C++数组

数组倒序

#include <iostream>
using namespace std;
#define  NUM 10
int a[NUM];
int main()
{
    for(int i = 0; i < NUM; ++i)
       cin >> a[i];
    for(int k = NUM - 1; k >= 0; --k)
       cout << a[k] << " "; //下标问题难掌握 
    return 0;
 }

筛法求素数

#include <iostream>
using namespace std;
int main()
{
    int Max_Num;
    cin >> Max_Num;
    bool isPrime[Max_Num+10];
    for(int i = 2; i <= Max_Num; ++i)   
        isPrime[i] = true;
    for(int k = 2; k <= Max_Num; ++k)
       {
               if(isPrime[k])              //检验该数是否被筛过 
               for(int m = 2 * k; m <= Max_Num; m = m + k)   //不要用乘法，保持下标简洁 
                  isPrime[m] = false; 
       }
    int flag = 0;
    for(int i = 2; i <= Max_Num; ++i)       //i与前几次循环不同，逐一增加 
       {
           if(isPrime[i]) 
           {
               printf("%2d ", i);
               flag++;
               if(flag == 5) 
               {
                   flag = 0;
                   printf("\n");
               }
           }
       }
    return 0;
}

数组取代分支结构

#include <iostream>
#include <string.h>
using namespace std;
int main()
{
    string weekdays[] = {"Monday", "Tuesday", "Wednesday", "Thursday"
    ,"Friday", "Saturday", "Sunday"};
    int n;
    cin >> n;
    if((n > 0) && (n <= 7))
    cout << weekdays[n-1];
    else
    cout << "Illegal";
    return 0;
  }

数组取代分支结构

#include <iostream>
#include <string.h>
using namespace std;
int main()
{
    string weekdays[] = {"Monday", "Tuesday", "Wednesday", "Thursday"
    ,"Friday", "Saturday", "Sunday"};
    int n;
    cin >> n;
    if((n > 0) && (n <= 7))
    cout << weekdays[n-1];
    else
    cout << "Illegal";
    return 0;
  }

判断2012.1.22（星期天）后星期几

#include <iostream>
using namespace std;
int monthdays[13] = {-1,31,28,31,30,31,30,31,31,30,31,30,31};
//巧妙绕过monthdays[0],避免被下标弄晕
int main()
{
    int year, month, day;
    int d = 0;
    cin >> year >> month >> day;//2012.1.1（星期天） 
    for(int y = 2012; y < year; y++)
    {
        if((y % 4 == 0 && y % 100 != 0 )||(y % 400) == 0 )
           d += 366;
        else
           d += 365;

    }
    if((year % 4 == 0 && year % 100 != 0 )||(year % 400) == 0 )
        monthdays[2] = 29;
    for(int m = 1; m < month; m++)
    d += monthdays[m];
    d = d + day;
    d = d % 7;
    cout << d - 1<< endl; 
    return 0;
 }

矩阵的乘法

#include <iostream>
using namespace std;
//开头定义好三个矩阵
#define ROWS 8
#define COLS 8
int a[ROWS][COLS];
int b[ROWS][COLS];
int c[ROWS][COLS];
int main()
{
     int i, j, k;
     int m, n;
     cin >> m >> n;

     for(i = 0; i < m; i++)
         for(j = 0; j < n; j++)
             cin >> a[i][j] ;
     int p, q;
     cin >> p >> q;

     for(i = 0; i < p; i++)
         for(j = 0; j < q; j++)
             cin >> b[i][j];

    int sum = 0;
    for(i = 0; i < m; i++)
    {
        for(j = 0; j < q; j++)
        {
            for(sum = 0, k = 0; k < n; k++)   //sum变量每次归0 
                sum += a[i][k] * b[k][j];
                c[i][j] = sum;
        }
    }                       //多层，套好括号 
    for(i = 0; i < m; i++)
    {
        for(j = 0; j < q; j++)
            {
                cout << c[i][j] << " "; 
            }
        cout << endl;
    }                           //括号的使用 
    return 0;                    
}

#许愿#