查找数字数组中缺少的元素

Question

I have array with numbers 1...31 (days at month) On my side I would like to find all missed numbers. This my solution but I not sure that is a good way.

var result = new List<int>();
int[] days = GetSelectedDays(); //I recive array with 1..31 with some missing elements sometimes (without "5" as example it depends if user has selected this number)
for (int i=0; i <30; i++)
{
  if (!days.Contains(i))
  result.Add(i);
}

Answer 1

您可以使用LINQ Except ：

var result = Enumerable.Range(1, 31).Except(days);

Answer 2

Looks like you are currently checking numbers in the range 0..29 as opposed to 1..31. The only suggestion I have is to change the for loop to:

for (int i=1; i <= 31; i++)

Answer 3

First off, when you're adding days of a month its probably easier to loop through a loop of DateTimes, ie

var missedDateDictionary = new Dictionary<DateTime, bool>(); // or other value
var date = new DateTime(2016, 01, 01);

for (int i = 0 ; i < 32 ; i++)
{
    var newDate = date.AddDays(i);

    //do something with adding / not adding date

    if (notDoneSomething)
        missedDateDictionary.Add(newDate, true)
}

The premise here is that each DateTime you're adding is unique.

If you cant guarantee that, your solution is pretty efficient but could use some optimization.

1. Initialize arary with max value: int[31] days = GetSelectedDays();

2. A month can have 31 days: for (int i = 1; i <32; i++)

3. If at all possible, fill result in the loop you're filling GetSelecteDays()