### 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;
long max_seconds = 30;
long i           = 2; // Start at 2
long found       = 0; // Number of primtes we've found

void setup() {
Serial.begin(57600);

while (!Serial) { }

start = millis();
}

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

if (prime == 1) {
Serial.print(i);
Serial.println(" is prime ");

found++;
}

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

if (max_seconds > 0 && (running_seconds >= max_seconds)) {
Serial.print("Found ");
Serial.print(found);
Serial.print(" primes in ");
Serial.print(max_seconds);
Serial.println(" seconds");
delay(60000);

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
}``````