Definition:Rounding

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Definition

Rounding is the process of approximation of a value of a variable to a multiple of a given power of whatever number base one is working in (usually decimal).


Let $n \in \Z$ be an integer.

Let $x \in \R$ be a real number.


Let $y \in \R$ such that:

$y = 10^n \floor {\dfrac x {10^n} + \dfrac 1 2}$

or:

$y = 10^n \ceiling {\dfrac x {10^n} - \dfrac 1 2}$

where $\floor {\, \cdot \,}$ denotes the floor function and $\ceiling {\, \cdot \,}$ denotes the ceiling function.


Then $y$ is defined as $x$ rounded to the nearest $n$th power of $10$.


Both of these definitions amount to the same thing, except for when $\dfrac x {10^n}$ is exactly halfway between $\floor {\dfrac x {10^n} }$ and $\ceiling {\dfrac x {10^n} }$.

How these instances is treated is known as the treatment of the half.


Rounding to Nearest Integer

When $n = 0$, the operation is referred to as rounding to the nearest integer:


Let $y \in \R$ such that:

$y = \floor {x + \dfrac 1 2}$


Then $y$ is defined as $x$ rounded to the nearest integer.


Treatment of Half

Consider the situation when $\dfrac x {10^n} + \dfrac 1 2$ is an integer.

That is, $\dfrac x {10^n}$ is exactly midway between the two integers $\dfrac x {10^n} - \dfrac 1 2$ and $\dfrac x {10^n} + \dfrac 1 2$.


Recall that the general philosophy of the process of rounding is to find the closest approximation to $x$ to a given power of $10$.

Thus there are two equally valid such approximations:

$\dfrac x {10^n} - \dfrac 1 2$ and $\dfrac x {10^n} + \dfrac 1 2$

between which $\dfrac x {10^n}$ is exactly midway.


There are a number of conventions which determine which is to be used.


Also known as

When $n < 0$, the terminology used is usually:

$x$ rounded to the nearest $m$th decimal place

where $m = -n$.


Examples

$72 \cdotp 8146$ to $2$ Decimal Places

$72 \cdotp 8146$ rounded to $2$ decimal places is $72 \cdotp 81$.

This is because $72 \cdotp 8146$ is closer to $72 \cdotp 81$ than $72 \cdotp 82$.


Sources