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IDL Reference Guide: Procedures and Functions |
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The FACTORIAL function computes the factorial N! For integers, the factorial is computed as (N) × (N – 1) × (N – 2) × ... × 3 × 2 × 1. For non-integers the factorial is computed using GAMMA( N+1 ).
This routine is written in the IDL language. Its source code can be found in the file factorial.pro in the lib subdirectory of the IDL distribution.
Result = FACTORIAL( N [, /STIRLING] [, /UL64] )
Returns the product of the non-negative scalar value or array of values.
A non-negative scalar or array of values.
| Note Large values of N will cause floating-point overflow errors. The maximum size of N varies with machine architecture. On machines that support the IEEE standard for floating-point arithmetic, the maximum value of N is 170. See MACHAR for a discussion of machine-specific parameters affecting floating-point arithmetic. |
Set this keyword to use Stirling's asymptotic formula to approximate N!:
where e is the base of the natural logarithm.
Set this keyword to return the results as unsigned 64-bit integers. This keyword is ignored if STIRLING is set.
| Note Unsigned 64-bit integers will overflow for values of N greater than 20. |
Compute 20!:
PRINT, FACTORIAL(20)
IDL prints: