# Floating Point Rounding

## Problem

You need to round a floating-point number to an integer.

## Solution

Use one of the functions round/1, ceiling/1, floor/1 and trunc/1. Note, the standard Erlang distribution does not come with either floor/1 or ceiling/1, but they can be easily implemented in terms of trunc/1

floor(X) when X < 0 -> T = trunc(X), case X - T == 0 of true -> T; false -> T - 1 end; floor(X) -> trunc(X). ceiling(X) when X < 0 -> trunc(X); ceiling(X) -> T = trunc(X), case X - T == 0 of true -> T; false -> T + 1 end. 1> floor(-4.3). -5 2> ceiling(-4.3). -4 3> trunc(-4.3). -4 4> round(-4.3). -4 5> floor(3.5). 3 6> ceiling(3.5). 4 7> trunc(3.5). 3 8> round(3.5). 4 9> round(7). 7

The procedures all return integers.

round/1 returns the closest integer to x, rounding to even when x is halfway between two integers. trunc/1 returns the integer closest to x whose absolute value is not larger than the absolute value of x. floor/1 returns the largest integer not larger than x. ceiling/1 returns the smallest integer not smaller than x.

For the floor and ceiling functions implemented above, T will always be less than or equal to X. We just need to check if there is a difference between the two in which case we will either increase or decrease the truncation depending if we want to floor or ceiling X.

*(Correction to floor/1 and ceiling/1 to check for negative or positive values. 2009-11-21 seetho)*

Further general information on math and rounding is available from:

MathWorld definition of the Floor Function. MathWorld definition of the Ceiling Function