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

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);

Answer 1

[John D'Errico]

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.

Answer 2

[Matt J]

if isprime(i)  

[Samuel Walker]

i took away the ~ sign but the value is still off

Answer 3

[Jorg Woehl]

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