Building an array while looping

Question

I have a for loop that loops over one array...

for i=1:length(myArray)

In this loop, I want to do check on the value of myArray and add it to another array myArray2 if it meets certain conditions. I looked through the MATLAB docs, but couldn't find anything on creating arrays without declaring all their values on initialization or reading data into them in one shot.

Many thanks!

Answer 1

I'm guessing you want something more complicated than

myArray = [1 2 3 4 5];
myArray2 = myArray(myArray > 3);

The easiest (but slowest) way to do what you're asking is something like

myArray2 = [];
for x = myArray
    if CheckCondition(x) == 1
        myArray2 = [myArray2 x]; %# grows myArray2, which is slow
    end;
end;

You can sort of optimize this with something like

myArray2 = NaN(size(myArray));
ctr = 0;
for x = myArray
    if CheckCondition(x) == 1
        ctr = ctr + 1;
        myArray2(ctr) = xx;
    end;
end;
myArray2 = myArray2(1:ctr); %# drop the NaNs

You might also want to look into ARRAYFUN.

Answer 2

For the most part, the way to do what you're describing is like mtrw said in the first example.

Let's say data = [1 2 3 4 5 6 7 8 9 10], and you want to get only the even numbers.

select = mod(data,2)==0; % This will give a binary mask as [0 1 0 1 0 1 0 1 0 1].

If you do data2=data(select), it will give you [2 4 6 8 10].

Of course, the shorter way to do this is as mrtw had in example 1:

data2=data(some_criteria);