Loop that generates an array of indices by omitting one index at each iteration

Question

Using a for loop, how would I write code to generate an array of indices where at each iteration k of the loop, I would generate an array of indices from [1, 2, 3, ... N] that excludes k from the set?

As an example, if I had k = 3 iterations, the first iteration would give me indices (2,3), the second iteration would give me indices (1,3) and finally the third iteration would give me indices (1,2).

Answer 1

Approach #1

You can use setdiff at each iteration to exclude the current iteration ID, like so -

for iteration_id = 1:3
    indices = setdiff(1:3,iteration_id)
end

Code run -

indices =
     2     3
indices =
     1     3
indices =
     1     2

---

Approach #2 (Vectorized)

You can employ a vectorized approach to generate all indices in one go, which could be easily used inside the loop(s) if you have to use those indices -

num_iters = 3; %// Number of iterations

all_indices = repmat([1:num_iters]',1,num_iters) %//'
all_indices(1:num_iters+1:end)=[]
valid_indices = reshape(all_indices,num_iters-1,[])'

Code run -

valid_indices =
     2     3
     1     3
     1     2

Answer 2

Another very simple way to do this:

N=3;
for k=1:N
    [1:k-1,k+1:N]
end