Use a function to determine whether a number is prime

method one:

#include<stdio.h>

int isPrime(int x);

int main(void)
{
   int x;
   scanf("%d",&x);
   if( isPrime(x) ){
       printf("%d是素数\n",x);
   }else{
       printf("%d不是素数\n",x);
   }
   
   return 0;
}

int isPrime(int x){
    int ret = 1;
    int i;
    if(x == 1){
        ret = 0;
    }
    for( i=2; i<x; i++){
        if( x % i == 0){
            ret = 0;
            break;
        }
    }
    return ret;
}

Method Two:

 Except for 2, all other even numbers are not prime numbers.

#include<stdio.h>

int isPrime(int x);

int main(void)
{
   int x;
   scanf("%d",&x);
   if( isPrime(x) ){
       printf("%d是素数\n",x);
   }else{
       printf("%d不是素数\n",x);
   }
   
   return 0;
}

int isPrime(int x){
    int ret = 1;
    int i;
    if(x == 1 || (x % 2 == 0 && x != 2)){
        ret = 0;
    }
    for( i = 3; i < x; i += 2){
        if( x % i == 0){
            ret = 0;
            break;
        }
    }
    return ret;
}

Improvement——>The program runs more efficiently 

#include<stdio.h>
#include<math.h>

int isPrime(int x);

int main(void)
{
   int x;
   scanf("%d",&x);
   if( isPrime(x) ){
       printf("%d是素数\n",x);
   }else{
       printf("%d不是素数\n",x);
   }
   
   return 0;
}

int isPrime(int x){
    int ret = 1;
    int i;
    if(x == 1 || (x % 2 == 0 && x != 2)){
        ret = 0;
    }
    for( i = 3; i < sqrt(x); i += 2){
        if( x % i == 0){
            ret = 0;
            break;
        }
    }
    return ret;
}

Method three:

 algorithm:

1. Let x be 2

2. Mark the numbers from 2x, 3x, 4x until ax<n as non-prime numbers

3. Let x be the next number that is not marked as non-prime, repeat 2; until all numbers have been tried.

 pseudocode:

1. Open prime[n] and initialize all its elements to 1. Prime[x] is 1, which means x is a prime number.

2. Let x=2

3. If x is a prime number, then for (i=2;x*i<n;i++) let prime[i*x]=0;

4. Let x++, if x<n, repeat 3, otherwise end

#include<stdio.h>

int main()
{
    const int maxNumber = 25;
    int isPrime[maxNumber];
    int i;
    int x;
    //数组初始化1
    for( i=0; i<maxNumber; i++){
        isPrime[i] = 1;
    }
    
    for( x=2; x<maxNumber; x++){
        if(isPrime[x]){
            for(i=2;i*x<maxNumber;i++){
                isPrime[i*x]=0;
            }
        }
    }
    
    for(i=2;i<maxNumber;i++){
        if(isPrime[i]){
            printf("%d\t",i);
        }
    }
    printf("\n");
    
    return 0;
}