MATLAB: HOW TO AVOID OVERWRITING WITH COMBINED FOR/IF LOOPS

combined if for 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

Best Answer

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;
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