You have defined B directly from a logical condition. When you do so, MATLAB checks for logical true values and defines accordingly; so the resulting variable is updated till the last logical true value.
Which is the 3232nd value here -
A=load('data.mat').A;
idx=find(A==2,1,'last')
%Example
x=1:10;
%size of y is not same as size of x
y(x<7)=0
To get the desired result, you should pre-allocate B with the same size as A and then apply the condition.
B=zeros(size(A));
B(A==2)=1;
size(B)
Also, you should consider what values those B elements take where elements of A are not equalto 2.
