Usenet post to alt.comp.lang.learn.c-c++ on 17.2.2009

> > Hello,
> > I have written a program (from exercise 13 in You can Program in
> > C++). The program is supposed to compute the 'Root Mean Square' of
> > a set of values. Unfortunately , i dont know a lot about maths at
> > the moment so i thought i better post my program here to check its
> > giving me the correct output.
> > 
> > This is the logic i used to write the program.
> > 1. Identify the Number of values that are to be input
> > 2. Make a total of the Square roots
> > 3. Then identify the average of the total (ie the Arithmentic
> > mean) 4. Identify the 'root mean square' - the square root of the
> > arithmetic mean.

Richard Heathfield replied:

Not quite.

I'll use your test values to indicate the procedure for calculating 
RMS.

> > I tested my program with these three values: 5 , 8 and 12.

Step 1: Square each value, and sum the squares:

n = 0     ---- number of data points, used later
Sum = 0.0
x = 5.0
++n       ---- n is now 1
x = x * x ---- x is now 25.0
Sum += x  ---- Sum is now 25.0
x = 8.0
++n       ---- n is now 2
x = x * x ---- x is now 64.0
Sum += x  ---- Sum is now 89.0
x = 12.0
++n       ---- n is now 3
x = x * x ---- x is now 144.0
Sum += x  ---- Sum is now 233.0

Step 2: Divide by the number of data points:

Mean = Sum / n ---- Mean is now 233.0 / 3 = 77.6667 (ish)

Step 3: Take the square root:

rms = sqrt(77.6667) ---- rms is now 8.81287

_________________________
In writing this program, I encountered the concept of 'Root Mean Square' for the first time. My initial attempt to write the program was flawed as I was not able to clearly and correctly identify the steps needed to calculate the value.

Instead of simply producing a sum of the squares of all values entered, I attempted to generate a sum of all the square roots of the values entered. Once the error was pointed out, I corrected the program code.