Calculating prime numbers using mod

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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'