Exercise 9 of 10 · Dictionaries

Town Hall Attendance by District

What you will make

A program that tracks how many residents from each district signed in to a town hall meeting and prints a per-district report.

The one new idea: Using a Dictionary<string,int> and ContainsKey to look up values safely by key

A dictionary maps a name to a value directly, which is a much more natural fit than an array or list when what you really have are labeled counts, not a plain sequence.

Go straight to the code ↓

Sometimes what you really have is a set of labeled values, not just a sequence — a count for each district, a score for each student. A Dictionary<TKey, TValue> stores exactly that kind of mapping: dictionary[key] = value adds or updates an entry, and dictionary[key] reads it back. But reading a key that was never added throws a KeyNotFoundException at runtime, so it's important to check dictionary.ContainsKey(key) first whenever a key's presence isn't guaranteed.

Worked example

Here's a dictionary mapping a workbook's chapter names to their page counts:

Dictionary<string, int> pagesByChapter = new Dictionary<string, int>();
pagesByChapter["Fractions"] = 12;
pagesByChapter["Geometry"] = 9;

string lookup = "Statistics";
int pages;
if (pagesByChapter.ContainsKey(lookup))
{
    pages = pagesByChapter[lookup];
}
else
{
    pages = 0;
}

Console.WriteLine($"{lookup}: {pages} pages");

Output

Statistics: 0 pages

Because "Statistics" was never added to pagesByChapter, ContainsKey returns false, so the code safely falls back to 0 instead of crashing.

Your turn

The starter code tracks town hall sign-ins per district and walks through a fixed list of district names, including "West", to print each one's count. But "West" was never added to the dictionary, so the direct lookup attendanceByDistrict[district] crashes as soon as the loop reaches it. Fix the lookup so it checks whether the key exists first, and treats a missing district as 0 sign-ins instead of crashing.

If something goes wrong

A KeyNotFoundException almost always means you indexed a dictionary with a key that was never inserted — check every key you're looking up against every key you actually added with dictionary[key] = value. If the total looks off by exactly the count you expected from a missing district, that's a sign the fallback branch isn't running, or isn't adding 0 the way you intended.

Write your code

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Things that often go wrong here

Looking up a dictionary key with [key] without checking ContainsKey first.: If the key was never added, this throws a KeyNotFoundException at runtime instead of failing gracefully.
Assuming every key you might want to check was definitely inserted.: In practice, some expected keys — like a district with zero sign-ins — may never get added in the first place.
Mixing up dictionary [ ] syntax with array indexing.: A dictionary's [ ] takes a key of the declared key type, not a positional index, unless that key type happens to be int.

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