簡單判斷某個數是否為素數

void main()
{  int n, j;
   scanf("%d",&n);
   for(j =2; j < n; j++) //for(j =2; j <= n-1; j++)
	   if(n%j == 0)
		   break;
   if(j >= n)
	   printf("%5d是素數\n", n);
   else
	   printf("%5d不是素數\n", n);
}

計算3-31內的素數

int fun(int  n)
{  int  i,j, count = 0;
   printf(
 
"\n3到%d的素數\n", n);
   for (i = 3; i <= n; i++) 
   {
      for (j = 2; j < i; j++)
         if (i%j == 0)
            break;
      if (j >= i)
      {  
		  count++; 
	      printf( count%15?  "%5d":"\n%5d",i);  
	  }
   }
   return  count;
}

void main()
{  
   int  n = 31, r;
   r = fun(n)
 
;
   printf("\n個數:%d\n", r);
}

在指定範圍內查詢素數

int fun(int m, int n)
{  
    int count=0, sum=0, j;
	for(m; m <= n; m++)
	{
		int temp = sqrt((float)m);
		for(j = 2; j <= temp; j++)
			if(m%j == 0)
				break;
		if(j > temp) //此處不能使用>=
		{ 
			if(m==1)
				count=count;	
			else
			{
				sum+=m;
 

				count++;
				printf("%5d", m);
			}			
		}
	}
	printf("\n和:%5d", sum);
	return count;
}

void main()
{  
   int m,n;
   scanf("%d %d",&m,&n);
   int count = fun(m, n);
   printf("  個數:%d\n", count);
}