to_d to always return 2 decimals places in ruby

Question

I'm dealing with currencies and I want to round down the number to 2 decimal places. Even if the number is 500.0, I would like it to be 500.00 to be consistent. When I do "500.00".to_d it converts it to 500.0.

Whats a good way of changing this behavior? I also use this method to round down to 2 digits and make sure it always has 2 decimals.

def self.round_down(x, n=2)
    s = x.to_s      
    l = s.index('.') ? s.index('.') + 1 + n : s.length
    s = s[0, l]
    s =  s.index('.') ? s.length - (s.index('.') + 1) == 1 ? s << '0' : s : s << '.00'      
    s.to_f
end

Solution 1

In addition to mcfinnigan's answer, you can also use the following to get 2 decimal places

'%.2f' % 500 # "500.00"

This use case is known as the string format operator

Solution 2

Since you are using Rails and this seems to be related to a view, there's number_with_precision:

number_with_precision(500, precision: 2)
#=> "500.00"

I18n.locale = :de
number_with_precision(500, precision: 2)
#=> "500,00"

For currencies I'd suggest number_to_currency:

number_to_currency(500)
#=> "$500.00"

Solution 3

Here's a hint. 500.00 is a representation of the number 500.0

Specifically, sprintf will help you:

irb(main):004:0> sprintf "%.2f", 500.0
=> "500.00"

Solution 4

Do not use floating point numbers to represent money. See this question for a good overview of why this is a bad idea.

Instead, store monetary values as integers (representing cents), or have a look at the money gem that provides lots of useful functionality for dealing with such values.

Solution 5

There was a requirement to round DOWN.

Most other answers round 500.016 UP to 500.02

Try:

def self.round_down(x, n = 2)
  "%.#{n}f" % x.to_d.truncate(n)
end

irb(main):024:0> x=500.0; '%.2f' % x.to_d.truncate(2)
=> "500.00"
irb(main):025:0> x=500.016; '%.2f' % x.to_d.truncate(2)
=> "500.01"

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Comments

[Camille Goudeseune]

As the proverb goes, Real Programmers can write C in any language! '%.2f' % 500 is the Ruby idiom.

[mcfinnigan]

@CamilleGoudeseune I blame it on far too many years of Java :/

[Lev Lukomsky]

sprintf is more explicit than % and I can easily get help about formatting parameters by pressing F1 on sprintf in my IDE

[webaholik]

Caveat: this will round down or UP, eg. 500.016 becomes 500.02

[webaholik]

Caveat: this will round down or UP, eg. 500.016 becomes 500.02

[jvnill]

yep expected behavior would be to follow the rules for rounding numbers

[ricks]

Just want to add that this answer covers a nil value, its great for .erb files.