# How to create a list of an array index

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Scragmore on 6 Nov 2011
I am doing a project Euler prime number puzzle but I do not wish to use the Matlab primes function , so I'm building my own.
I am going to use the standard sieve to produce my primes but I wish to store these in a logic array. How do I get the index/cell location of all true values into an array/output as these will be my prime numbers.
Thanks
AD
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### Accepted Answer

Image Analyst on 7 Nov 2011
Can't you just use a counter that increments when you find a prime number?
if itsAPrimeNumber
numberOfPrimesFound = numberOfPrimesFound + 1;
listOfPrimes(numberOfPrimesFound) = yourCurrentNumber;
end
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Scragmore on 7 Nov 2011
If possible I would like to do it this way, learning to do bitwise calculations, bigger array/smaller memory allocation, how I want to use the function ect.
so if I have an array;
ary = [1 1 0 1 0 1 0 0 0 1 .....]
I would like to return;
return ary(true)
ans = 2 3 5 7 11 .......
Image Analyst on 7 Nov 2011
Just do this
primesList = find(ary)+1;
The find() function tells you which elements are non-zero or true.

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