descending Dictionary Exception foreach IComparable KeyValuePair Lambda List orderby Sort ToList var

`Sort` `Dictionary`

Dictionary has no Sort method. If we need to loop through the Dictionary contents in sorted order, we must separately acquire the elements and sort them.

Sorting approaches

We can sort a Dictionary with the Keys and Values properties and a List instance. Some approaches are slower than others.

An example

To begin, we sort the string keys in a Dictionary. In the output, the string "apple" should come before the string "zebra."

Step 1 An example Dictionary is created. The var implicit typed keyword is used to reduce syntactic redundancy.

Step 2 We access the Keys property. Then we use the ToList extension method and the Sort instance method.

Step 3 The resulting List is looped through using the foreach-loop construct. We print each key and its value.

using System;
using System.Collections.Generic;
using System.Linq;

// Step 1: create dictionary.
var dictionary = new Dictionary<string, int>();
dictionary.Add("car", 2);
dictionary.Add("zebra", 0);
dictionary.Add("apple", 1);

// Step 2: get keys and sort them.
var list = dictionary.Keys.ToList();
list.Sort();

// Step 3: loop through keys.
foreach (var key in list)
{
    Console.WriteLine("{0}: {1}", key, dictionary[key]);
}apple: 1
car: 2
zebra: 0

`Sort` values

This program adds keys to a Dictionary and then sorts them by their values. Dictionary instances are not initially sorted. We use the orderby keyword in a query statement.

Note The dictionary has string keys, and int values. We will reorder the values to go from lowest to highest.

Result It displays results, using foreach to iterate through and display each KeyValuePair. No additional lookups are required.

using System;
using System.Collections.Generic;
using System.Linq;

// Example dictionary.
var dictionary = new Dictionary<string, int>(5);
dictionary.Add("cat", 1);
dictionary.Add("dog", 0);
dictionary.Add("mouse", 5);
dictionary.Add("eel", 3);
dictionary.Add("programmer", 2);

// Order by values.
// ... Use LINQ to specify sorting by value.
var items = from pair in dictionary
                orderby pair.Value ascending
                select pair;

// Display results.
foreach (KeyValuePair<string, int> pair in items)
{
    Console.WriteLine("{0}: {1}", pair.Key, pair.Value);
}

// Reverse sort.
// ... Can be looped over in the same way as above.
items = from pair in dictionary
        orderby pair.Value descending
        select pair;dog: 0
cat: 1
programmer: 2
eel: 3
mouse: 5

`OrderBy`

Another option is the OrderBy extension method in System.Linq. This approach compiles to the same code as the orderby query expression.

Tip Here the syntax is shorter. It requires only one lambda expression and method call.

Info The program could (with certain changes) raise a KeyNotFoundException. We can trap those errors with exception handling.

using System;
using System.Collections.Generic;
using System.Linq;

var items = new Dictionary<int, int>();
items.Add(-1, 0);
items.Add(0, 1);
items.Add(-2, 0);
items.Add(3, 1);

// Use OrderBy method.
foreach (var item in items.OrderBy(i => i.Key))
{
    Console.WriteLine(item);
}[-2, 0]
[-1, 0]
[0, 1]
[3, 1]

Notes, other methods

Other methods I found involve more steps, code or complexity. There is nothing wrong with those methods, although they vary in efficiency.

`Sort` `string` values

Sorting strings would work just as well. Sort() uses the interface implementations of the types. Its syntax is the same for any type that List.Sort() could sort.

Summary

Dictionary is optimized for fast lookups, not for specific looping mechanisms. It helps in lookup-heavy programs, but hinders programs that require sorted keys.

Sort Dictionary