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Improved sieve_of_eratosthenes.cpp (#933)

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* Update sieve_of_eratosthenes.cpp

* removed unwanted spaces

* removed init function

Co-authored-by: Krishna Vedala <7001608+kvedala@users.noreply.github.com>
This commit is contained in:
Rajiv Ranjan Singh 2020-07-10 20:32:59 +05:30 committed by GitHub
parent bf2dac6de2
commit ebd13a7e24
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1 changed files with 21 additions and 28 deletions
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49
math/sieve_of_eratosthenes.cpp
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@ -10,25 +10,21 @@
* @see primes_up_to_billion.cpp prime_numbers.cpp * @see primes_up_to_billion.cpp prime_numbers.cpp
*/ */
#include <iostream> #include <iostream> // for io operations
/** Maximum number of primes */
#define MAX 10000000
/** array to store the primes */
bool isprime[MAX];
/** /**
* This is the function that finds the primes and eliminates * This is the function that finds the primes and eliminates
* the multiples. * the multiples.
* @param N number of primes to check
* @param [out] isprime a boolean array of size `N` identifying if `i`^th number is prime or not
*/ */
void sieve(uint32_t N) { void sieve(uint32_t N, bool *isprime) {
isprime[0] = false; isprime[0] = true;
isprime[1] = false; isprime[1] = true;
for (uint32_t i = 2; i <= N; i++) { for (uint32_t i = 2; i * i <= N; i++) {
if (isprime[i]) { if (!isprime[i]) {
for (uint32_t j = (i << 1); j <= N; j += i) { for (uint32_t j = (i << 1); j <= N; j = j + i) {
isprime[j] = false; isprime[j] = true;
} }
} }
} }
@ -36,10 +32,12 @@ void sieve(uint32_t N) {
/** /**
* This function prints out the primes to STDOUT * This function prints out the primes to STDOUT
* @param N number of primes to check
* @param [in] isprime a boolean array of size `N` identifying if `i`^th number is prime or not
*/ */
void print(uint32_t N) { void print(uint32_t N, const bool *isprime) {
for (uint32_t i = 1; i <= N; i++) { for (uint32_t i = 2; i <= N; i++) {
if (isprime[i]) { if (!isprime[i]) {
std::cout << i << ' '; std::cout << i << ' ';
} }
} }
@ -47,19 +45,14 @@ void print(uint32_t N) {
} }
/** /**
* Initialize the array * Main function
*/ */
void init() {
for (uint32_t i = 1; i < MAX; i++) {
isprime[i] = true;
}
}
/** main function */
int main() { int main() {
uint32_t N = 100; uint32_t N = 100;
init(); bool *isprime = new bool[N];
sieve(N); sieve(N, isprime);
print(N); print(N, isprime);
delete[] isprime;
return 0; return 0;
} }