c# - 按索引返回数字的方法

Question

我想编写一个方法，在不使用时按索引(place)返回数字(num)的数字 仅数组循环和条件。

例如:

1. 对于输入 num = 17489、place = 1，该方法返回 9
2. 但是如果索引不存在，如num = 17489，place = 6返回-1

如果我设置 i = 0，我的代码中的某些内容不起作用 情况 1) 有效，但 2) 不行如果我设置 i = 1 情况 2) 有效，但 1) 不行请帮忙找出问题所在

    public static int Digit_by_place(int num, int place)
    {
        int digit = 0, i = 0;

        // the lop run until the place in the 
        while (num > 0 && i <= place)number
        {
            digit = num % 10;
            num = num / 10;
            i++;

            // if there is no didgit in the input place return -1
            if (i < place && num == 0)
            {
                return -1;
            }

            // return the last digit from left (the bigest)
            if (num == 0 && digit > 0 && i == place)
            {
                return digit;
            }
        }

        return digit;
    }

Answer 1

我们可以利用 int 最多可以有 10 位数字这一事实。

代码:

public static int Digit_by_place(int num, int place) {
  if (place <= 0 || place > 10)   // place must be in [1..10] range
    return -1;
  else if (num == 0)              // special case num == 0
    return place == 1 ? 0 : -1;

  for (int i = 1; i < place; ++i)  
    num /= 10;

  return num == 0 ? -1 : Math.Abs(num % 10);
}

演示:

using System.Linq;
...

Tuple<int, int>[] tests = new Tuple<int, int>[] {
  Tuple.Create( 17489,  1),
  Tuple.Create( 17489,  2),
  Tuple.Create( 17489,  3),
  Tuple.Create( 17489,  4),
  Tuple.Create( 17489,  5),
  Tuple.Create( 17489,  6),
  Tuple.Create(-17489,  1),
  Tuple.Create(-17489,  6),
  Tuple.Create( 17489,  0),
  Tuple.Create( 17489, -1),
  Tuple.Create(-17489, -1),
  Tuple.Create( 17489,  5),
  Tuple.Create(-17489,  5),
  Tuple.Create(     0,  1),
  Tuple.Create(     0,  4),
};

var report = string.Join(Environment.NewLine, tests
  .Select(test => 
     $"Digit_by_place({test.Item1,6}, {test.Item2,2}) = {Digit_by_place(test.Item1, test.Item2),2}"));

Console.Write(report);

结果:

Digit_by_place( 17489,  1) =  9
Digit_by_place( 17489,  2) =  8
Digit_by_place( 17489,  3) =  4
Digit_by_place( 17489,  4) =  7
Digit_by_place( 17489,  5) =  1
Digit_by_place( 17489,  6) = -1
Digit_by_place(-17489,  1) =  9
Digit_by_place(-17489,  6) = -1
Digit_by_place( 17489,  0) = -1
Digit_by_place( 17489, -1) = -1
Digit_by_place(-17489, -1) = -1
Digit_by_place( 17489,  5) =  1
Digit_by_place(-17489,  5) =  1
Digit_by_place(     0,  1) =  0
Digit_by_place(     0,  4) = -1