List, nested

A List can have elements of List type. This is a jagged list, similar in syntax to a jagged array. Lists are not by default multidimensional in C#.

2D list solution

To store more complex shapes of data, we can develop multidimensional Lists. We use an easily-developed and expandable data structure.

Requirements

Consider this shape: the first row has 3 elements, the next row just has 1, and then we have a row with 4 elements. We require a nested list solution that allows this data.

0 1 2
2
1 2 3 4

Example code

The C# List provides an underlying and internal array where it stores its elements. But it always specifies a one-dimensional and zero-based array in its internal structure.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Part 1: add rows and columns to the List.
        List<List<int>> list = new List<List<int>>();
        var rand = new Random();
        for (int i = 0; i < 3; i++)
        {
            // Put some integers in the inner lists.
            List<int> sublist = new List<int>();
            int top = rand.Next(1, 4);
            for (int v = 0; v < top; v++)
            {
                sublist.Add(rand.Next(1, 5));
            }
            // Add the sublist to the top-level List.
            list.Add(sublist);
        }
        // Display the List.
        Display(list);
    }

    static void Display(List<List<int>> list)
    {
        // Part 2: loop over and display everything in the List.
        Console.WriteLine("Elements:");
        foreach (var sublist in list)
        {
            foreach (var value in sublist)
            {
                Console.Write(value);
                Console.Write(' ');
            }
            Console.WriteLine();
        }
        // Part 3: display element.
        Console.WriteLine("Element at 1, 0:");
        Console.WriteLine(list[1][0]);
        // Part 4: display total count.
        int count = 0;
        foreach (var sublist in list)
        {
            count += sublist.Count;
        }
        Console.WriteLine("Count:");
        Console.WriteLine(count);
    }
}
Elements:
1 3
2 2 2
2
Element at 1, 0:
2
Count:
6

A discussion

As for performance, a nested List is relatively fast. It internally uses zero-based arrays, and the List type itself is fast.

Summary

We used a nested List that is essentially a jagged or 2D List data type with the generic List class. The code here is not ideal. But it helps in prototyping or experimental work.