Matlab for loop trouble

Question

I'm trying to understand for loops better in Matlab since for some reason they don't work like other languages.

values = zeros(1,7);
for i =[1, 4, 10, 15, 22, 24, 36]
        variable = function(input, input, i);
        values(i) = function(input, variable);
end

So values becomes a 1x36 double where for i I get results, but any number that isn't i gets a column set to 0. Why does it still add columns for values I didn't choose for i?

Thanks

Answer 1

You are preallocating values with 7 entries. When you assign a value at entry 10, entries 8 and 9 must be created too, and Matlab fills them with 0.

Here's an example:

>> clear
>> values = ones(1,7) % preallocate with ones
values =
     1     1     1     1     1     1     1
>> values(4) = 40
values =
     1     1     1    40     1     1     1
>> values(10) = 100 % the array will have to grow
values =
     1     1     1    40     1     1     1    0     0   100

Note also that having an array growing is not desirable, as it negatively impacts speed. It's better to preallocate to the final size, whenever it is known in advance.

---

Thanks to @Steve for the edit

If you want the results to be stored in consecutive entries, consider using a counting variable, e.g. k for your indexing which increases by 1 each step of the loop.

values = zeros(1,7);
% k stores the number of steps of the loop
k = 0;
for i =[1, 4, 10, 15, 22, 24, 36]
        % increment k
        k = k+1;
        variable = function(input, input, i);
        values(k) = function(input, variable);
end