From: Dan Nachbar <dan@...>
Date: 2011-10-22T20:57:28+09:00
Subject: Re: power of 2 code

On Oct 22, 2011, at 12:37 AM, Ray DarkScythe wrote:

> hello I am new here and new to ruby (no programming experience but
> willing to learn)
> 
> I am following "Daniel Carrera" tutorial I am already at while loops and
> fairly I understand of it but the one thats bugging me is the exercise
> of the power of 2.
...

I would make three changes to your code.

First, in your code you have a variable named "number".
Actually, there are a pair of numbers. I think one
reason you are having trouble is that you have confused
yourself about what "number" is supposed to be doing.

So I suggest that you use more descriptive variable
names such as (for this example) "limit" and "exponent".

Secondly, I'd do the exponentiation explicitly rather
than implicitly the way your code does.

The third problem you are having is a classic
"off by one" error. You test ever-increasing values
until one is too big.  You then print out the
excessively large value. But the desired result is last
value that is not too big. In other words, you report
a value that has been increased one too many times.
(Off-by-one errors are extremely common when looping.)

So, using more descriptive variable names, explicit
exponentiation, and reworking the while loop to
"test ahead" to avoid the "off by one" problem:

puts "Enter a maximum value: "
limit = gets.chomp.to_i
exponent = 0
while (2 ** (exponent+1)) <  limit
  exponent += 1
end
puts exponent.to_s + " is the highest " +  \
  "power of 2 less than " + limit.to_s

That's actually a tricky bit of coding.  I have to admit that
I initially tripped over the off-by-one issue while giving
it a try. So don't be surprised that it gave you some
trouble.

Have fun and keep plugging away.

PS - You may have noticed that Ruby programmers often use
very short variable names. So, for the above, some might
use "l" and "e".  This tendency is largely bravado. I
suggest you stick with longer variable names while learning.
You can save the keystrokes when you are more experienced
and want to show off your refined poetic sensibilities.

Dan Nachbar