Thread Subject:

Applying equation to Matrix

I want to apply equation to a matrices elements individualy.
ie if any element in C is less than 50 i want it to change element to 0
If it is greater than 50 but less than 600 i want it to apply the equation below to the element
and less than 1100 and greater than 600 apply the last equation to the element

ie working its way through the matrix

if 0<C<50;
C=0;
elseif 600>C>=50;
C=(.2666*C+155.29);
elseif 1100>C>=600;
C=(.4041*C+106.8);
else
C=C
end

"Fred O'Con" <barabus12@hotmail.com> wrote in message <i4rm8j$gd7$1@fred.mathworks.com>...
> I want to apply equation to a matrices elements individualy.
> ie if any element in C is less than 50 i want it to change element to 0
> If it is greater than 50 but less than 600 i want it to apply the equation below to the element
> and less than 1100 and greater than 600 apply the last equation to the element
>
> ie working its way through the matrix
>
> if 0<C<50;
> C=0;
> elseif 600>C>=50;
> C=(.2666*C+155.29);
> elseif 1100>C>=600;
> C=(.4041*C+106.8);
> else
> C=C
> end

Hi Fred, There are many ways to do this. One way is to use logical indexing.

C = randi(1300,20,20);
C(C<50) = 0;
% I use > 0 here because you've already set everything less than 50 to zero
C(C>0 & C<600) = 0.2666*C(C>0 & C<600)+155.29;

And so on.

Wayne

"Wayne King" <wmkingty@gmail.com> wrote in message <i4roqm$rqs$1@fred.mathworks.com>...
> "Fred O'Con" <barabus12@hotmail.com> wrote in message <i4rm8j$gd7$1@fred.mathworks.com>...
> > I want to apply equation to a matrices elements individualy.
> > ie if any element in C is less than 50 i want it to change element to 0
> > If it is greater than 50 but less than 600 i want it to apply the equation below to the element
> > and less than 1100 and greater than 600 apply the last equation to the element
> >
> > ie working its way through the matrix
> >
> > if 0<C<50;
> > C=0;
> > elseif 600>C>=50;
> > C=(.2666*C+155.29);
> > elseif 1100>C>=600;
> > C=(.4041*C+106.8);
> > else
> > C=C
> > end
>
> Hi Fred, There are many ways to do this. One way is to use logical indexing.
>
> C = randi(1300,20,20);
> C(C<50) = 0;
> % I use > 0 here because you've already set everything less than 50 to zero
> C(C>0 & C<600) = 0.2666*C(C>0 & C<600)+155.29;
>
> And so on.
>
> Wayne
- - - - - - - - - -
  Care must be taken when applying such vectorized changes sequentially. Notice that the C numbers lying in [600,1100) are changed into numbers within the [50,600) range, so the [50,600)-transformation must occur before the [600,1100)-transformation. Otherwise some values would undergo double changes.

  One easy, safe way to avoid such inadvertent changes in general is to say

t1 = C<50; t2 = 50<=C & C<600; t3 = 600<=C & C<1100;
C(t1) = 0;
C(t2) = .2666*C(t2)+155.29;
C(t3) = .4041*C(t3)+106.8;

This is safe because the t-values are unaffected by any of the subsequent changes in C. (We assume here that the t-values are all disjoint.)

Roger Stafford

Thanks very much Roger that is great.