Anonymous Functions for updating arrays.

I use many anonymous functions for updating elements within arrays. However generally this requires me to write this kind of thing:

Anew = A;
Anew(i) = anon_f(A,i);

However I feel it should be possible to redefine the function so that all I need is:

Anew = anon_f2(A,i);

Of course, I could always just use child functions instead... An example function:

    f = @(state,i)(2*ceil(rem(sum(state(:,i).^3),1)) - 1);

Answer 1

[Walter Roberson]

anon_f2 = @(Variable,idx) subsasgn(Variable, struct('type','()','subs',{{idx}}), anon_f(Variable,idx));

Note that your preferred syntax expects the index to not only select which output location is used but also forms one of the arguments into anon_f to choose the proper output value. I can't quite picture a case in which that would be useful ?

[Chris]

Thanks, this works - I didn't know about the subsasgn function.

I'm not sure I understand your comment though. I guess you're pointing out that, (at least it works for the function f above) I could redefine it so that instead:

anon_f2 = @(Variable,idx) subsasgn(Variable, struct('type','()','subs',{{idx}}), anon_f(Variable(:,idx)));

I guess this is a little neater.

[Walter Roberson]

Your index appears to be a linear index for the purpose of the assignment, but you are using it as a column selector of the variable for the purpose of the anonymous function. I do see in your sample routine with the rem() and sum() calls that Yes, you extract a column and produce a single value, but:

(A) If the output is the same for all values in the same column, then why is the output only assigned to a single value instead of to a column ?

(B) Are you sure that you want to select a column from the variable but then assign to the row of the same number, since linear indexing goes down rows? If ANew = A; ANew(3) = sum(A(:,3)) then the ANew(3) means ANew(3,1) not some slot in the third column.

(C) Watch out for non-square A, different number of rows and columns is going to mess you up...

[Chris]

Sorry - mis-typed above, should be Anew(i,i) = ...I think that solves most of those issues. As to why I only want to update the values on the diagonal...I don't want to explain the whole problem. Thanks again.

[Walter Roberson]

Ah. To change (i,i) make the 'subs' into {{idx, idx}}