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-   -   Find the sum of all the prime numbers (http://www.go4expert.com/articles/sum-prime-t2049/)

shabbir 29Nov2006 18:02

Re: Find the sum of all the prime numbers
 
There is other method also which does not require sqrt.

Quote:

Originally Posted by wiki
One need not actually calculate the square root; once one sees that the quotient is less than the divisor, one can stop


pradeep 29Nov2006 18:16

Re: Find the sum of all the prime numbers
 
Interesting, so we will find the factor of n within any of the number in sqrt(n) if not then its a prime number.

friendsforniraj 2Dec2006 17:57

Re: Find the sum of all the prime numbers
 
let me tell you the logic now
it is very simple
look if k is any no.
then k can be represented as k=i*j
where i and are k's factor
now i varies from 1to k
and j varies from k to 1
but for prime no. we check for i=2 to k/2
acordingly j will varies from (k/2) to 2
now if 2 is a factor of k
then automatically k/2 is also a factor and so on
that is when i increases, j decreases
so we continue like this then there is condition that both i and j becomes equal
so i*i=k
so i=sqrt(k)
but if k is not a square of any no. then i increases upto a integer just less than sqrt iof k
thats all what i did
now try this problem
for a given no. print all its prime factors that is
if input is 12 out put should be 2*2*3
using the hint that the lowest factor of any no. other than one is always prime
try it

shabbir 4Dec2006 10:45

Re: Find the sum of all the prime numbers
 
friendsforniraj thats a good logic explained.

friendsforniraj 5Dec2006 17:40

Re: Find the sum of all the prime numbers
 
thank u sir
but ma name is niraj

shabbir 6Dec2006 11:13

Re: Find the sum of all the prime numbers
 
Quote:

Originally Posted by friendsforniraj
thank u sir
but ma name is niraj

I will try putting that from now on.

rahul.mca2001 6Mar2008 13:53

Re: Find the sum of all the prime numbers
 
why do we need a square root

asadullah.ansari 7Mar2008 20:19

Re: Find the sum of all the prime numbers
 
Quote:

Originally Posted by friendsforniraj
let me tell you the logic now
it is very simple
look if k is any no.
then k can be represented as k=i*j
where i and are k's factor
now i varies from 1to k
and j varies from k to 1
but for prime no. we check for i=2 to k/2
acordingly j will varies from (k/2) to 2
now if 2 is a factor of k
then automatically k/2 is also a factor and so on
that is when i increases, j decreases
so we continue like this then there is condition that both i and j becomes equal
so i*i=k
so i=sqrt(k)
but if k is not a square of any no. then i increases upto a integer just less than sqrt iof k
thats all what i did
now try this problem
for a given no. print all its prime factors that is
if input is 12 out put should be 2*2*3
using the hint that the lowest factor of any no. other than one is always prime
try it

Excellent explaination. Now any one can understand either he is new or experience

oleber 8Mar2008 12:51

Re: Find the sum of all the prime numbers
 
for(j=2;j<=sqrt(i);j++) :cryin:

is similar to

for(j=2;j*j <= i;j++) :p

so, no sqrt in here since it is really slow. At least if was some years ago.

oleber 8Mar2008 12:54

Re: Find the sum of all the prime numbers
 
for getting the prime number try

http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

No devisions in here


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