Start learning free

LESSON 134 OF 674 · DICTIONARIES - CHALLENGES

Add a Contact in C#: Adding a Dictionary Key Only If Missing

Practice Read the full explanation

You get a phone book phoneBook, a Dictionary from names to phone numbers. You also get a name and a phone number.

Add name with its number to the phone book, and return the phone book.

Only add name when it isn't in the phone book yet. A name that is already there keeps its old number. The order of the entries doesn't matter.

 

Example 1:

Input: phoneBook = {"Ana": "555-0142", "Ben": "555-0199"}, name = "Cara", number = "555-0123"
Output: {"Ana": "555-0142", "Ben": "555-0199", "Cara": "555-0123"}
Explanation: "Cara" isn't in the phone book yet, so she is added.

Example 2:

Input: phoneBook = {"Ana": "555-0142", "Ben": "555-0199"}, name = "Ben", number = "555-0100"
Output: {"Ana": "555-0142", "Ben": "555-0199"}
Explanation: "Ben" is already in the phone book. He keeps his old number, "555-0199".

Example 3:

Input: phoneBook = {}, name = "Max", number = "555-0100"
Output: {"Max": "555-0100"}
Explanation: The phone book was empty. Now it has one contact.

 

Constraints:

  • phoneBook can be empty.
  • Names are single words made of English letters.
  • Every phone number has three digits, a dash and four digits, like "555-0142".
  • Two people can share a phone number.
STEP 1 OF 3
    Ctrl + Enter to check

    C# · DICTIONARIES - CHALLENGES

    Add Contact in C#, explained

    Problem overview

    The method AddContact receives a phone book, a Dictionary<string, string> from names to phone numbers, plus a name and a phone number. It adds the name with its number and returns the phone book. The one rule: a name that is already in the book keeps its old number. The order of the entries does not matter, and the book may start empty.

    With Ana and Ben in the book and the new name Cara, Cara is added with her number. With the name Ben and a new number "555-0100", nothing changes, and Ben keeps "555-0199". With an empty book and the name Max, the result has one contact, Max.

    Approach

    Assigning to a dictionary key, phoneBook[name] = number, does two different things depending on the key. If the key is new, it adds the entry. If the key exists, it replaces the old value. The task wants the first behavior only, so the assignment must be guarded: first ask whether the key is there, with ContainsKey, and assign only when it is not. The ! operator reverses the answer, so the guard is !phoneBook.ContainsKey(name).

    An empty book needs no special case. ContainsKey on an empty dictionary is false, so the new name is added.

    Step by step

    1. Write the guard. An if that is true when name is not a key of phoneBook. Close its braces right away so the next step has a place to go.
    2. Store the number inside it. phoneBook[name] = number. Because the key is new, this adds the entry.
    3. Return phoneBook after the if. It holds the new contact, or it is as it was when the name was already in it.

    The solution in C#

    class Solution {
    public Dictionary<string, string> AddContact(Dictionary<string, string> phoneBook, string name, string number) {
        if (!phoneBook.ContainsKey(name)) {
            phoneBook[name] = number;
        }
        return phoneBook;
    }
}

    Running it on the three examples gives the dictionaries below. The lesson's tests print a dictionary as key: value lines sorted by key, and these outputs use the same form.

    Ana: 555-0142
Ben: 555-0199
Cara: 555-0123

    Ana: 555-0142
Ben: 555-0199

    Max: 555-0100

    The C# details worth knowing:

    • The index setter replaces. That is why the guard is needed. Without it, phoneBook["Ben"] = "555-0100" would overwrite Ben's number.
    • Add throws on a duplicate. phoneBook.Add("Ben", "555-0100") throws an ArgumentException when Ben exists, so it is no replacement for the check.
    • TryAdd does the same job in one call. phoneBook.TryAdd(name, number) adds the entry when the key is missing and returns false without touching the book when it is there. The lesson teaches the explicit ContainsKey check first, because it shows the logic.
    • The book is changed in place. Dictionary is a reference type, so the caller's book is the one that grows. The method returns the same object.

    Edge cases and common mistakes

    • No guard. A bare phoneBook[name] = number; overwrites an existing contact. On the second example Ben would end up with "555-0100".
    • A missing !. if (phoneBook.ContainsKey(name)) only changes numbers that exist, and never adds a new contact.
    • Using Add alone. It crashes on a name that is already there.
    • Checking the number. Two people can share a number, so the check is on the name (the key), not on the value. ContainsValue is the wrong tool here.
    • An empty book. It works with the same code.

    Testing

    • Ana and Ben, add "Cara" with "555-0123": three contacts.
    • Ana and Ben, add "Ben" with "555-0100": Ben keeps "555-0199".
    • An empty book, add "Max": one contact.
    • Add a second name with the same number as an existing contact: both are kept.
    Keep going with C#500+ step-by-step lessons, from your first line of code to coding interviews. Free to start.
    Start learning free