How to generate and calculate with empty values of matrix-elements

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Emerson De Souza 2011-8-28

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PROBLEM:

I want to use logical functions to perform calculations between

matrices and don't know how to generate an empty value NaN.

EXAMPLE:

I use the two matrices below and if the conditions are not satisfied,

the output should be empty:

clear all;

close all;

m=rand(10,5);

n=rand(10,5);

T = zeros(size(m)); % Make another array to fill up...

for i = 1:numel(m)

    if m(i)>.5  && n(i)>.5
       T(i) = m(i+1)+n(i+1);
    else
       T(i) = 'NaN';
    end

end

However, I obtain the following error:

??? In an assignment A(I) = B, the number of elements in B and

I must be the same.

I hope someone know the correction of this mistake

Thank you in advance

Emerson

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Oleg Komarov 2011-8-28

Please format the code as indicated here: http://www.mathworks.com/matlabcentral/answers/13205-tutorial-how-to-format-your-question-with-markup

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Paulo Silva 2011-8-28

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T(i) = 'NaN';

is wrong, your arrays have numeric values but you are trying to put one string in the array, you can't mix numeric values with strings on arrays, do this instead:

T(i) = NaN;

See this simple example:

a=[1 2;3 4];a(1)='NaN'; %error because of what I told you

a=[1 2;3 4];a(1)=NaN; %No error

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Emerson De Souza 2011-8-28

Thank you Paulo,

I learned it. On the other hand, I obtain the problem with the assigning to T(i) (see comments from Oleg).

I'm trying to reformulate the problem.

Thank you for your help

Emerson

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Oleg Komarov 2011-8-28

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You can write directly:

NaN

instead of

'NaN'

but my suggestion would be to preallocate NaNs directly and delete the else part:

T = NaN(size(m))

The other problem is that you are assigning to T(i) the next values in m and n, but if i = 50 then the index will exceed the matrix dimensions.

So, what do you want to accomplish here? (you can also avoid the loop)

EDIT replaced & with |:

m = rand(10,5);
n = rand(10,5);
T = [m(2:end,:) + n(2:end,:); NaN(1,size(m,2))];
T(m(1:end-1,:) <= .5 | n(1:end-1,:) <= .5) = NaN;

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Emerson De Souza 2011-8-30

I don't know what is wrong, but it is not working consistently for all elements. Sometimes is correct, but not always.

Could you please have a look on it?

Thanks

Emerson

Oleg Komarov 2011-8-30

You have to be careful when checking m and n, if their first element are both bigger than .5 then the first element of T is the sum of the SECOND elements of m and n.

After correcting for the | I do get consistent results. Try to replace in my example:

m = rand(3);

n = rand(3);

You'll be able to visualize and compare more easily.

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