see, i have this error in my code saying: ??? Attempted to access x(0); index must be a positive integer or logical. Error in ==> abc at 14 x(i)=mod(g^x(i-1), p);

clear all; close all; clc;

p = input('Introduceti numarul prim p= ');

g = input('Introduceti numarul prim g= ');

Nr=p-1;

x=zeros(1, Nr);

for i=1:Nr
    x(i)=mod(g^x(i-1), p);
end;

     disp('Rezultatul este: ');
  disp(x);

[Walter Roberson]

Duplicates http://uk.mathworks.com/matlabcentral/answers/230110-see-i-have-this-error-in-my-code-saying-attempted-to-access-x-0-index-must-be-a-positive-int which also has answers

Answer 1

[Brendan Hamm]

You start the loop at i=1, but then try and index x(i-1), which is the same as x(0). MATLAB starts indexes at 1 (Mathematical indexing, not programmer indexing).

That is

x = [1 2 3 4];
x(1)
ans = 
    1
x(0)
Subscript indices must either be real
positive integers or logicals.

[Andreea Alexandra]

ok, I got it, thank you so much. how could I solve it? the thing is, that is the range this alg gives me in it's theory..

Answer 2

[Walter Roberson]

You have x(i)=mod(g^x(i-1), p); . When "i" is 1, that is mod(g^x(1-1),p) which is mod(g^x(0),p) . However there is no element #0 of a vector.

You need to think more about your initial conditions, of what is to happen on the very first iteration.

[Andreea Alexandra]

Thank you very much. I understand now. I'll think more about it.

[Walter Roberson]

I suspect what you want is

x(1) = 1;
for i=2:Nr
    x(i)=mod(g^x(i-1), p);
end