# prime numbers

posted 4 years ago

i am getting a strange output from my class

when howMany = 20 i get the following output

Prime Numbers

1

2

3

5

5

5

7

7

7

7

7

9

11

11

11

11

11

11

11

11

11

when howMany = 20 i get the following output

Prime Numbers

1

2

3

5

5

5

7

7

7

7

7

9

11

11

11

11

11

11

11

11

11

SCJP

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posted 4 years ago

i think i figured it out maybe. i am using longs so the % operator rounds results?

SCJP

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posted 4 years ago

Not sure what you mean by that, but whatever you mean, no that's not it.

Why does that output surprise you? What would you expect instead? You should add print statements or use a debugger so you can see what's actually happening?

Also note that when checking for divisors of X, you only need to go up to the square root of X.

Randall Twede wrote:i think i figured it out maybe. i am using longs so the % operator rounds results?

Not sure what you mean by that, but whatever you mean, no that's not it.

Why does that output surprise you? What would you expect instead? You should add print statements or use a debugger so you can see what's actually happening?

Also note that when checking for divisors of X, you only need to go up to the square root of X.

posted 4 years ago

thats right that is not the problem. i will try some System.out.println

you are also correct that i dont have to loop so many times

i see the problem now. it is doing exactly what i told it too

thanks for the tip about only needing to go up to the square root

you are also correct that i dont have to loop so many times

i see the problem now. it is doing exactly what i told it too

thanks for the tip about only needing to go up to the square root

SCJP

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posted 4 years ago

ok, i solved that one. now i am trying to solve problem3 a projectEuler.net

here is my solution of original problem

the new problem is this: What is the largest prime factor of the number 600851475143 ?

even using longs the number is too big. and Math.sqrt() requires a double and the number is too big

here is my solution of original problem

the new problem is this: What is the largest prime factor of the number 600851475143 ?

even using longs the number is too big. and Math.sqrt() requires a double and the number is too big

SCJP

Visit my download page

posted 4 years ago

600851475143 does fit in a long, but if you write it in source code you have to put an "L" at the end of it: long value = 600851475143

If you don't, Java is going to interpret 600851475143 as an

To solve this problem, you should do something smarter than checking if all numbers smaller than 600851475143 are primes. That's the trick of this Project Euler exercise!

**L**;If you don't, Java is going to interpret 600851475143 as an

`int`literal instead of a`long`, and it's going to tell you the number is too big to fit in an`int`.To solve this problem, you should do something smarter than checking if all numbers smaller than 600851475143 are primes. That's the trick of this Project Euler exercise!