Sum of all the prime numbers under 100, sum of the ten smallest prime numbers.

As the title says i have to create a program which sums all the prime numbers under 100, and sums the ten smallest prime numbers. I can't use the built in functions like: sum, prime, etc. I tried making a program using those functions and it worked.

I'm thinking about something like this:

i have to write a program that detects a prime number, since a prime number only can be divided by 1 and itself, i need to write a program that detects that.

Then i need to customize it so that it detects all the prime numbers under 100.

Then it needs to sum all of the prime numbers under 100 and displays the "sum".

For the "ten smallest prime numbers", i have no idea to be honest..

Cheers!

Answer 1

[Torsten]

Oh come on ...

summe = 0;
nprime = 0;
for i = 2:100
  istprime = 1;
  for j = 2:i-1
    if mod(i,j)==0
      istprime = 0;
      break;
    end
  end
  if istprime == 1
    nprime = nprime + 1;
    summe = summe + i;
    if nprime == 10
      summe_first_ten = summe ;
    end 
  end
end

Best wishes

Torsten.

Answer 2

[Walter Roberson]

For the 10 smallest prime numbers, check whether 1 is prime, check whether 2 is prime, check whether 3 is prime, and so on, continuing until you have 10 prime numbers. A while loop.

Hint: if you have already found all the primes under 100, then you do not need to repeat that work; for example if it turns out there are 7 primes under 100 then you could continue on from 100 until you found as many more as you needed.

[Andreas Larsson]

Yes I understand, but my problem right now is to write a program that "detects" if the number is a prime number or not.

[Guillaume]

Writing a primality test is not particularly difficult. As per the article, test if the number can be divided by all integers (or just the odd ones) up to the square root of the number.

[Torsten]

A very primitive approach to test whether a number "n" is prime or not is to use MATLAB's "mod" function. If mod(n,i) == 0 for some 2<=i<=n-1, n is not prime.

Best wishes

Torsten.

[Andreas Larsson]

this is how far i've come:

for i=2:100
   for j=2:100
      if(~mod(i,j)) 
         break; % if factor found, not prime
      end 
   end
   if(j > (i/j))
      fprintf('%d is prime\n', i);
   end
end

Now i just need to sum them all together, i can't use sum function.

Answer 3

[Torsten]

summe = 0;
for i = 2:100
  istprime = 1;
  for j = 2:i-1
    if mod(i,j)==0
      istprime = 0;
      break;
    end
  end
  if istprime == 1
    summe = summe + i;
  end
end

Best wishes

Torsten.

[Andreas Larsson]

Thank you!!

However, how should i do it, for it to calculate the sum of the 10 first prime numbers?

Answer 4

[Elias Gule]

Does this help

primes_ = primes(100);  
sum_    = sum(primes_(1:10));