Ruby: Round Down with some decimal places

Memorized the way to round down number to the nth decimal places in Ruby.

The number like 0.81235 (Float) has function floor, which round down the number to zeal number, can’t round down to some decimal places.


  • Ruby 2.2.3p173
  • OS: Ubuntu 15.04


I detected 3 ways below.

Use BigDecimal

BigDecimal has the function floor, which can round down to decimal number. When it comes to create BigDecimal object, String parameter is required.

Move up decimal point and down

If you want to round down to nth decimal number, multiple ( 10^n ) and round down and divide it by ( 10^n ).

Round after subtraction

Map the number which is equal to or more than 0.1 and less than 0.2, to the number which is equal to or more than 0.05 and less than 0.15, and round it. Then it’s rounded down 2nd decimal number.

When the argument num is positive, the result is correct. But the argument num is zero, the result is incorrect.

The following code is true for argument num equal to or more than zero.

But this has another problem. dec_floor(0.01, 2) returns 0.009999999999999998.

Bad Way

Convert to String with sprintf

sprintf rounds the number and makes String. It’s not round down.

It’s rounding off, so subtract half and use sprintf makes rounded down number, but I don’t recommend.