Set the display precision of a float in Ruby

Question

Is it possible to set the display precision of a float in Ruby?

Something like:

z = 1/3
z.to_s    #=> 0.33333333333333
z.to_s(3) #=> 0.333
z.to_s(5) #=> 0.33333

Or do I have to override the to_s method of Float?

Solution 1

z.round(2) or x.round(3) is the simplest solution. See http://www.ruby-doc.org/core-1.9.3/Float.html#method-i-round.

That said, that will only ensure that it is no more than that many digits. In the case of 1/3 that is fine, but if you had say 0.25.round(3) you will get 0.25, not 0.250.

Solution 2

You can use sprintf:

sprintf("%0.02f", 123.4564564)

Solution 3

I would normally just do the conversion in open code, something like:

puts "%5.2f" % [1.0/3.0] 

Ruby calls Kernel#format for expressions like this, because String has a core operator % defined on it. Think of it as printf for Ruby if that rings any bells for you.

Solution 4

Rubocop recommends using #format over #sprintf and using annotated string tokens.

The syntax for #format is

%[flags][width][.precision]type

Example:

# Ensure we store z as a float by making one of the numbers a float.
z = 1/3.0

# Format the float to a precision of three.
format('%<num>0.3f', num: z)
# => "0.333"

format('%<num>0.5f', num: z)
# => "0.33333"

# Add some text to the formatted string
format('I have $%<num>0.2f in my bank account.', num: z)
# => "I have $0.33 in my bank account."

References:

• https://www.rubydoc.info/github/bbatsov/RuboCop/RuboCop/Cop/Style/FormatString
• https://www.rubydoc.info/github/bbatsov/RuboCop/RuboCop/Cop/Style/FormatStringToken

Solution 5

You can use puts

z = #{'%.3f' % z}

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