A(I) = B, the number of elements in B and I must be the same

Hello,

I know I'm missing something fairly obvious, but I'm getting the error "A(I) = B, the number of elements in B and I must be the same" from a loop in my code:

    for j = 1:num_footstrike
        myMarkerCycle(j) = yy(start_fs(j):end_fs(j));
    end

'num_footstrike' is a row vector, as are 'start_fs', 'end_fs' and 'yy'. Any thoughts?

Answer 1

[Evan]

Assuming the sections of yy that you're assigning to myMarkerCycle are all the same size, try this:

for j = 1:num_footstrike
   myMarkerCycle(j,:) = yy(start_fs(j):end_fs(j));
end

Otherwise, you might need to go some other route, such as using a cell array for myMarkerCycle instead of a matrix.

[Amanda]

Ah, I see. The sections of yy are slightly varied in their lengths. So you would recommend using a cell array? I'm not familiar with how those work, but I'll look into it. Thanks!

[Evan]

No problem!

As for the cell array, they have their advantages and disadvantages. They allow you to store multiple datatypes and (more relevant to your case) arrays of variable size in a single array. However, if you end up with very large cell arrays or lots of operations on the cells, you can suffer performance losses and large memory use.

If you for some reason ran into either of these problems, there's one other way to create your array, but it would require padding your rows with zeros to make them all the same length. If you know that none of your data will be zero, this wont be a problem, but if zeros are a possibility, this wouldn't be a very convenient route.

Answer 2

[Jan]

As Evan has pointed out already, this is the method to store the vectors in a cell:

myMarkerCycle = cell(1, num_footstrike)
for j = 1:num_footstrike
   myMarkerCycle{j} = yy(start_fs(j):end_fs(j));
end

This is clean and efficient when num_footstrikes is small, e.g. < 10'000. For large arrays it is worth to consider, if the current storage method isn't the most efficient solution already: A dense data vector yy and two index vectors start_fs and end_fs.