RE: Cube root

["Richard Jackson" <richardjackson@xxxxxxxxxxxxxxxxxx>]

There is no "easy and good approximation" technique for cube roots.  Here are three methods to do what you want. 

 

Using system APIs, convert input argument to log(10), divide by 3, convert back. 

 

Use the power API function to raise the input argument to the 0.333... power. 

 

Write an RPG program:

 

Assign some value to a variable called tolerance. 

Using some technique that you invent, compute a seed value from input argument and assign it to a variable named work. 

TAG: Cube variable work into a variable named cube. 

Subtract the input value from cube and assign to a variable named difference. 

If the absolute value of difference is greater than tolerance,

    if difference is positive, adjust work down

    if difference is negative, adjust work up

    Go to tag

else

    the current value of work is the cube root of input argument

end

 

You should be able to make this into a nice tight little loop, I wrote it in this style because we don't have to have any arguments about syntax or language.  The RPG program is, in fact, the missing approximation technique noted in the first sentence.  You will note that it uses a technique called successive approximation.  It is good but not easy - it may require a lot of CPU cycles in the cubing function to discover the answer. 

 

Richard Jackson

(speaking only for myself)

 

 -----Original Message-----
From: owner-rpg400-l@midrange.com [mailto:owner-rpg400-l@midrange.com]On Behalf Of Walter Hesius
Sent: Wednesday, February 14, 2001 3:06 PM
To: RPG400-L@midrange.com
Subject: Cube root

Hi all,

 

I would like to know a way to calculate the cube root of a number. We are on V4R4.

 

Thanx.