Removing non-prime numbers from a vector - c++

Question

I have a simple function that "primes a list" i.e. it returns the passed vector but without all non-primes. Essentially it removes non-primes from the vector and returns the updated vector. But my function just returns the vector without the numbers 0 and 1. Assume the vector is sorted in ascending order(0, 1, 2, 3, ... ) The basic structure of my function follows:
#include<vector>
#include<algorithms>
#include "cmath.h"
.
.
.
std::vector<int> prime_list(std::vector<int> foo){
int limit_counter = 1;
const int p_limit = sqrt(foo.at(foo.size() - 1));
for(int w = 0; w < foo.size(); w++){
if(foo.at(w) <= 1)
foo.erase(foo.begin() + w);
}
for(int i : foo){
do{
if(i % limit_counter == 0)
foo.erase(std::remove(foo.begin(), foo.end(), i), foo.end());
limit_counter++;
}
while(limit_counter < p_limit);
}
return foo;
}

Answer 1

const int p_limit = sqrt(foo.at(foo.size() - 1));
This will initialize the limit once, based on the last element in the list. You have to do that for each element being test for prime.
More importantly, limit_counter should be initialized for each i
Ignoring the problem with iterators, you can fix it with this pseudo code.
std::vector<int> prime_list(std::vector<int> foo)
{
foo.erase(std::remove(foo.begin(), foo.end(), 0), foo.end());
foo.erase(std::remove(foo.begin(), foo.end(), 1), foo.end());
for (int i : foo)
{
int limit_counter = 1;
const int p_limit = static_cast<int>(sqrt(i));
do
{
if (i % limit_counter == 0)
{
//undefined behavior, this might destroy the iterator for `i`
foo.erase(std::remove(foo.begin(), foo.end(), i), foo.end());
}
limit_counter++;
} while (limit_counter < p_limit);
}
return foo;
}
For an easier and safer solution, just create a new list and add the primes to it. Or, create a duplicate list primes and go through the loop in foo
std::vector<int> prime_list(std::vector<int> &foo)
{
std::vector<int> primes = foo;
primes.erase(std::remove(primes.begin(), primes.end(), 0), primes.end());
primes.erase(std::remove(primes.begin(), primes.end(), 1), primes.end());
for (int n : foo)
for (int k = 2, limit = static_cast<int>(sqrt(n)); k <= limit; k++)
if (n % k == 0)
primes.erase(std::remove(primes.begin(), primes.end(), n), primes.end());
return primes;
}

Answer 2

There are basically 2 possible solutions.
You can either erase none prime values from the std::vector
or copy just prime values to a new std::vector
For both solutions it is necessary to have a function, which detects, if a number is a prime or not. There are tons of such functions in the internet. Please select the best for your requirements.
Then, you need to think, if the erase/remove_if or the copy_if solution is better for you.
With erase/remove_if you are shifting a lot of memory, so, if you use it, then maybe better to start from the end.
On the other hand, copy_if may also invoke memory reallocation and copying, because of push_back and back_inserter actions.
So, sorry to say, it depends . . .
Please see below an example with both solutions.
#include <iostream>
#include <vector>
#include <algorithm>
// Some of the many isPrime functions, available in the internet
// Basically, for this example, it does not matter which
bool isPrime(int number){
if(number < 2) return false;
if(number % 2 == 0) return false;
for(int i=3; (i*i)<=number; i+=2)
if(number % i == 0 ) return false;
return true;
}
// Version with erase/remove idiom
// Yes, no reference for input parameter. Data shall be copied
auto primeVector1(std::vector<int> data) {
// Remove all none primes
data.erase(std::remove_if(data.begin(), data.end(), [](const int i){ return not isPrime(i);}),data.end());
return data;
}
// Version with copying prime data to new vector
auto primeVector2(std::vector<int>& data) {
std::vector<int> result{};
// Copy primes to new vector
std::copy_if(data.begin(), data.end(), std::back_inserter(result), [](const int i){ return isPrime(i);});
return result;
}
// Test/Driver code
int main() {
std::vector test{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23};
for (const int i : primeVector1(test)) std::cout << i << ' '; std::cout << '\n';
for (const int i : primeVector2(test)) std::cout << i << ' '; std::cout << '\n';
}