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Diffstat (limited to 'C_C++/IsPrime.c')
| -rw-r--r-- | C_C++/IsPrime.c | 49 |
1 files changed, 0 insertions, 49 deletions
diff --git a/C_C++/IsPrime.c b/C_C++/IsPrime.c deleted file mode 100644 index c7c7630..0000000 --- a/C_C++/IsPrime.c +++ /dev/null @@ -1,49 +0,0 @@ -#include <stdio.h> -#include <stdbool.h> -#include <math.h> - -bool isPrime(unsigned long num, unsigned long *divisible) { - // The formula 6n +- 1, explained in the comment below, only finds primes above 3 - if (num <= 1) return false; - if (num <= 3) return true; - - if (num % 2 == 0) { - *divisible = 2; - return false; - } - if (num % 3 == 0) { - *divisible = 3; - return false; - } - - // All prime numbers follow the formula 6n +- 1 (where n >= 1). All composite numbers are made up of prime numbers, so this way, we skip iterating through a lot of nums. - // The biggest number we need to check is the square root of num. Otherwise, we're gonna have situations like 5 * 20 and 20 * 5. - // Source (didn't look at example code): https://en.wikipedia.org/wiki/Primality_test#Simple_methods - - // It must be n - 1 <= max, and not n + 1 <= max, because there are some cases, where n + 1 > sqrt(num) but n - 1 == sqrt(num) - // For example: 289, 323, 2209, 10201, 22201 - for (unsigned long n = 6, max = sqrt(num); n - 1 <= max; n += 6) { - if (num % (n - 1) == 0) { - *divisible = n - 1; - return false; - } - if (num % (n + 1) == 0) { - *divisible = n + 1; - return false; - } - } - return true; -} - -int main() { - char numberInput[20]; - printf("Natural number, up to 2^64-1: "); - fgets(numberInput, 20, stdin); - - unsigned long number = 0, divisible = -1; - sscanf(numberInput, "%lu", &number); - if (isPrime(number, &divisible)) - printf("Your number is prime!"); - else - printf("Your number is not prime! It is divisible by %ld!\n", divisible); -} |
