Calculating prime numbers using mod

Hi, I have a code that its supposed to calculate all prime numbers from an input of n numbers. However, in the first loop, the code uses "mod" to check if the number is divisible by any number from 2 to sqrt of n and after dividing by 2 it does not proceed to 3 and discard multiples of 3, 5 and so for. How can I fix this issue?

n = 20;                     
L = 2:n;                    

index = 2;

for k = 2:floor(sqrt(n))

while index < n

if (mod(L(index),k) == 0)
    L(index) = 0;

end

index = index + 1;

end

end

L';
Prime=L(L~=0);
Prime'

Answer 1

[Walter Roberson]

You need to reset index at the beginning of the for loop.

[Jose Trevino]

how do I do that? That's what I was trying with no luck