Question

note that the code below is obtained from Matlab documentation except for the bottom portion.

function B = nway(A,n)
% Compute average of every N elements of A and put them in B.

if ((mod(numel(A),n) == 0) && (n>=1 && n<=numel(A)))

B = ones(1,numel(A)/n);
    k = 1;   

    for i = 1 : numel(A)/n
         B(i) = mean(A(k + (0:n-1)));
         k = k + n;

    end

What does the for loop code mean, especially the following line?

for i = 1 : numel(A)/n

and how does the i work by inserting it in B(i)?

Answer 1

A for-loop in Matlab is constructed as

for iterationVariable = listOfValues
    do something
end 

The iterationVariable will take on the value of the first column of listOfValues in the first iteration of the loop, then the value of the second column, etc. You can then use iterationValue in your calculations.

for i=1:numel(A)/n

will therefore set the value of i to 1,2,3... up to the value of "number of elements of A divided by n".

B(i)

is an indexing operation, that returns the ith element of the array B.

As @HighPerformanceMark suggests, I very much recommend trying out these expressions at the command line, or to work through the “getting started” section of the excellent Matlab documentation.