HOW TO AVOID OVERWRITING WITH COMBINED FOR/IF LOOPS

Hello everybody ;

I have a problem with combinaison of loops for and if.

Let's say that I have this matrix which is combined of positiv and negativ numbers.

A=[ 1 4 7 -8 -1 -7; -2 1 3 4 5 9; 1 2 5 -8 -7 4;1 2 4 -4 7 -2; -3 5 -7 -1 1 1]

If I want to set a condition to every element depending on its sign, and add 3 for negative numbers while substracting 7 to positive numbers

I wrote then the following code

B=zeros(size(A))

for i=1:length(A)
    if A(i)<0
        B(i)=A(i)+3
    else 
        B(i)=A(i)-7
    end 
end

the problem is the overwriting of the matrix B (which I don't understand why) and the non-saving of the results also.

Any help please

thank you

Answer 1

[Jan]

length(A) is 6, so you process the first 6 elements only. To process the complete matrix:

for i = 1:numel(A)
  ...

By the way: a vectorized version:

B = A + 3;
B(A >= 0) = A(A >= 0) - 10;

Or:

B          = zeros(size(A));
match      = (A < 0);
B(match )  = A(match) + 3;
B(~match ) = A(~match ) - 7;

[heir ancestors]

Thank you very much :)