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Recursive Factorial Function ( n ! ) Expand / Collapse
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Posted Wednesday, October 3, 2007 6:00 PM
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Comments posted to this topic are about the item Recursive Factorial Function ( n ! )


Post #406547
Posted Thursday, May 7, 2009 11:11 AM
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Abslolutely ridiculous example.

Bad advice.

This is the most inefficient way of calculating factorials.

At least if there was a warning that this is intended to show how recursion works along with a warning to novices that in the case of a factorial this definitely not the proper way of doing and why.
Post #712248
Posted Tuesday, August 31, 2010 12:02 PM
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thanks for the help
Post #978250
Posted Friday, April 12, 2013 10:01 AM
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A better solution with no limits.

Thanks to:
http://2smart4school.com/tsql-stored-procedure-to-get-factorial-of-a-given-number/


CREATE PROCEDURE Factorial (@num INT) AS
BEGIN

DECLARE @fact int, @query varchar(255)
SET @fact = 1
IF(@num = 0)
BEGIN
SET @fact = 1
END
ELSE
BEGIN
WHILE(@num >0)
BEGIN
SET @fact = @fact * @num
SET @num = @num -1
END
END
RETURN @fact
END




Post #1441779
Posted Friday, April 12, 2013 10:04 PM


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davidrudd (4/12/2013)
A better solution with no limits.

Thanks to:
http://2smart4school.com/tsql-stored-procedure-to-get-factorial-of-a-given-number/


CREATE PROCEDURE Factorial (@num INT) AS
BEGIN

DECLARE @fact int, @query varchar(255)
SET @fact = 1
IF(@num = 0)
BEGIN
SET @fact = 1
END
ELSE
BEGIN
WHILE(@num >0)
BEGIN
SET @fact = @fact * @num
SET @num = @num -1
END
END
RETURN @fact
END



Not quite true. That stored procedure is limited to a factorial of only 12 because of the INT datatype. Because it's a proc, it's difficult to use in a non-RBAR environment. And I'm not sure that I'd trust anyone's code that blatantly had an unused variable in it.

I guess I don't understand why people insist on recalculating that which will not change. For example, no matter how many times you calculate it, 170! will always return the same number. So why not calculate it just once and store it in a "helper" table?

Here's how to make a Factorial "helper" table.

--===== Create the table with columns for N and N!.
-- This will prepopulate the values of N, as well.
SELECT TOP 171
N = IDENTITY(INT,0,1),
[N!] = CAST(0 AS FLOAT)
INTO dbo.Factorial
FROM sys.all_columns
;
--===== Add the quintessential PK for max performance of future lookups
ALTER TABLE dbo.Factorial
ADD CONSTRAINT PK_Factorial
PRIMARY KEY CLUSTERED (N) WITH FILLFACTOR = 100
;
--===== Declare a variable that well need to keep track of the previous product.
DECLARE @Factorial FLOAT;

--===== Update the table with factorials.
UPDATE f
SET @Factorial = [N!] = CASE WHEN N > 0 THEN @Factorial * N ELSE 1 END
FROM dbo.Factorial f WITH (TABLOCKX, INDEX(1))
OPTION (MAXDOP 1)
;
--===== Show our work
SELECT * FROM dbo.Factorial ORDER BY N;


Then all you have to do is join to the factorial table for any number of rows in a set based fashion instead of recalculating the same thing over and over.


--Jeff Moden
"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

(play on words) "Just because you CAN do something in T-SQL, doesn't mean you SHOULDN'T." --22 Aug 2013

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