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# How many prime Exist between two numbers?

Greenhorn
Posts: 10
I have to count, how many prime numbers exist, but it give me wrong answer: Exactly 1254 prime numbers exist between 1 and 10,000.

can you tell me why? Thank you!

Result:

Exactly 1254 prime numbers exist between 1 and 10,000.

Winston Gutkowski
Bartender
Posts: 10527
64
Niloo Rad wrote:something is wrong with my code and i can't figure it out! will you help me to find it?

Well, you haven't given us all of it. Where is MAX_PRIME defined? Without that and your isPrime() method, it's a bit tough to work out exactly what's wrong.

However, there is one simple optimization: you DON'T have to check every number.
In fact, (tip) ALL primes greater than 3 are equal to 6k ± 1, where k is a positive integer.

Winston

Greenhorn
Posts: 10
JUST UPDATED MY CODE

Campbell Ritchie
Sheriff
Posts: 50225
79
Niloo Rad wrote:JUST UPDATED MY CODE

Don’t That makes Winston’s reply look nonsense. It means nobody can work out the changes. Please repost the original code and a new post with the changes. And read this.

Roger Wolfenden
Greenhorn
Posts: 20

[complete solution removed]

Winston Gutkowski
Bartender
Posts: 10527
64
Niloo Rad wrote:JUST UPDATED MY CODE

And I'm afraid it's still wrong. What does isPrime(49), or indeed isPrime(9), return?

Winston

Roger Wolfenden
Greenhorn
Posts: 20
@Winston would you like to elaborate on what is wrong with the code?
• isPrime(9) = 29
• isPrime(49) = 229

• Interesting too, I notice that my code has been removed by @Winston. I'm just wondering why. Have I broken some code of etiquette?

[Edit by Winston] See below.

Winston Gutkowski
Bartender
Posts: 10527
64

Hi Roger.

I'm sure your motives were great, but we don't like people to post ready-made solutions here - particularly in the 'Beginners' forum.
It's much better to guide someone to their own solution. Please read the NotACodeMill (←click) page.

I've kept your original post, and if my colleagues ask me to, I'll restore it.

Thanks.

Winston

Winston Gutkowski
Bartender
Posts: 10527
64
Roger Wolfenden wrote:@Winston would you like to elaborate on what is wrong with the code?
• isPrime(9) = 29
• isPrime(49) = 229

• I was referring to Niloo's code, not yours, and specifically to his isPrime() method.

Winston

Roger Wolfenden
Greenhorn
Posts: 20
OK @Winston understand and accept your logic.

FYI, I too am a newbie but quite liked the, "Let's find all the primes between 0 and 1000 challenge." Hence my code.

Winston Gutkowski
Bartender
Posts: 10527
64
Roger Wolfenden wrote:OK @Winston understand and accept your logic.

No probs. And please, feel free to help. We just don't like beginners to be spoon-fed.

Winston

Wendy Gibbons
Bartender
Posts: 1111
so you pass in 3 numbers assign them to variables with very bad names a,b,c so we have no idea what you intended to do with them, you then do nothing with them.

the controlling loop in countPrimes always goes from 2 to MAX_PRIME.

this is just a nice layout thing...
you do not need to indent the validation try catch blocks within each other

Campbell Ritchie
Sheriff
Posts: 50225
79
This sounds like a project Euler question. Is it?
I would suggest you create an array and use a sieve of Eratosthenes to work out whether each number is prime. Then count the primes. 0 and 1 don’t count. Much more effecient than an isPrime method.