Home » Calculating entropy of a string

# Calculating entropy of a string

## Solutons:

Won’t this work too?

``````string name = "lltt";
int uniqueCharacterCount = name.Distinct().Count();
``````

will return 2

``````public static int Entropy(this string s)
{
HashSet<char> chars = new HashSet<char>(s);
return chars.Count;
}
``````

I also came up with this, based on Shannon entropy.

In information theory, entropy is a measure of the uncertainty associated with 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, usually in units such as bits.

It is a more “formal” calculation of entropy than simply counting letters:

``````/// <summary>
/// returns bits of entropy represented in a given string, per
/// http://en.wikipedia.org/wiki/Entropy_(information_theory)
/// </summary>
public static double ShannonEntropy(string s)
{
var map = new Dictionary<char, int>();
foreach (char c in s)
{
if (!map.ContainsKey(c))
else
map[c] += 1;
}

double result = 0.0;
int len = s.Length;
foreach (var item in map)
{
var frequency = (double)item.Value / len;
result -= frequency * (Math.Log(frequency) / Math.Log(2));
}

return result;
}
``````

Results are:

```"abcdefghijklmnop" = 4.00
"Hello, World!" = 3.18
"hello world" = 2.85
"123123123123" = 1.58
"aaaa" = 0
```

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