Collections

Dictionary Lookup

A dictionary maps keys to values for fast lookup by name.

dictionary A dictionary stores key-value pairs.

Dictionary Lookup

key
DictionaryLookup.cs
Replay: real traced execution (multi-file project)
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string key = "go";
        Dictionary<string, int> scores = new Dictionary<string, int>
        {
            ["csharp"] = 94,
            ["go"] = 88
        };
        bool found = scores.ContainsKey(key);
        int value = found ? scores[key] : 0;

        Console.WriteLine($"key={key}");
        Console.WriteLine($"found={found}");
        Console.WriteLine($"value={value}");
    }
}

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string key = "csharp";
        Dictionary<string, int> scores = new Dictionary<string, int>
        {
            ["csharp"] = 94,
            ["go"] = 88
        };
        bool found = scores.ContainsKey(key);
        int value = found ? scores[key] : 0;

        Console.WriteLine($"key={key}");
        Console.WriteLine($"found={found}");
        Console.WriteLine($"value={value}");
    }
}

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string key = "ruby";
        Dictionary<string, int> scores = new Dictionary<string, int>
        {
            ["csharp"] = 94,
            ["go"] = 88
        };
        bool found = scores.ContainsKey(key);
        int value = found ? scores[key] : 0;

        Console.WriteLine($"key={key}");
        Console.WriteLine($"found={found}");
        Console.WriteLine($"value={value}");
    }
}

  1. key ← go, scores ← System.Collections.Generic.Dictionary`2[System.String,System.Int32]

    5{6    static void Main()7    {8        string key→ go = "go"; //@key="csharp", "ruby"9        Dictionary<string, int> scores→ System.Collections.Generic.Dictionary`2[System.String,System.Int32] = new Dictionary<string, int>10        {11            ["csharp"] = 94,12            ["go"] = 8813        };14        bool found→ True = scoresSystem.Collections.Generic.Dictionary`2[System.String,System.Int32].ContainsKey(keygo);15        int value→ 88 = foundTrue ? scores[key]88 : 0;1617        Console.WriteLine($"key={keygo}");18        Console.WriteLine($"found={foundTrue}");19        Console.WriteLine($"value={value88}");20    }
    outputkey=go
found=True
value=88

  1. key ← csharp, scores ← System.Collections.Generic.Dictionary`2[System.String,System.Int32]

    5{6    static void Main()7    {8        string key→ csharp = "csharp";9        Dictionary<string, int> scores→ System.Collections.Generic.Dictionary`2[System.String,System.Int32] = new Dictionary<string, int>10        {11            ["csharp"] = 94,12            ["go"] = 8813        };14        bool found→ True = scoresSystem.Collections.Generic.Dictionary`2[System.String,System.Int32].ContainsKey(keycsharp);15        int value→ 94 = foundTrue ? scores[key]94 : 0;1617        Console.WriteLine($"key={keycsharp}");18        Console.WriteLine($"found={foundTrue}");19        Console.WriteLine($"value={value94}");20    }
    outputkey=csharp
found=True
value=94

  1. key ← go, scores ← System.Collections.Generic.Dictionary`2[System.String,System.Int32]

    5{6    static void Main()7    {8        string key→ go = "go"; //@key="csharp", "ruby"9        Dictionary<string, int> scores→ System.Collections.Generic.Dictionary`2[System.String,System.Int32] = new Dictionary<string, int>10        {11            ["csharp"] = 94,12            ["go"] = 8813        };14        bool found→ True = scoresSystem.Collections.Generic.Dictionary`2[System.String,System.Int32].ContainsKey(keygo);15        int value→ 88 = foundTrue ? scores[key]88 : 0;1617        Console.WriteLine($"key={keygo}");18        Console.WriteLine($"found={foundTrue}");19        Console.WriteLine($"value={value88}");20    }
    outputkey=go
found=True
value=88

Follow the Lookup

  1. The dictionary starts with scores for csharp and go.
  2. The selected key is "go".
  3. ContainsKey(key) checks whether that key exists.
  4. When the key is found, scores[key] reads the matching value.
  5. When the key is missing, the fallback value is 0. | Key | Found? | Value used | | --- | --- | --- | | go | true | 88 | | ruby | false | 0 |

Exercise: DictionaryLookup.cs

Look up a score by key, print whether it was found, and use 0 when the key is missing