<< problem 6 - Sum square difference | Largest product in a series - problem 8 >> |
Problem 7: 10001st prime
(see projecteuler.net/problem=7)
By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
What is the 10001st prime number?
My Algorithm
A prime number is an integer number p>=2 that can only be divided by 1 and by itself (p).
2 is the smallest prime number and the only even prime number, too (all other prime numbers are odd).
Each number x can be split into its prime factors, that means we check for all primes p<x whether x mod p == 0.
If that test fails for all those primes, then x is a prime number and can be added to our std::vector
.
Alternative Approaches
Take a look at my toolbox for other prime sieves or even precomputed lookup tables.
Wikipedia lists a few faster algorithms (en.wikipedia.org/wiki/Prime_number), too.
On my website create.stephan-brumme.com/eratosthenes/ you can find parallelized code that computes
all 50847534 prime numbers below 1 billion in less than a second.
Note
Actually we can abort the loop if p>=sqrt{x} (which is p^2>=x) to speed up the program.
And since all primes are odd - except for 2 - I simply add 2 to the list of primes and then scan
only odd numbers, beginning with 3 (and increment 2).
Interactive test
You can submit your own input to my program and it will be instantly processed at my server:
This is equivalent toecho "1 6" | ./7
Output:
Note: the original problem's input 10001
cannot be entered
because just copying results is a soft skill reserved for idiots.
(this interactive test is still under development, computations will be aborted after one second)
My code
… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too.
#include <iostream>
#include <vector>
int main()
{
// compute the first 10001 primes
std::vector<unsigned int> primes;
primes.reserve(10001);
primes.push_back(2);
for (unsigned int x = 3; primes.size() <= 10000; x += 2)
{
bool isPrime = true;
for (auto p : primes)
{
// found a divisor ? => abort
if (x % p == 0)
{
isPrime = false;
break;
}
// no larger prime factors possible ?
if (p*p > x)
break;
}
// yes, we have a new prime
if (isPrime)
primes.push_back(x);
}
// processing all test cases is now just a plain lookup
unsigned int tests;
std::cin >> tests;
while (tests--)
{
unsigned int x;
std::cin >> x;
// just look up the x-th prime
// with a little twist: vector's index is zero-based, therefore "off by one"
x--;
if (x < primes.size())
std::cout << primes[x] << std::endl;
else
std::cout << "ERROR" << std::endl;
}
return 0;
}
In order to run my code, executeecho "1 10001" | ./euler-0007
(input format usually follows Hackerrank's requirements)
This solution contains 5 empty lines, 7 comments and 2 preprocessor commands.
Benchmark
The correct solution to the original Project Euler problem was found in less than 0.01 seconds on an Intel® Core™ i7-2600K CPU @ 3.40GHz.
(compiled for x86_64 / Linux, GCC flags: -O3 -march=native -fno-exceptions -fno-rtti -std=gnu++11 -DORIGINAL
)
See here for a comparison of all solutions.
Note: interactive tests run on a weaker (=slower) computer. Some interactive tests are compiled without -DORIGINAL
.
Changelog
February 23, 2017 submitted solution
March 28, 2017 added comments
Hackerrank
see https://www.hackerrank.com/contests/projecteuler/challenges/euler007
My code solves 5 out of 5 test cases (score: 100%)
Difficulty
Project Euler ranks this problem at 5% (out of 100%).
Hackerrank describes this problem as easy.
Note:
Hackerrank has strict execution time limits (typically 2 seconds for C++ code) and often a much wider input range than the original problem.
In my opinion, Hackerrank's modified problems are usually a lot harder to solve. As a rule thumb: brute-force is rarely an option.
Similar problems at Project Euler
Problem 10: Summation of primes
Note: I'm not even close to solving all problems at Project Euler. Chances are that similar problems do exist and I just haven't looked at them.
Links
projecteuler.net/thread=7 - the best forum on the subject (note: you have to submit the correct solution first)
Code in various languages:
C# www.mathblog.dk/project-euler-problem-7/ (written by Kristian Edlund)
C github.com/eagletmt/project-euler-c/blob/master/1-9/problem7.c (written by eagletmt)
Java github.com/nayuki/Project-Euler-solutions/blob/master/java/p007.java (written by Nayuki)
Javascript github.com/dsernst/ProjectEuler/blob/master/7 10001st prime.js (written by David Ernst)
Go github.com/frrad/project-euler/blob/master/golang/Problem007.go (written by Frederick Robinson)
Mathematica github.com/nayuki/Project-Euler-solutions/blob/master/mathematica/p007.mathematica (written by Nayuki)
Haskell github.com/nayuki/Project-Euler-solutions/blob/master/haskell/p007.hs (written by Nayuki)
Scala github.com/samskivert/euler-scala/blob/master/Euler007.scala (written by Michael Bayne)
Perl github.com/gustafe/projecteuler/blob/master/007_010_Two-problems-concerning-primes.pl (written by Gustaf Erikson)
Those links are just an unordered selection of source code I found with a semi-automatic search script on Google/Bing/GitHub/whatever.
You will probably stumble upon better solutions when searching on your own.
Maybe not all linked resources produce the correct result and/or exceed time/memory limits.
Heatmap
Please click on a problem's number to open my solution to that problem:
green | solutions solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too | |
yellow | solutions score less than 100% at Hackerrank (but still solve the original problem easily) | |
gray | problems are already solved but I haven't published my solution yet | |
blue | solutions are relevant for Project Euler only: there wasn't a Hackerrank version of it (at the time I solved it) or it differed too much | |
orange | problems are solved but exceed the time limit of one minute or the memory limit of 256 MByte | |
red | problems are not solved yet but I wrote a simulation to approximate the result or verified at least the given example - usually I sketched a few ideas, too | |
black | problems are solved but access to the solution is blocked for a few days until the next problem is published | |
[new] | the flashing problem is the one I solved most recently |
I stopped working on Project Euler problems around the time they released 617.
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I scored 13526 points (out of 15700 possible points, top rank was 17 out of ≈60000 in August 2017) at Hackerrank's Project Euler+.
My username at Project Euler is stephanbrumme while it's stbrumme at Hackerrank.
Look at my progress and performance pages to get more details.
Copyright
I hope you enjoy my code and learn something - or give me feedback how I can improve my solutions.
All of my solutions can be used for any purpose and I am in no way liable for any damages caused.
You can even remove my name and claim it's yours. But then you shall burn in hell.
The problems and most of the problems' images were created by Project Euler.
Thanks for all their endless effort !!!
<< problem 6 - Sum square difference | Largest product in a series - problem 8 >> |