在范围内出现的数字的计数数

Question

输入左限号、右限号和要计数的数字。此代码仅适用于整数，即从0到n。此代码返回在给定范围内出现的数字n的数目。请检查此代码并提出改进建议。

#include <iostream>

int count_n(int l, int r, int n)
{
    if (r <= l)
    {
        std::cerr << "Right limit must greater then left limit\n";
        return 0;
    }

    if (l < 0)
    {
        std::cerr << "Only positive integers\n";
        return 0;
    }

    int count = 0;
    for (int i = l; i <= r; ++i)
    {
        int i_copy = i;
        while (i_copy != 0)
        {
            if (i_copy % 10  == n)
            {
                count++;
            }
            i_copy = i_copy / 10;
        }
    }
    return count;
}

int main()
{
    int left_limit, right_limit, num;
    std::cout << "Enter left limit, right limit and the number(Only positive numbers)\n";
    std::cin >> left_limit >> right_limit >> num;

    int result = count_n(left_limit, right_limit, num);
    std::cout << result << "\n";
}

输出：

Enter left limit, right limit and the number(Only for positive numbers)
1 20 0
2

Answer 1

除了这之外，您还应该指定n ( digit )的验证。另外，将验证分组为单独的功能可能更好。

void check_input(int min_left, int max_right, int digit) {
  if (max_right <= min_left)
    throw std::invalid_argument("right is <= left, expected right > left");
  if (min_left < 0)
    throw std::invalid_argument("left < 0, expected left > 0");
  if (digit <= 0)
    throw std::invalid_argument("digit <= 0");
    //throw std::invalid_argument("number <= 0");  
  if (digit > 9)
    throw std::invalid_argument("digit > 9");

  ////number:
  //if (digit > max_right)
  //  throw std::invalid_argument("number > right limit");
}

int count_digit_entries(int min_left, int max_right, int digit) {
//...
}

int main() {
  int left_limit, right_limit, num;
  std::cout << "Enter left limit, right limit and the number(Only positive numbers)\n";
  std::cin >> left_limit >> right_limit >> num;

  check_input(left_limit, right_limit, num);

  int result = count_digit_entries(left_limit, right_limit, num);

  std::cout << "\nTotal: " << result << "\n";
}

你可能不仅想要找到数字，还想找到一个完整的数字。

//...


#define DEBUG
#ifdef  DEBUG
#define debug(x) x
#else 
#define debug(x)
#endif


int count_digit_entries(int min_left, int max_right, int search_number) {
  int count = 0;

  int mod = 10;
  while(search_number / mod > 0) mod = mod * 10; // 279 -> 1000

  for (int current_number = min_left, right = max_right; current_number < right; ++current_number) {
    int copied_current_number = current_number; // 542793

    while (copied_current_number >= search_number) {
        // 542793 - 279 % 1000    >0 
        // 54279 - 279 % 1000     =0

      if ( ( (copied_current_number - search_number) % mod ) == 0 ) { 
        debug(std::cout << current_number << " ");
        ++count;
      };
      copied_current_number = copied_current_number / 10; 
      // 542793 -> 54279

    };
  };

  return count;
};

输出：

Enter left limit, right limit and the number(Only positive numbers)
0 1000 25
25 125 225 250 251 252 253 254 255 256 257 258 259 325 425 525 625 725 825 925 
Total: 20