Add percentage-calculation functions

[sharkdp]

>>> 72 € |> increase_by(15%)

    = 82.8 €    [Money]

>>> percentage_change(3.2 kg, 4.7 kg)

    = 46.875 %

Additional ideas / TODOs:

• Special syntax for increase_by/decrease_by?
• Special/custom error message when attempting to do something like 72 € - 15%.

closes #494

[Goju-Ryu]

This looks great, but the name of percentage_chance is a bit confusing to me. I had to reread some examples to figure out what the intent was.

Maybe something like percentage_difference would be more clear?

So the current:

assert_eq(percentage_chance(40 dollar, 50 dollar), +25%)

Would be this instead:

assert_eq(percentage_difference(50 dollar, 40 dollar), +25%)
# or with pipeline 
assert_eq((40 dollar |> percentage_difference(50 dollar)), +25%)

I’m not sure if it is exactly right, but I think naming it something without the word chance would make it more easily understood.

[sharkdp]

percentage_chance

without the word chance

Is this a typo or did you misread? The function is called "percentage_change".

[Goju-Ryu]

Is this a typo or did you misread? The function is called "percentage_change".

It wasn't a typo, I see now that I misread it multiple times over. It makes a lot more sense with the naming now that I actually read it right.

[andylima]

Thank you for adding this. 🙂 It's a bit verbose though, so I'm looking forward to that special syntax for increase_by / decrease_by — What do you have in mind?

[sharkdp]

Thank you for adding this. 🙂 It's a bit verbose though, so I'm looking forward to that special syntax for increase_by / decrease_by — What do you have in mind?

I like those -%/+% operators that you proposed in #494. Maybe we could open a new ticket for those additional tasks? We can merge this as-is, I think.

[andylima]

I like those -%/+% operators that you proposed in #494. Maybe we could open a new ticket for those additional tasks? We can merge this as-is, I think.

That's great! 🙂 Do you want me to open the new ticket?

[sharkdp]

That'd be great — thanks.