nqthqn / elm-round

Round floats (mathematically/commercially) to a given number of decimal places

elm-round

This library converts a Float to a String with ultimate control how many digits after the decimal point are shown and how the remaining digits are rounded. It rounds, floors and ceils the "common" way (ie. half up) or the "commerical" way (ie. half away from zero).

Example:

x = 3.141592653589793

round 2 x -- "3.14"
round 4 x -- "3.1416"

ceiling 2 x -- "3.15"
floor 4 x -- "3.1415"

The given number of digits after decimal point can also be negative.

x = 213.14

round -2 x -- "200"
round -1 x -- "210"

ceiling -2 x -- "300"
floor -3 x -- "0"

Commercial rounding means that negative and positive numbers are treated symmetrically. It affects numbers whose last digit equals 5. For example:

x = -0.5

round 0 x -- "0"
roundCom 0 x -- "-1"

floor 0 x -- "-1"
floorCom 0 x -- "0"

ceiling 0 x -- "0"
ceilingCom 0 x -- "-1"

Have a look at the tests for more examples!

Why couldn't you just do x * 1000 |> round |> toFloat |> (flip (/)) 1000 in order to round to 3 digits after comma? Due to floating point arithmetic it might happen that it results into someting like 3.1416000000001, although we just wanted 3.1416.

Under the hood this library converts the Float into a String and rounds it char-wise. Hence it's safe from floating point arithmetic weirdness.

Installation

From the root of your Elm project run

elm package install myrho/elm-round

Import it in your Elm modules:

import Round 

Releases

Version	Notes
1.0.4	Upgrade to Elm 0.19
1.0.3	Fix issues with number in scientific notation, complete rewrite.
1.0.2	Given number of digits after decimal point can be negative.
1.0.1	Upgrade to Elm 0.18
1.0.0	First official release, streamlined API and tests, docs added