multiplication of empty 0X0 double and scalar (1X1)

Hello,

I am trying to calculate sum of certain maximum elements of arrays. When some of these variables are not available in the program (due to if statement), then the maximum value of element of these variable array comes out to be 0X0 double (as I have by default defined the variable as a cell array of zeros).

The problem I am facing is an error "Unable to perform assignment because the indices on the left side are not compatible with the size of the right side."

I think the error is due to the multiplication part of two variables on the right side of equation i.e. - 0X0 double and one scalar. The other (left) side of equation (sum) is supposed to produce the scalar value.

How to handle this and get the scalar output on left side by ignoring these "absent" variables value (assigned 0 X0 double and a scalar)???

Can anybody help with this small issue and guide me on this, please?

A=cell(1,5);

B=cell(1,5);

C=cell(1,5);

a=1;

b=2;

c=3;

if.......

% A, B or C is not used and modified.

% Say A is not used or modified.

end...

AA=max(A);

BB=max(B);

CC=max(C);

DD=max(D);

sum= max(AA*a+BB*b+CC*c+DD*d);

% AA is now 0X0 double and a is 1 (1X1 double).

% Is this causing problem???

[Walter Roberson]

You can check with isempty()

However, you cannot take max() of a cell .

Answer 1

[the cyclist]

One possible solution is to use a try-catch structure. The "try" block will be the code that is generally executed, but if that section fails (due to the empty array), then the "catch" block will be executed instead.

[Rohit Mangalekar]

Thank you for your answers @the cyclist and @Walter Roberson.

@the cyclist It still means that I will have to provide alternative equation in 'catch' section? I was hoping to get it resolved within the same equation.

@Walter Roberson. Sorry, I could not understand exactly how the checking will help. But I think even after checking with isempty, I will again have to provide alternative equation to calculate the value of sum if the variable is empty.

[the cyclist]

You are saying "Do two different things for these two different cases", so I don't see any way around the fact that you will effectively have to code this in a way that treats the two cases differently. That being said, it may be possible to code a single line of code that seems a bit more elegant in how it handles the two cases.

What would be helpful is if you could post functional code that works in the one case, and gives the error in the other one. (As @Walter Roberson has pointed out, your code above is going to fail because A is a cell array, and you are taking the max of it.)

[Walter Roberson]

if isempty(sum) ;sum=0; end

do that before you attempt to assign sum into a subscripted variable.

... and don't use "sum" as a variable name. Chances are too high that you will end up also wanting to use the function sum() .And besides, it confuses readers.

[Rohit Mangalekar]

Thank you both @the cyclist and @Walter Roberson. I changed the logic I was trying to use and it worked.

I just didnt know exactly how this isempty could be used.

isempty is what I used , to check if the AA,BB,CC (the maximum array) are vacant. If they are null, then I assigned them a scalar 0. Because multiplying other scalar (a,b,c) with null vector was causing a problem , as I said in the question itself. If any of the AA,BB or CC was empty- the single equation- as given in the original question/query by me, does not work due to this error. So as @the cyclist said, I would have required to add multiple conditions either using try-catch or if-else. Both created large number of cases. So I avoided them.

Thank you both of you.

P.S. the max function is giving what I actually wanted i.e. the array made of maximum values from each all the columns of the cell array(converted to matrix using cell2mat) not the maximum array. So I didnt have any problem using max on cell arrray.