1. The method
/*
Here isPrime function to test if the entered positive integer
number is prime or not. The related algorithm is the following:
int isPrime(int x){
int i;
int is_prime=1;
for(i = 2; i <= x/2; i++){
if((x % i) == 0)
is_prime = 0;
}
return is_prime;
}
The integer 2 is prime. To test whether an integer "x" is prime, we
calculate its modulo (x,n); n varies from 2 to x/2. Note that the
division of x by the numbers between x/2 and x( x excluded) gives
float numers between 2 and 1, then in this range, we have never
mod(x,n) = 0. It is then reasonable to stop evaluating the
modulo at x/2.
2. Example in C language:
*/
#include
int isPrime(int x){
int i;
int is_prime=1;
for(i = 2; i <= x/2; i++){
if((x % i) == 0)
is_prime = 0;
}
return is_prime;
}
int main()
{
float number;
float decimal_part;
int integer_part;
printf("n Enter a positive integer: -->");
scanf("%f", &number);
integer_part= (int)number;
decimal_part = number - integer_part;
if(number <=0)
printf("n Enter a positive integer! n");
else if(decimal_part !=0)
printf("n Enter an integer! n");
else if(isPrime(integer_part) == 1)
printf("n The entered number %d is prime.n",integer_part);
else
printf("n The entered number %d is not prime.n",integer_part);
return 0;
}
3. Example in Fortran90 language:
LOGICAL FUNCTION is_it_prime(Number)
IMPLICIT NONE
INTEGER :: Number
INTEGER :: Div
IF (Number == 2) THEN
is_it_prime = .TRUE.
ELSE IF (MOD(Number, 2) == 0) THEN
is_it_prime = .FALSE.
ELSE
Div = 3
DO
IF (MOD(Number,Div)==0) EXIT
Div = Div + 2
END DO
is_it_prime = Div == NUMBER
! Divide untill Divisor = Number, to make sure that the integer is prime
END IF
END FUNCTION is_it_prime
! Main program
!-------------
PROGRAM is_prime
IMPLICIT NONE
INTEGER :: Number
LOGICAL :: is_it_prime
!Get an integer:
WRITE(*,*) "n Enter an integer: --> "
READ(*,*) Number
IF (is_it_prime(Number)) THEN
WRITE(*,*) "The number", Number , " is prime. n"
else
WRITE(*,*) "The number", Number , " is not prime. n"
END IF
END PROGRAM is_prime
