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codecaine · (This post was last modified: 12-04-2009 10:17 AM by codecaine.)

What is the sum of all prime numbers below 5000?
Answer is: 1548136

Code:

#returns true if number is prime else false

def is_prime(num):

    tempnum = 0;

    if num == 2: 

        return True;

    if num < 2:

        return False;

    for x in range(3,num,2):

        tempnum = num % x;

        if tempnum == 0:

            return False;

    return True;

#sum starts with the value of 2 because 2 is a prime number    

sum = 2;

#skip al even numbers because non are prime except 2

for num in range(1, 5001, 2):

    #add number to sum if the number is prime

    if is_prime(num):

        sum = sum + num;

print('Prime sum: ' + str(sum));

note for the isprime function all you need to do is go up to the square root of a number for the loop. I will save time for large numbers. I did not include it in this source because I didn't really need it

***drdebcol*** · 12-04-2009, 11:17 PM

Very good solution i must say.
I made that in Pascal too :

Code:

program sum_of_primes_below_5000;

uses wincrt;

var

i,s:longint;

function prime(k:longint):boolean;

var

  i,g:longint;

  b:boolean;

begin

  if k mod 2 = 0 then

  if k=2 then

  prime:=true

  else

  prime:=false

  else

  begin

  if k=3 then

  prime:=true

  else

  begin

  b:=false;

  g:=round(sqrt(k));

  i:=3;

  repeat

    begin

      if k mod i = 0 then

      b:=true;

      i:=i+2;

    end;

  until (i>g) or (b=true);

  if b=true then

    prime:=false

  else

    prime:=true;

  end;

end;

end;

begin

s:=0;

for i:=2 to 5000 do

  if prime(i) then

    s:=s+i;

  writeln(s);

end.

I have the same result as you have !

codecaine · 12-05-2009, 12:42 AM

awesome code Smile