Grow a task collection with List

A learner adds another task after the plan is created. What should grow? Use a list, then inspect what changes after adding and removing items.

Predict the changing task list

An array has a fixed length. A List<string> is useful when tasks will be added or removed. The angle-bracket part tells C# that this list holds strings; List<int> would hold integers. using System.Collections.Generic; makes the List type name available.

Predict every output line before running this complete program.

Read the list-creation syntax before running: new List<string> { ... } creates a list with the initial strings written between the braces. new List<int>() creates an empty list of integers; use Add to put values in it. Here new means create an instance, and <string> or <int> selects the element type.

using System;
using System.Collections.Generic;

List<string> tasks = new List<string> { "Read", "Practise" };
tasks.Add("Review");
bool removed = tasks.Remove("Read");

Console.WriteLine(removed);
Console.WriteLine($"Count: {tasks.Count}");

foreach (string task in tasks)
{
    Console.WriteLine(task);
}

Trace membership

Add appends an item. Remove searches for the supplied value, removes its first matching occurrence, and returns whether it found one. That bool result lets your program report a useful message when a requested task does not exist.

A list uses Count for its current number of elements. If you access an item by index, valid positions are 0 through Count - 1. Check Count before reading a position that might not exist. Prefer foreach when you simply want to display all items, including when the list might be empty.

Growing means the list can change its membership; it does not mean every index is valid. Add first, then read a position that exists. A list containing two tasks still has no item at index 2.

Filter into a separate result

Do not add or remove list items while a foreach loop is traversing that same list. Such changes invalidate the traversal and can cause an exception. A simple beginner approach is to leave the source list alone and put the values you want into a new list.

using System;
using System.Collections.Generic;

List<int> recordedMinutes = new List<int> { 15, 0, 20, 0, 10 };
List<int> positiveMinutes = new List<int>();

foreach (int minutes in recordedMinutes)
{
    if (minutes > 0)
    {
        positiveMinutes.Add(minutes);
    }
}

foreach (int minutes in positiveMinutes)
{
    Console.WriteLine(minutes);
}

Try it independently

Start with Read, Read and Review. Remove Read once, then display numbered tasks. Predict two remaining tasks: Read and Review. Removing by value removes the first occurrence only. Use index + 1 for the displayed human number while keeping the actual index zero-based.

using System;
using System.Collections.Generic;

List<string> tasks = new List<string> { "Read", "Read", "Review" };
tasks.Remove("Read");

for (int index = 0; index < tasks.Count; index++)
{
    Console.WriteLine($"{index + 1}. {tasks[index]}");
}

Explain back

Explain why a task editor is a good use for List rather than a fixed-length array. Then explain the difference between a zero-based index and the number shown to a person. Finally, describe how the filter avoids changing the list it is traversing.

References: Microsoft — List.Remove and list traversal rules.