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---
category: Algorithms & Data Structures
name: Asymptotic Notation
contributors:
- ["Jake Prather", "http://github.com/JakeHP"]
- ["Divay Prakash", "http://github.com/divayprakash"]
---
# Asymptotic Notations
## What are they?
Asymptotic Notations are languages that allow us to analyze an algorithm's
running time by identifying its behavior as the input size for the algorithm
increases. This is also known as an algorithm's growth rate. Does the
algorithm suddenly become incredibly slow when the input size grows? Does it
mostly maintain its quick run time as the input size increases? Asymptotic
Notation gives us the ability to answer these questions.
## Are there alternatives to answering these questions?
One way would be to count the number of primitive operations at different
input sizes. Though this is a valid solution, the amount of work this takes
for even simple algorithms does not justify its use.
Another way is to physically measure the amount of time an algorithm takes to
complete given different input sizes. However, the accuracy and relativity
(times obtained would only be relative to the machine they were computed on)
of this method is bound to environmental variables such as computer hardware
specifications, processing power, etc.
## Types of Asymptotic Notation
In the first section of this doc we described how an Asymptotic Notation
identifies the behavior of an algorithm as the input size changes. Let us
imagine an algorithm as a function f, n as the input size, and f(n) being
the running time. So for a given algorithm f, with input size n you get
some resultant run time f(n). This results in a graph where the Y axis is the
runtime, X axis is the input size, and plot points are the resultants of the
amount of time for a given input size.
You can label a function, or algorithm, with an Asymptotic Notation in many
different ways. Some examples are, you can describe an algorithm by its best
case, worse case, or equivalent case. The most common is to analyze an
algorithm by its worst case. You typically don't evaluate by best case because
those conditions aren't what you're planning for. A very good example of this
is sorting algorithms; specifically, adding elements to a tree structure. Best
case for most algorithms could be as low as a single operation. However, in
most cases, the element you're adding will need to be sorted appropriately
through the tree, which could mean examining an entire branch. This is the
worst case, and this is what we plan for.
### Types of functions, limits, and simplification
```
Logarithmic Function - log n
Linear Function - an + b
Quadratic Function - an^2 + bn + c
Polynomial Function - an^z + . . . + an^2 + a*n^1 + a*n^0, where z is some
constant
Exponential Function - a^n, where a is some constant
```
These are some basic function growth classifications used in various
notations. The list starts at the slowest growing function (logarithmic,
fastest execution time) and goes on to the fastest growing (exponential,
slowest execution time). Notice that as 'n', or the input, increases in each
of those functions, the result clearly increases much quicker in quadratic,
polynomial, and exponential, compared to logarithmic and linear.
One extremely important note is that for the notations about to be discussed
you should do your best to use simplest terms. This means to disregard
constants, and lower order terms, because as the input size (or n in our f(n)
example) increases to infinity (mathematical limits), the lower order terms
and constants are of little to no importance. That being said, if you have
constants that are 2^9001, or some other ridiculous, unimaginable amount,
realize that simplifying will skew your notation accuracy.
Since we want simplest form, lets modify our table a bit...
```
Logarithmic - log n
Linear - n
Quadratic - n^2
Polynomial - n^z, where z is some constant
Exponential - a^n, where a is some constant
```
### Big-O
Big-O, commonly written as **O**, is an Asymptotic Notation for the worst
case, or ceiling of growth for a given function. It provides us with an
_**asymptotic upper bound**_ for the growth rate of runtime of an algorithm.
Say `f(n)` is your algorithm runtime, and `g(n)` is an arbitrary time
complexity you are trying to relate to your algorithm. `f(n)` is O(g(n)), if
for some real constant c (c > 0), `f(n)` <= `c g(n)` for every input size
n (n > 0).
*Example 1*
```
f(n) = 3log n + 100
g(n) = log n
```
Is `f(n)` O(g(n))?
Is `3 log n + 100` O(log n)?
Let's look to the definition of Big-O.
```
3log n + 100 <= c * log n
```
Is there some constant c that satisfies this for all n?
```
3log n + 100 <= 150 * log n, n > 2 (undefined at n = 1)
```
Yes! The definition of Big-O has been met therefore `f(n)` is O(g(n)).
*Example 2*
```
f(n) = 3*n^2
g(n) = n
```
Is `f(n)` O(g(n))?
Is `3 * n^2` O(n)?
Let's look at the definition of Big-O.
```
3 * n^2 <= c * n
```
Is there some constant c that satisfies this for all n?
No, there isn't. `f(n)` is NOT O(g(n)).
### Big-Omega
Big-Omega, commonly written as **Ω**, is an Asymptotic Notation for the best
case, or a floor growth rate for a given function. It provides us with an
_**asymptotic lower bound**_ for the growth rate of runtime of an algorithm.
`f(n)` is Ω(g(n)), if for some real constant c (c > 0), `f(n)` is >= `c g(n)`
for every input size n (n > 0).
### Note
The asymptotic growth rates provided by big-O and big-omega notation may or
may not be asymptotically tight. Thus we use small-o and small-omega notation
to denote bounds that are not asymptotically tight.
### Small-o
Small-o, commonly written as **o**, is an Asymptotic Notation to denote the
upper bound (that is not asymptotically tight) on the growth rate of runtime
of an algorithm.
`f(n)` is o(g(n)), if for any real constant c (c > 0), `f(n)` is < `c g(n)`
for every input size n (n > 0).
The definitions of O-notation and o-notation are similar. The main difference
is that in f(n) = O(g(n)), the bound f(n) <= g(n) holds for _**some**_
constant c > 0, but in f(n) = o(g(n)), the bound f(n) < c g(n) holds for
_**all**_ constants c > 0.
### Small-omega
Small-omega, commonly written as **ω**, is an Asymptotic Notation to denote
the lower bound (that is not asymptotically tight) on the growth rate of
runtime of an algorithm.
`f(n)` is ω(g(n)), if for any real constant c (c > 0), `f(n)` is > `c g(n)`
for every input size n (n > 0).
The definitions of Ω-notation and ω-notation are similar. The main difference
is that in f(n) = Ω(g(n)), the bound f(n) >= g(n) holds for _**some**_
constant c > 0, but in f(n) = ω(g(n)), the bound f(n) > c g(n) holds for
_**all**_ constants c > 0.
### Theta
Theta, commonly written as **Θ**, is an Asymptotic Notation to denote the
_**asymptotically tight bound**_ on the growth rate of runtime of an algorithm.
`f(n)` is Θ(g(n)), if for some real constants c1, c2 (c1 > 0, c2 > 0),
`c1 g(n)` is < `f(n)` is < `c2 g(n)` for every input size n (n > 0).
∴ `f(n)` is Θ(g(n)) implies `f(n)` is O(g(n)) as well as `f(n)` is Ω(g(n)).
Feel free to head over to additional resources for examples on this. Big-O
is the primary notation use for general algorithm time complexity.
### Ending Notes
It's hard to keep this kind of topic short, and you should definitely go
through the books and online resources listed. They go into much greater depth
with definitions and examples. More where x='Algorithms & Data Structures' is
on its way; we'll have a doc up on analyzing actual code examples soon.
## Books
* [Algorithms](http://www.amazon.com/Algorithms-4th-Robert-Sedgewick/dp/032157351X)
* [Algorithm Design](http://www.amazon.com/Algorithm-Design-Foundations-Analysis-Internet/dp/0471383651)
## Online Resources
* [MIT](http://web.mit.edu/16.070/www/lecture/big_o.pdf)
* [KhanAcademy](https://www.khanacademy.org/computing/computer-science/algorithms/asymptotic-notation/a/asymptotic-notation)
* [Big-O Cheatsheet](http://bigocheatsheet.com/) - common structures, operations, and algorithms, ranked by complexity.
|
