C# LINQ Introduction

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Filtering data with lambda expressions

Lambda expressions are anonymous functions you can define inline.

Func<int, bool> isGreaterThanZero = x => x > 0;

Console.WriteLine(isGreaterThanZero(10)) // true
Console.WriteLine(isGreaterThanZero(-4)) // false

Here, x => x > 0 is the lambda expression.

The => operator is the lambda operator.
To the left of the lambda operator you see the input parameters (x).
To the right of the lambda operator you see the expression that is executed when calling the function (x > 0).

The types of input argument and return value are not explicitly stated in the lambda expression. They are inferred by the context. In the example above we assign the lambda expression x => x > 0 to a variable of type Func<int, bool>, which means a function with an int as input (x) and a bool as output (x > 0).

Lambda expressions applied

Filtering with lambda expressions

// {
using System;
using System.Collections.Generic;
namespace Answer
{
    public class FilterWithLambda
    {
// }
        public IEnumerable<int> KeepPositiveNumbers(IEnumerable<int> numbers)
        {
            Func<int, bool> greaterZero = x => x > 0;
            return FilterNumbers(numbers, greaterZero);
        }
        
        public IEnumerable<int> KeepNegativeNumbers(IEnumerable<int> numbers)
        {
            return FilterNumbers(numbers, x => x < 0);
        }
        public IEnumerable<int> FilterNumbers(IEnumerable<int> numbers, Func<int, bool> filterPredicate)
        {
            var filteredNumbers = new List<int>();
            foreach (var number in numbers)
            {
                if (filterPredicate(number))
                {
                    filteredNumbers.Add(number);
                }
            }
            return filteredNumbers;
        }
// {
    }
}
// }

The code explained

FilterNumbers is unchanged.

We removed the two explicitly defined filter methods IsGreaterThanZero and IsSmallerThanZero and put their content directly as lambda expressions into KeepPositiveNumbers and KeepNegativeNumbers.

In KeepPositiveNumbers we assign the lambda expression to a variable. In KeepNegativeNumbers we directly pass the lambda expression as argument to FilterNumbers.

It can't hardly get simpler

KeepPositiveNumbers and KeepNegativeNumbers are now really tight - that's fine.

The only method with some complexity left is FilterNumbers. Here, we have to enumerate the original numbers and add them to filteredNumbers if our filterPredicate evaluates to true.

But isn`t the concept of filtering a collection of elements always the same, no matter if we have a collection of numbers, or timestamps, or any other type of objects?

It is! And finally we are at the heart of this tutorial: LINQ

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6/14 Filtering data with lambda expressions

Filtering data with lambda expressions

Lambda expressions applied

The code explained

It can't hardly get simpler

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