loop: stop if value repeats

In my case this is based on

N = Number 
G = Guess
x = 1:N

R= mod(G^x,N)

if R repeats I want the calculation mod(G^x,N) to stop and tell me the steps P that it needed to to get to R again

for example if R = 1, 4 3, 2, 1 -stop, P = 4

[Niklas Kurz]

noone comes up with a solution?

[Jan]

The question is not clear yet. This makes it hard to find an answer.

Answer 1

[Jan]

N = Number; 
G = Guess;
x = 1:N;

R = nan(1, N);
found = N;
for k = 1:N
  R(k) = mod(G^x(k), N);
  if any(R(1:k-1) == R(k))
      found = k;
      break
  end
end
R = R(1:found)

Answer 2

[Bob Thompson]

I'm assuming you have all four of these commands within some kind of loop. Without the loop I have not tested what I'm going to suggest adding.

To do what you're asking you need to record all the values of R, this can be done by indexing R. If you aren't running an integer for loop, starting at 1, then you'll need to add a counter for each time you loop.

Then you just need an if check to determine if the loop needs to be broken.

N = Number
G = Guess
x = 1:N

R(count) = mod(G^x,N)
if sum(R==R(count))>1
    P = length(R)-1;
    break
end
end

[Niklas Kurz]

How comes the value inside the sum can get greater then one? Looks to me like logical 0 and 1. Also I didn't build an actual loop before that. Assume N and G are constants, how a functional loop would be written then? By indexing, do you mean a for loop?

[Bob Thompson]

Mmm, I see where you're getting the multiple values for R from now.

I'm not sure why you want to stop the 'loop' if you don't actually have one. Do you just want the first index of R which contains a duplicate?

[Niklas Kurz]

I want the loop to stop if it found a repeated number because I need it to count the period in order that I can continue processing wih it.