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From: Roger Stafford on 24 Jun 2010 22:28
"dhaskantha " <dharen10(a)gmail.com> wrote in message <i00qvf$16s$1(a)fred.mathworks.com>...
> Thank you. Roger. Could you show me an example of 20 generating random normal values between 0 and 1 using the while loop? Your help is much appreciated. I am not very familiar with the while loop.
> Thank you, once again.
- - - - - - - - - - -
I'll use the example you quoted of normrnd(0,1) with its mean 0 and variance 1, and choosing only values of it between 0 and 1. This would be a rather strange undertaking because more than half of the outputs of normrnd would be rejected this way. However it will show you the method of selection with a while-loop. As you see, the while-loop keeps repeating until it has received 20 of the random outputs that fall within range.

N = 20; % The desired number of outputs of normrnd in [0,1]
x = zeros(N,1); % The results are stored here
n = 0; % The number already selected
while n < N % Keep looping until N values in range have occurred
t = normrnd(0,1);
if 0<=t & t<=1 % Is it in range?
n = n+1;
x(n) = t; % If so, accept it
end
end % Exit when n is equal to N

Please bear in mind that the resulting distribution in x would be a far cry from being normal! The entire left side and a great section of the right side of the standard "bell curve" would be missing.

Roger Stafford
From: James Tursa on 2 Jul 2010 17:10
"Roger Stafford" <ellieandrogerxyzzy(a)mindspring.com.invalid> wrote in message <i0147o$k3c$1(a)fred.mathworks.com>...
> "dhaskantha " <dharen10(a)gmail.com> wrote in message <i00qvf$16s$1(a)fred.mathworks.com>...
> > Thank you. Roger. Could you show me an example of 20 generating random normal values between 0 and 1 using the while loop? Your help is much appreciated. I am not very familiar with the while loop.
> > Thank you, once again.
> - - - - - - - - - - -
> I'll use the example you quoted of normrnd(0,1) with its mean 0 and variance 1, and choosing only values of it between 0 and 1. This would be a rather strange undertaking because more than half of the outputs of normrnd would be rejected this way. However it will show you the method of selection with a while-loop. As you see, the while-loop keeps repeating until it has received 20 of the random outputs that fall within range.
>
> N = 20; % The desired number of outputs of normrnd in [0,1]
> x = zeros(N,1); % The results are stored here
> n = 0; % The number already selected
> while n < N % Keep looping until N values in range have occurred
> t = normrnd(0,1);
> if 0<=t & t<=1 % Is it in range?
> n = n+1;
> x(n) = t; % If so, accept it
> end
> end % Exit when n is equal to N

Or, for a somewhat vectorized version of this rejection scheme:

N = 20;
x = randn(N,1);
z = (x < 0) | (x > 1);
s = sum(z);
while s
x(z) = randn(s,1);
z = (x < 0) | (x > 1); % not optimized, doing extra work here
s = sum(z);
end

James Tursa
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