[FIXED] How to round Decimals to the First Significant Figure in Ruby

Issue

I am attempting to solve an edge case to a task related to a personal project.

It is to determine the unit price of a service and is made up of the total_amount and cost.

Examples include:

# 1
unit_price = 300 / 1000 # = 0.3

# 2
unit_price = 600 / 800 # = 0.75

# 3
unit_price = 500 / 1600 # = 0.3125

For 1 and 2, the unit_prices can stay as they are. For 3, rounding to 2 decimal places will be sufficient, e.g. (500 / 1600).round(2)

The issue arises when the float becomes long:

# 4
unit_price = 400 / 56000 # = 0.007142857142857143

What's apparent is that the float is rather long. Rounding to the first significant figure is the aim in such instances.

I've thought about using a regular expression to match the first non-zero decimal, or to find the length of the second part and apply some logic:

• unit_price.match ~= /[^.0]/
• unit_price.to_s.split('.').last.size

Any assistance would be most welcome

Solution

One should use BigDecimal for this kind of computation.

require 'bigdecimal'

bd = BigDecimal((400.0 / 56000).to_s)
#⇒ 0.7142857142857143e-2
bd.exponent
#⇒ -2

Example:

[10_000.0 / 1_000, 300.0 / 1_000, 600.0 / 800,
                   500.0 / 1_600, 400.0 / 56_000].
  map { |bd| BigDecimal(bd.to_s) }.
  map do |bd|
    additional = bd.exponent >= 0 ? 0 : bd.exponent + 1 
    bd.round(2 - additional) # THIS
  end.
  map(&:to_f)
#⇒ [10.0, 0.3, 0.75, 0.31, 0.007]

Answered By - Aleksei Matiushkin

Answer Checked By - Katrina (PHPFixing Volunteer)