C# Advent 2020

common-mistakes-where.cs

// Generate numbers from -10 .. 100
var numbers = Enumerable.Range(-10, 111);

// Determines all positive numbers that are divisible by 6
var positiveDivisbleBySix = numbers
                            .Where(x=>x > 0) // iterates through the whole list (111 comparisons, returning 100 results)
                            .Where(x=>x % 2 == 0) // iterates through the new list (100 comparisons, returning 50 results)
                            .Where(x=>x % 3 == 0); // iterates through the new list (50 comparisons, returning 16 results)
// Overall metrics: 261 comparisons over 111 total elements)

// Generate numbers from -10 .. 100
var numbers = Enumerable.Range(-10, 111);

var positiveDivisibleBySix = numbers.Where(x => x > 0 && x % 2 == 0 && x % 3 == 0);


Func<T, bool> CombinePredicateWithAnd<T>(Func<T, bool> a, Func<T, bool> b)
{
  return x => a(x) && b(x);
}

Func<int, bool> isPositive = x => x > 0;
Func<int, bool> isEven = x => x % 2 == 0;
Func<int, bool> isDivisibleByThree = x => x % 3 == 0;
var predicates = new List<Func<int, bool>> {isPositive, isEven, isDivisibleByThree};

var combinedPredicate = predicates.Aggregate(...., ....);

Func<int, bool> combinedPredicateWithAnd(Func<int, bool> a, Func<int, bool> b)
{
  return x => ...;
}

Func<int, bool> combinedPredicateWithAnd(Func<int, bool> a, Func<int, bool> b)
{
  return x => ... && ...;
}

Func<int, bool> combinedPredicateWithAnd(Func<int, bool> a, Func<int, bool> b)
{
  return x => a(x) && b(x);
}

Func<int, bool> andIdentity = _ => true;
var combinedPredicate = predicates.Aggregate(andIdentity, CombinePredicateWithAnd);

// Define the predicates
Func<int, bool> isPositive = x => x > 0;
Func<int, bool> isEven = x => x % 2 == 0;
Func<int, bool> isDivisibleByThree = x => x % 3 == 0;
var predicates = new List<Func<int, bool>> {isPositive, isEven, isDivisibleByThree};

// Defining the Aggregate functions
Func<int, bool> andIdentity = _ => true;
Func<int, bool> combinedPredicateWithAnd(Func<int, bool> a, Func<int, bool> b)
{
  return x => a(x) && b(x);
}

// Combining the predicates
Func<int, bool> combinedAndPredicate = predicates.Aggregate(andIdentity, combinedPredicateWithAnd);

// The new solution
// Generate numbers from -10 .. 100
var numbers = Enumerable.Range(-10, 111);
var positiveDivisbleBySix = numbers.Where(combinedAndPredicate);