problem with index when using List<T>

This code simumates matches between 3 teams. each team plays against the two others. Winning gives 3 points, equal gives 1 point to each team. I did the same program with arrays and there i had no error, but using List<>T, I always get an error at line

points[i] += 3; or points[j] += 3; the index is out of range or cannot be negative ...

Thanks for help.

using System;
using System.Collections.Generic;
using System.Linq;
class Oef16
{
public static void Main()
{
int match;
Random rand = new Random();
List teams = new List
{ "team A", "team B", "team C"};

List points = new List { };

for (int i = 0; i <= teams.Count - 1; i++)
{
for (int j = i + 1; j <= teams.Count - 1; j++)
{
match = rand.Next(1, 4); // 1 = team A wins, 2 = team B wins, 3 = even
switch (match)
{
case 1:
points[i] += 3;
break;
case 2:
points[j] += 3;
break;
default:
points[i] += 1;
points[j] += 1;
break;
}
}
Console.ReadLine(); // stoppen na alle matchen van elke ploeg
}
int m = points.Max();
int p = points.IndexOf(m);

Console.WriteLine("Final ranking: the winner is " + teams[p] + " with " + points[p] + " points.");

List teampoint = new List { };
for (int i = 0; i <= teams.Count - 1; i++)
{
if (points[i] < 10)
teampoint[i] = "0" + points[i] + " " + teams[i];
else
teampoint[i] = points[i] + " " + teams[i];
}

teampoint.Sort();
teampoint.Reverse();

foreach (string i in teampoint)
Console.WriteLine(i);
}
}

Hi,

the issue here is that you're trying to access an element in an empty list.

The points list has been initialized but it's still empty, so wherever you try to access points[i] you get an exception because there are no elements at index i. You can't access element in a list before you've added them.

You can add elements to a list during its initialization, or using methods like Add or AddRange.

After you've added some elements to the list, you can access them using the index like you were trying to do.

Lists have a different behavior than arrays: arrays are initialized with a fixed number of elements, while lists are initially empty.

One way to solve the problem could be to add some elements to the list when you create it:

List<int> points = new List<int>() { 0, 0, 0 };

Notice that points should have the same number of elements as the teams list.

In the same way, you should also apply this fix to the teampoint list, or you'll get the same error.

Another tip: instead of managing three separate lists and having to make sure they have the same number of elements, you can create a custom class to hold all the information, and use a single list of those objects instead. For example:

private class TeamScore
{
public string TeamName { get; set; }
public int Score { get; set; }
public TeamScore(string name)
{
TeamName = name;
}
public override string ToString()
{
if (Score < 10)
return "0" + Score + " " + TeamName;
else
return Score + " " + TeamName;
}
}
public static void Main()
{
int match;
Random rand = new Random();
List teams = new List
{ new TeamScore("team A"), new TeamScore("team B"), new TeamScore("team C")};
...

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

Riccardo Rigutini4y ago
Accepted answer
Hi,
the issue here is that you're trying to access an element in an empty list.
The points list has been initialized but it's still empty, so wherever you try to access points[i] you get an exception because there are no elements at index i. You can't access element in a list before you've added them.
You can add elements to a list during its initialization, or using methods like Add or AddRange.
After you've added some elements to the list, you can access them using the index like you were trying to do.
Lists have a different behavior than arrays: arrays are initialized with a fixed number of elements, while lists are initially empty.

One way to solve the problem could be to add some elements to the list when you create it:
List<int> points = new List<int>() { 0, 0, 0 };
Notice that points should have the same number of elements as the teams list.
In the same way, you should also apply this fix to the teampoint list, or you'll get the same error.

Another tip: instead of managing three separate lists and having to make sure they have the same number of elements, you can create a custom class to hold all the information, and use a single list of those objects instead. For example:
private class TeamScore
{
    public string TeamName { get; set; }
    public int Score { get; set; }

    public TeamScore(string name)
    {
        TeamName = name;
    }

    public override string ToString()
    {
       if (Score < 10)
            return "0" + Score + " " + TeamName;
        else
            return Score + " " + TeamName;
    }
}

public static void Main()
{
    int match;
    Random rand = new Random();
    List teams = new List
        { new TeamScore("team A"), new TeamScore("team B"), new TeamScore("team C")};
    ...
Valerie Meunier4y ago
Thanks.