Why size variable in case of big values is not correct?

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I need to find the n-th prime number, in this case, I use the Eratosthenes Algorithm, but I have some misunderstanding of size variable why is it giving an error. Prime numbers are growing by some pattern starting from 5, I used that pattern to estimate the size.

C++

<pre>#include <iostream>
#include <vector>
#include <cmath>

long long getNthPrime(int n)
{
    long long size=n+8*(n+2);

    std::vector<char>is_prime(size+1, 't');
    is_prime[0]=is_prime[1]='f';
    int count=0;
    for(long long i=2; i<=size; i++)
    {
        if(is_prime[i]=='t')
        {
            count++;
            if(count==n)
            {
                return i;
            }
            for(long long j=i*i; j<=size; j+=i)
            {
                is_prime[j]='f';
            }
        }
    }

}

int main() {
    int k;
    int n;
    std::cin >> k;
    int arr[k];

    for (int i = 0; i < k; i++) {
        std::cin >> n;
        arr[i] = n;
    }
    for (int i = 0; i < k; i++) {
        std::cout << getNthPrime(arr[i]) << std::endl;
    }

}

What I have tried:

I have tried by debugging the code.

Posted 17-Oct-18 17:42pm

Member 13277493

Updated 17-Oct-18 20:26pm

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Comments

Patrice T 17-Oct-18 23:49pm

and you plan to tell the error message ?

Member 13277493 18-Oct-18 0:04am

no, I just need to print the n-th prime number for each input data

Richard MacCutchan 18-Oct-18 3:49am

What error? Please do not expect people to guess what you see on your screen.

Rick York 18-Oct-18 20:41pm

When finding primes, the standard tactic is to check 2, then 3, and then skip by 2 (not 1!). This is because after checking for factors of 2 there is no need to check any other even numbers. There will still be a few redundant checks but a lot fewer than if you check the evens also.

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OriginalGriff · Accepted Answer · 2018-10-17T20:26:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!