MATLAB - Random Number Generation

Mush · #1 $Old$ February 17th, 2009, 11:09 AM

Hi.

Basically I want to generate four random numbers in a certain interval, and they can't be equal (unlikely to happen, but just incase!).

So let's imagine, that, for example, that interval is [0.5,1]. I want to generate a number in that interval, use it in a calculation, then generate another number on that interval, and use that in the same calculation, etc.

So:

Code:

a1 = some number in [0.5,1]
operation on a1
a2 = some number in [0.5,1] not equal to a1
operation on a2
a3= some number in [0.5,1] not equal to a1 or a2
operation on a3
a4 = some number in [0.5,1] not equal to a1 or a2 or a3
operation on a4

I know that the 'rand' command generates a number on the interval [0 1]. Hrm. But I can't find any way to use that to meet my end.

Mush · #2 $Old$ February 17th, 2009, 03:28 PM

I wrote something that works.

Code:

a = rand;
        if a<0.5
            a = a+0.5;
        end
        
   b = rand;
        if b<0.5
            b = b+0.5;
        end
        if b == a
            while b == a
                b = rand;
                 if b<0.5
            b = b+0.5;
                 end
            end
        end
        
    c = rand;
        if c<0.5
            c = c+0.5;
        end
        if c == a
            while c == a
                c = rand;
                 if c<0.5
            c= c+0.5;
                 end
            end
        end
        if c == b
            while c == b
                c = rand;
                 if c<0.5
            c = c+0.5;
                 end
            end
        end

It works, but I'll be glad to hear of anything simpler?!

CaptainBlack · #3 $Old$ February 17th, 2009, 09:11 PM

Quote:

Originally Posted by Mush $View Post$

Hi.

Basically I want to generate four random numbers in a certain interval, and they can't be equal (unlikely to happen, but just incase!).

So let's imagine, that, for example, that interval is [0.5,1]. I want to generate a number in that interval, use it in a calculation, then generate another number on that interval, and use that in the same calculation, etc.

So:

Code:

a1 = some number in [0.5,1]
operation on a1
a2 = some number in [0.5,1] not equal to a1
operation on a2
a3= some number in [0.5,1] not equal to a1 or a2
operation on a3
a4 = some number in [0.5,1] not equal to a1 or a2 or a3
operation on a4

I know that the 'rand' command generates a number on the interval [0 1]. Hrm. But I can't find any way to use that to meet my end.

Let the interval be (b,t) then somethink like:

Code:

function rv=RandIntvl(n,m,b,t)
%
%  function to produce a matrix of size nxm of random numbers
%  uniformly distributed on (b,t)
%
    if t<=b
       disp('error: b must be greater than t')
       rv=[];
       return
    end
 
    rv=rand(n,m);
    rv=rv*(t-b)+b;

should do part of the trick (warning: untested code).

CB

Mush · #4 $Old$ February 18th, 2009, 04:33 AM

[quote=CaptainBlack;268127]Let the interval be (b,t) then somethink like:

Code:

function rv=RandIntvl(n,m,b,t)
%
%  function to produce a matrix of size nxm of random numbers
%  uniformly distributed on (b,t)
%
    if t<=b

:
:

How is it ensured that they are uniformly distributed? Does matlab do this by default when you create a matrix/vector of random numbers?

CaptainBlack · #5 $Old$ February 18th, 2009, 07:03 AM

Quote:

Originally Posted by Mush $View Post$

How is it ensured that they are uniformly distributed? Does matlab do this by default when you create a matrix/vector of random numbers?

Code:

 
>help rand

CB