用筛法求之N内的素数。
#include<stdio.h>
#include<math.h>
int main()
{
    int j,i,flag,N;
    double k;
    scanf("%d",&N);
    for(i=2;i<=N;i++)
    {
        k=sqrt(i);
        j=2;
        flag=0;
        while(i<=k&&!flag)
            if(i%j==0)
                flag=1;
            else
                j++;
            if(!flag)
            {
                printf("%d\n",i);
            }
    }
    printf("\n");
    return 0;
}


输入一行字符，分别统计出其中英文字母、空格、数字和其它字符的个数。
#include<stdio.h>  
int main()  
{  
    char ch;
    int a=0,b=0,c=0,d=0;
    while((ch=getchar())!='\n')
    {
      if(ch>='a'&&ch<='z')
      {
        a++;
      }
      else if(ch==' ')
      {
         b++;
      }
      else if(ch>='0'&&ch<='9')
      {
        c++;
      }
      else
          d++;
    }
    printf("%d\n%d\n%d\n%d\n",a,c,b,d);
    return 0;  
}  

#include<stdio.h>
#include<math.h>
int main()
{
    int j, i, flag, N;
    double k;
    scanf("%d", &N);
    for (i = 2; i <= N; i++)
    {
        k = sqrt(i);
        j = 2;
        flag = 0;
        while (j <= k&&!flag)
        {
            if (i%j == 0)
            {
                flag = 1;
                break;
            }
            j++;
        }
        if (!flag)
        {
            printf("%d\n", i);
        }
    }
    printf("\n");
    return 0;
}

#include<stdio.h>  
int main()
{
    char ch;
    int a = 0, b = 0, c = 0, d = 0;
    while ((ch = getchar()) != '\n')
    {
        if ((ch >= 'a'&&ch <= 'z') || (ch >= 'A'&&ch <= 'Z'))
        {
            a++;
        }
        else if (ch == ' ')
        {
            b++;
        }
        else if (ch >= '0'&&ch <= '9')
        {
            c++;
        }
        else
            d++;
    }
    printf("%d\n%d\n%d\n%d\n", a, c, b, d);
    return 0;
}

我明白了！谢谢你啦