathematics for the same function: max {final_total, 0}. The difference is just notation and context. Once you see this max expression once or twice you can read it as follows: "final_total, but at least zero".
Perhaps if you use this expression more than once you can add another at_least_zero method or something like in Shiko's solution.

Answer 2

Thinking we can extend the Numeric class?
class Numeric
def non_negative
self > 0 ? self : 0
end
end
class Calculator
def initialize(subtotal: subtotal, discount: discount)
#subtotal = subtotal
#discount = discount
end
def total
(#subtotal - #discount).non_negative
end
end

Answer 3

A plain if statement might be easier to understand:
def total
if discount > subtotal
0
else
subtotal - discount
end
end

Answer 4

Some performance numbers:
user system total real
[i, 0.0].max 0.806408 0.001779 0.808187 ( 0.810676)
0.0 if i < 0.0 0.643962 0.001077 0.645039 ( 0.646368)
0.0 if i.negative? 0.625610 0.001680 0.627290 ( 0.629439)
Code:
require 'benchmark'
n = 10_000_000
Benchmark.bm do |benchmark|
benchmark.report('[value, 0.0].max'.ljust(18)) do
n.times do |i|
a = [-1*i, 0.0].max
end
end
benchmark.report('0.0 if value < 0.0'.ljust(18)) do
n.times do |i|
a = 0.0 if -1*i < 0.0
end
end
benchmark.report('0.0 if value.negative?'.ljust(18)) do
n.times do |i|
a = 0.0 if (-1*i).negative?
end
end
end

Answer 5

Just to clarify more, we need to add classes to be extended in core_ext.rb . file :
1) Create core_ext.rb file under config\initializers folder in your project.
2) Paste below as mentioned by #songyy in his answer:
class Numeric
def non_negative
self > 0 ? self : 0
end
end
Reference:
https://guides.rubyonrails.org/plugins.html#extending-core-classes