Ruby output numbers to 2 decimal places - json

Question

I'm having trouble serializing my ruby object to json, more specifically the format of the numbers.
I have written an rspec test to illustrate my issue more precisely.
expected = '{ "foo": 1.00, "bar": 4.50, "abc": 0.00, "xyz": 1.23 }'
it 'serializes as expected' do
my_hash = { "foo": 1, "bar": 4.5, "abc": 0, "xyz": 1.23}
expect(my_to_json_method(my_hash)).to eq expected
end
This is the case that I am having trouble with. I can use the sprintf but how do I get the string output as shown in the above example?

Answer 1

First of all, you should not use floats to represent monetary values. So instead, let's use a more appropriate type: (there's also the Ruby Money gem)
require 'bigdecimal'
my_hash = {
foo: BigDecimal.new('1.00'),
bar: BigDecimal.new('4.50'),
abc: BigDecimal.new('0.00'),
xyz: BigDecimal.new('1.23')
}
There are several options to represent monetary values. All of the following JSON strings are valid according to the JSON specification and all require special treatment upon parsing. It's up to you to choose the most appropriate.
Note: I'm implementing a custom to_json method to convert the BigDecimal instances to JSON using Ruby's default JSON library. This is just for demonstration purposes, you should generally not patch core (or stdlib) classes.
1. Numbers with fixed precision
This is what you asked for. Note that many JSON libraries will parse these numbers as floating point values by default.
class BigDecimal
def to_json(*)
'%.2f' % self
end
end
puts my_hash.to_json
Output:
{"foo":1.00,"bar":4.50,"abc":0.00,"xyz":1.23}
2. Numbers as strings
This will work across all JSON libraries, but storing numbers as strings doesn't look quite right to me.
class BigDecimal
def to_json(*)
'"%.2f"' % self
end
end
puts my_hash.to_json
Output:
{"foo":"1.00","bar":"4.50","abc":"0.00","xyz":"1.23"}
3. Numbers as integers
Instead of representing monetary values as fractional numbers, you simply output the cents as whole numbers. This is what I usually do.
class BigDecimal
def to_json(*)
(self * 100).to_i.to_s
end
end
puts my_hash.to_json
Output:
{"foo":100,"bar":450,"abc":0,"xyz":123}

Answer 2

User Sprintf
sprintf('%.2f', 5.5)
And simply interpolate into your JSON as an ERB template.

Answer 3

You can use, sprintf and can take as many decimal points as you needed by mentioning %.(number)f.
Eg: For two decimals, %.2f
Here is a real implementation,
2.2.2 :019 > test = { "foo": (sprintf "%.2f","1.11"), "bar": (sprintf "%.2f","4.55"), "abc": (sprintf "%.2f","0.2") }
=> {:foo=>"1.11", :bar=>"4.55", :abc=>"0.20"}
Here is the reference

Answer 4

puts '{' << my_hash.map { |k, v| %Q|"#{k}": #{"%.2f" % v}| }.join(', ') << '}'
#⇒ {"foo": 1.00, "bar": 4.50, "abc": 0.00, "xyz": 1.23}