Find a value by name with a dictionary

How many minutes belong to Monday—and what if Monday has no entry? A lookup by name needs a clear missing-key path.

An array or list answers a position question: what is at index 0? A dictionary answers a name question: what value belongs to Monday? In Dictionary<string, int>, each key is a string and each value is an int. The type names inside angle brackets tell C# what this collection accepts. A key identifies one entry; different keys may have equal values.

Predict before running

Write the three output lines before running this complete program. It changes the existing Monday entry and asks about a different day.

using System;
using System.Collections.Generic;

Dictionary<string, int> minutesByDay = new Dictionary<string, int>();
minutesByDay.Add("Monday", 20);
minutesByDay["Monday"] = 35;

if (minutesByDay.TryGetValue("Tuesday", out int minutes))
{
    Console.WriteLine($"Tuesday: {minutes}");
}
else
{
    Console.WriteLine("No Tuesday session.");
}

Console.WriteLine($"Monday: {minutesByDay["Monday"]}");
Console.WriteLine($"Entries: {minutesByDay.Count}");

Draw keys and values separately

minutesByDay
  key          value
  "Monday" --> 35

lookup "Tuesday" --> not found
Count = 1

Add means add a new key. Assigning through minutesByDay["Monday"] means set the value for that key; it also adds a new entry if that key is missing. Reading an indexer for a missing key throws an exception. For a lookup that may reasonably be missing, use TryGetValue: its bool result tells you whether the key was found, and its out variable receives the value. Use the value inside the successful branch.

Your turn: a useful lookup

Read the condition before coding: day != null means the input is not null. && means both conditions must be true. It checks the left side first and skips the right side when the left side is false. This short-circuit behaviour prevents a lookup with a null key.

Build a lookup with Monday = 25 and Tuesday = 0. Ask for a day name. A recorded zero must print zero; an absent day must say there is no entry. Write an attempt first, then compare it with this complete solution. Enter the day names exactly as shown; this dictionary uses its default case-sensitive string matching.

using System;
using System.Collections.Generic;

Dictionary<string, int> minutesByDay = new Dictionary<string, int>();
minutesByDay.Add("Monday", 25);
minutesByDay.Add("Tuesday", 0);

Console.WriteLine("Day name:");
string? day = Console.ReadLine();
if (day != null && minutesByDay.TryGetValue(day, out int minutes))
{
    Console.WriteLine($"Recorded minutes: {minutes}");
}
else
{
    Console.WriteLine("No entry for that day.");
}

Run it with Monday, Tuesday and Wednesday. Expect respectively Recorded minutes: 25, Recorded minutes: 0, and No entry for that day after the prompt. An empty line or end of input also takes the missing-entry path. The null check prevents passing a null key.

Explain it back

Explain why a list index and a dictionary key solve different problems. Then explain why testing only whether minutes equals zero loses information. Before continuing, change the values, keep one stored zero, and test one absent day again. Keep the key name and stored value in separate columns in your trace so you can see which detail changed.

References: Microsoft - Dictionary and TryGetValue.