Arduino calculate prime numbers2010-05-03

I wrote some code to stress test my Arduino. The code below will make your Arduino calculate prime numbers.

``````long start       = 0;
int  max_seconds = 30;
long i           = 2; // Start at 2
long found       = 0; // Number of primes we've found

void setup() {
Serial.begin(115200);

while (!Serial) { }

Serial.println("");
Serial.print("Running prime benchmark for ");
Serial.print(max_seconds);
Serial.println(" seconds...");

start = millis();
}

void loop() {
int prime = is_prime(i); // Check if the number we're on is prime

if (prime == 1) {
found++;

if (found % 2000 == 0) {
Serial.print(".");
}

if (found % 50000 == 0) {
Serial.print("\n");
}
}

int running_seconds = (millis() - start) / 1000;

if (max_seconds > 0 && (running_seconds >= max_seconds)) {
Serial.print("\nFound ");
Serial.print(found);
Serial.print(" primes in ");
Serial.print(max_seconds);
Serial.println(" seconds");
delay(10000);

i     = 2;
found = 0;
start = millis();
}

i++;
}

int is_prime(long num) {
// Only have to check for divisible for the sqrt(number)
int upper = sqrt(num);

// Check if the number is evenly divisible (start at 2 going up)
for (long cnum = 2; cnum <= upper; cnum++) {
long mod = num % cnum; // Remainder

if (mod == 0) {
return 0;
} // If the remainer is 0 it's evenly divisible
}

return 1; // If you get this far it's prime
}
``````
Replies
random 2022-02-28 11:43am - No Email - Logged IP: 2003:e0:71b:8800:758c:cfb0:6762:1ca5

Should be 3 times faster

``````
uint32_t start       = 1;
uint32_t  max_seconds = 10;
uint32_t i           = 11; // Start at 11
uint16_t found       = 4; // Number of primes we've found, start with 4 as there are 4 primes prior 11

void setup() {
Serial.begin(115200);

while (!Serial) { }

Serial.println("");
Serial.print("Prime benchmark for ");
Serial.print(max_seconds);
Serial.println(" seconds...");

start = millis();
}

void loop() {

is_prime(); // to cancel out all numbers modulo 2 or 3 = 0
i+=2;       // Odd + 2 = odd
is_prime();
i+=4;       //9%3=0 so 11+2+2 would be %3 = 0 so we add here 4 instead of 2
//we could filter out other Prime dividables as 5,7,... but this woud be a mess here

if (millis() >= (start + ((max_seconds)*1000))) {
Serial.print("nFound ");
Serial.print(found);
Serial.print(" primes in ");
Serial.print(max_seconds);
Serial.println(" seconds");
Serial.print(" searched to  ");
Serial.print(i-4);
delay(5000);

i     = 11;
found = 4;
start = millis();
}

}

boolean is_prime() { // could be void, but my esp did not like it so here with boolean
// Only have to check for divisible for the sqrt(number)
uint32_t upper = sqrt(i);

// Check if the number is evenly divisible (start at 2 going up)
for (uint32_t cnum = 5; cnum ``````