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# Shannon Entropy

##### This online calculator computes Shannon entropy for a given event probability table and for a given message
Timur2013-06-04 09:40:17
In information theory, entropy is a measure of the uncertainty in a random variable. In this context, the term usually refers to the Shannon entropy, which quantifies the expected value of the information contained in a message.
The formula for entropy was introduced by Claude E. Shannon in his 1948 paper "A Mathematical Theory of Communication".

$H(X)= - \sum_{i=1}^np(x_i)\log_b p(x_i)$

Minus is used because for values less than 1 logarithm is negative, however, since

$-\log a = \log \frac{1}{a}$,

formula can be expressed as

$H(X)= \sum_{i=1}^np(x_i)\log_b \frac{1}{p(x_i)}$

Expression
$\log_b \frac{1}{p(x_i)}$
is also called an uncertainty or surprisal, the lower the probability $p(x_i)$, i.e. $p(x_i)$ → 0, the higher the uncertainty or the surprise, i.e. $u_i$ → ∞, for the outcome $x_i$.

Formula, in this case, expresses the mathematical expectation of uncertainty and that is why information entropy and information uncertainty can be used interchangeably.

There are two calculators below - one computes Shannon entropy for given probabilities of events, second computes Shannon entropy for given symbol frequencies for given message.

Shannon Entropy
Event probability table
Import data.
"One of the following characters is used to separate data fields: tab, semicolon (;) or comma(,)":
Entropy, bits:

Shannon Entropy
Entropy, bits:

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