C language programming - classic process control program

Classic examples of program flow control

1. C language determines and finds prime numbers (prime numbers can only be divisible by 1 and itself)

Method 1: Determine whether n is divisible by 1~n-1

#include <stdio.h>

int main()
{
   
    
    
    int i, n;
    printf("please input a number:"); 
    scanf("%d", &n);
    for (i = 2; i < n ; i++)
    {
   
    
    
        if (n%i == 0)
            break;
    }
    if (n <=1 ) 
    printf("This is not a prime\n");//n<=1的数当然是非素数
    else  if (i < n) 
    printf("This is not a prime.\n"); 
    else 
    printf("This is a prime.\n");
    return 0;
 
}

Find all prime numbers between 100 and 200

#include<stdio.h>
int main() {
   
    
    
	int i, j, k;
	for (i = 101; i < 201;i++) 
	{
   
    
    
		k = 1;
		for (j = 2; j <i;j++)
		{
   
    
    
			if (i%j == 0) 
			{
   
    
    
				k = 0;
				break;
			}
		}
		if (k == 1)
			printf("%d \n", i);
	}
		
		
	return 0;
}

Method 2: Determine whether n is divisible by an integer between 2 and √n

#include<stdio.h>
#include<math.h>
int main()
{
   
    
    
    int n,i;
    double k;
    printf("please input a number:"); 
    scanf("%d", &n);
    k = sqrt(n);
    for (i = 2; i <= k;i++)
    {
   
    
    
        if (n%i