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- Re: How to calculate the factorial of a number in ...

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shrirangphadke

Path Finder

02-11-2016
01:18 AM

1 Solution

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javiergn

SplunkTrust

02-12-2016
05:23 AM

You could try an approximation by using the natural logarithm (see formula in picture below).

That should work fine when n is not too big, if n is big enough you might have approximation errors. Using the Stirling's approximation is more accurate in this case but it won't be easy to implement with the Splunk built-in commands.

See an example below for the natural logarithm approximation when n=5:

```
| stats count
| fields - count
| eval n = 5
| eval ki = mvrange(1, n+1)
| mvexpand ki
| eval ln_ki = ln(ki)
| eventstats sum(ln_ki) as sum_ln_ki by n
| eval n_factorial = round(exp(sum_ln_ki))
| stats first(n_factorial) as n_factorial by n
```

Alternative that performs the exponential calculations at the end and might improve performance. Give it a go too:

```
| stats count
| fields - count
| eval n = 5
| eval ki = mvrange(1, n+1)
| mvexpand ki
| eval ln_ki = ln(ki)
| eventstats sum(ln_ki) as sum_ln_ki by n
| stats count by n, sum_ln_ki
| eval n_factorial = round(exp(sum_ln_ki))
| fields - count, sum_ln_ki
```

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Richfez

SplunkTrust

02-12-2016
05:43 AM

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javiergn

SplunkTrust

02-12-2016
05:23 AM

You could try an approximation by using the natural logarithm (see formula in picture below).

That should work fine when n is not too big, if n is big enough you might have approximation errors. Using the Stirling's approximation is more accurate in this case but it won't be easy to implement with the Splunk built-in commands.

See an example below for the natural logarithm approximation when n=5:

```
| stats count
| fields - count
| eval n = 5
| eval ki = mvrange(1, n+1)
| mvexpand ki
| eval ln_ki = ln(ki)
| eventstats sum(ln_ki) as sum_ln_ki by n
| eval n_factorial = round(exp(sum_ln_ki))
| stats first(n_factorial) as n_factorial by n
```

Alternative that performs the exponential calculations at the end and might improve performance. Give it a go too:

```
| stats count
| fields - count
| eval n = 5
| eval ki = mvrange(1, n+1)
| mvexpand ki
| eval ln_ki = ln(ki)
| eventstats sum(ln_ki) as sum_ln_ki by n
| stats count by n, sum_ln_ki
| eval n_factorial = round(exp(sum_ln_ki))
| fields - count, sum_ln_ki
```

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martin_mueller

SplunkTrust

09-13-2020
04:49 AM

Here's Stirling's Approximation in SPL: `| eval n! = sqrt(2*pi()*n)*pow(n/exp(1), n)`

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javiergn

SplunkTrust

02-15-2016
01:45 AM

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shrirangphadke

Path Finder

02-15-2016
11:07 PM

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stanwin

Contributor

02-11-2016
05:14 AM

The splunk eval functions dont offer factorial computator.

You could create a custom command & offload the factorial generation logic to the python code.

http://docs.splunk.com/Documentation/Splunk/6.2.3/AdvancedDev/Searchscripts

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ichaer_splunk

Splunk Employee

02-11-2016
03:43 AM

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