“索引超出范围异常”错误

Question

I'm trying to find the first missing positive integers in an array and i keep on getting this error message. I'm quite new to programming and i can't figure out the reason for this. Can anyone help me find the problem? Also, when the input is [1], it also throws an exception.

This is my code:

public static int solution(int[] A)
{
    Array.Sort(A);
    List<int> missing = new List<int>();
    List<int> An = new List<int>();

    foreach (var r in A)
    {
        if (r > 0)
        {
            An.Add(r);
        }

    }
    int lengthList = (An[An.Count]) - (An[0]);

    for (int i = An[0]; i <= lengthList - 1; i++)
    {
        if (An[i + 1] != An[i] + 1)
        {
            missing.Add(An[i] + 1);
            An.Insert(i + 1, An[i] + 1);
        }
    }
    if (missing != null)
    {
        return missing[0];
    }
    else
        return 0;
}

Answer 1

You have several flaws in your code. The most important is

 int lengthList = (An[An.Count]) - (An[0]);

What should should that be? The length of An - the number of items in An you can iterate over - is given by An.Count .

And since lists are 0-indexed, your index i should walk from 0 ( not An[0] ) to An.Count-1 . So your code should look something like this:

for (int i = 0; i < An.Count; i++) {
    if (An[i + 1] != An[i] + 1) {
       missing.Add(An[i] + 1);
       An.Insert(i + 1, An[i] + 1); // this will break your algorithm
}

i<An.Count avoids accessing the index after the last item.

Note that this only fixes this IndexOutOfRangeException . I don't know if the algorithm does what you want at all. You insert elements to An while iterating over it (line An.Insert... ). This seems wrong to me.

Answer 2

You are having a off-by-one error:

In your for loop you use this exit condition:

i <= lengthList - 1

ON a list with for example 10 items that would iterate until 9.

Then you access the array like this:

An[i + 1]

In the given example you would access An[10] which does not exist in a 10-item array/list.

Your correct exit statement for the loop is:

i < lengthList - 1

Yur calculation of lengthList seems to be also wrong. Change it to:

int lengthList = An.Count;

Your initial code

int lengthList = An[An.Count] - An[0];

...gives you also an ArgumentOutOfRangeException . It also makes no sense. You are accessing the content of the list, not its length .

Sidenote:

Your inital loop condition

int i = An[0]

...is probably also wrong. Did you mean this?

int i = 0

Answer 3

your problem code:

1) (An[An.Count]) fix that: An[An.Count-1]

2) int lengthList = (An[An.Count]) - (An[0]); fix that: int lengthList = An.Count;