Operators And Expressions

Collections and LINQ in C#

In numerical programming, we rarely work with single values. Collections allow us to group related data—such as a vector of residuals or a list of material properties. LINQ (Language Integrated Query) provides a powerful, declarative way to filter, transform, and analyze these collections.

1. Common Collection Types

C# provides several specialized collections. While arrays are the standard for fixed-size mathematical data, other collections offer dynamic resizing and key-based lookups.

List of Essential Collections

Type	Namespace	Category	Description
Array (T[])	System	Reference type	Fixed-size, high-performance contiguous memory.
List<T>	System.Collections.Gen	Reference type	Dynamically resizable array for iterative growth.
Dictionary<K,V>	System.Collections.Gen	Reference type	Key-value pairs for fast lookups.
HashSet<T>	System.Collections.Gen	Reference type	Unordered set of unique elements.

// Arrays: Best for Matrix data (Fixed size)
double[] vector = { 1.0, 0.0, 0.0 };

// Lists: Best for Solver History (Dynamic size)
List<double> residuals = [];
residuals.Add(0.125);
residuals.Add(0.001);

// Dictionary: Best for Material Properties mapping
Dictionary<string, double> materials = [];
materials["Steel_E"] = 210e9;

2. Introduction to LINQ

LINQ allows you to perform query operations directly on collections. It simplifies tasks like finding the maximum error in a vector or extracting specific nodes from a mesh using a functional approach.

double[] data = { -1.5, 0.2, 4.5, 10.1, -0.5 };

// Filtering: Get only positive values
var positive = data.Where(x => x > 0);

// Transformation: Get absolute values
var absolute = data.Select(Abs);

// Aggregation: Find the maximum value
double maxVal = data.Max();

// Conversion: Force evaluation into an array
double[] result = [..positive];

3. Deferred Execution

A vital concept in LINQ is that queries are not executed when they are defined. They are executed when you “materialize” them (by using foreach, .ToArray(), or .ToList()). This is known as lazy evaluation.

Examples

Example 1 : Filtering Convergence Data

In this scenario, we have a list of error values captured from a solver that is struggling to reach a steady state. We want to programmatically extract only the “successful” steps—those where the error dropped below our predefined tolerance—to verify how often our algorithm performed well.

List<double> errors =  [0.5, 0.01, 0.0002, 1.5, 0.00001];
double tolerance = 0.001;

// Find all errors that meet the tolerance criteria
var convergedEntries = errors.Where(e => e < tolerance).ToArray();
Console.WriteLine($"Found {convergedEntries.Length} converged steps.");

Ouput

Found 2 converged steps.

Example 2 : Statistical Analysis

Imagine you have completed a simulation and possess a vector of residuals representing the difference between your guess and the true solution. Before proceeding, you need to calculate the average error to understand the general accuracy and the “Total Energy” (sum of squares) to assess the stability of the system.

double[] residuals = [0.02, 0.05, 0.01, 0.08, 0.03];
double averageError = residuals.Average();
double totalEnergy = residuals.Sum(r => r * r); // Sum of squares
Console.WriteLine($"Average: {averageError}, Energy: {totalEnergy}");

Ouput

Average: 0.038, Energy: 0.0103

Example 3 : Mapping Node IDs

In Finite Element Analysis (FEA), nodes are often assigned unique identification numbers that aren’t necessarily sequential. By using a Dictionary, we can map these arbitrary Node IDs to their physical coordinates on a 1D beam, allowing for instant lookup without searching through a massive list.

var nodes = new Dictionary<int, double>
{
    { 101, 0.0 },
    { 102, 0.5 },
    { 103, 1.0 }
};

if (nodes.ContainsKey(102))
{
    Console.WriteLine($"Node 102 position: {nodes[102]}");
}

Ouput

Node 102 position: 0.5

Example 4 : Generating Sequences

Setup is a major part of numerical modeling. Instead of writing cumbersome loops to initialize an identity-like vector or an index map, we use LINQ generators to create a range of integers or a repeating set of initial guesses in a single line of code.

// Generate a sequence of 100 integers for indexing
int[] indices = [..Enumerable.Range(0, 100)];

// Generate a constant initial guess vector
double[] initialGuess = [..Enumerable.Repeat(1.0, 5)];

Performance Note

While LINQ is expressive, it can introduce overhead due to allocations. For the “hot-path” of a numerical solver (like inner loops of matrix multiplication), traditional for-loops remain the preferred choice.