# Finding the sum of the first n primes and to use a while function

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Samuel Walker on 7 Mar 2021
Edited: John D'Errico on 7 Mar 2021
clear % This scripts attempts to find the sum of the first n primes
n=input('Enter a value for n');
i = 1;
prime_count=0;
sum = 0;
while prime_count <= n
if ~isprime(i)
% isprime returns TRUE if i is a prime
prime_count = prime_count + 1;
sum = sum + i;
end
i = i + 1;
end
fprintf('The sum of the first %d primes is %d\n', n, sum);

John D'Errico on 7 Mar 2021
Edited: John D'Errico on 7 Mar 2021
You stopped your while loop the wrong way. As it is, if you allow prime_count to be less than or EQUAL to n, then it adds ONE more prime into the sum. And that is one prime too many.
Next, NEVER use sum as a variable. If you do, then your next post here will be to ask in an anguished voice, why the function sum no longer works. Do not use existing function names as variable names.
The following code (based on yours) does now work:
n = 10;
i = 1;
prime_count=0;
psum = 0;
while prime_count < n
if isprime(i)
% isprime returns TRUE if i is a prime
prime_count = prime_count + 1;
psum = psum + i;
end
i = i + 1;
end
prime_count
prime_count = 10
psum
psum = 129
Does it agree?
plist = primes(1000);
sum(plist(1:10))
ans = 129
Of course.

Matt J on 7 Mar 2021
if isprime(i)
Samuel Walker on 7 Mar 2021
i took away the ~ sign but the value is still off

Jorg Woehl on 7 Mar 2021
Edited: Jorg Woehl on 7 Mar 2021
There are two issues in your code:
1. You are testing if i is not a prime number, when you actually want to test whether it is a prime number, according to the statements that are executed in this case. Therefore, change ~isprime(i) to isprime(i).
2. The while loop should continue until n prime numbers have been found. Yet your loop execution condition is prime_count <= n, which means that even if prime_count is equal to n the loop would continue to run until prime_count is n+1! Change prime_count <= n to prime_count < n and you are good to go.
Although your code will work as a script, it would be nicer to put it into a function where the user is directly supplying n as opposed to being asked to enter it during execution. It also avoids the need for a clear statement, as functions operate in their own variable space.
function sum = firstNPrimes(n)
i = 1;
prime_count=0;
sum = 0;
while prime_count < n
if isprime(i)
% isprime returns TRUE if i is a prime
prime_count = prime_count + 1;
sum = sum + i;
end
i = i + 1;
end
end