Add A006880 Number of primes sequence (TheAlgorithms#284)

OleksiiHe · siriak · web-flow · commit d73ae5f56f67 · 2022-02-11T14:19:25.000+02:00
Co-authored-by: Andrii Siriak &lt;siryaka@gmail.com&gt;
diff --git a/Algorithms.Tests/Sequences/NumberOfPrimesByPowersOf10SequenceTests.cs b/Algorithms.Tests/Sequences/NumberOfPrimesByPowersOf10SequenceTests.cs
@@ -0,0 +1,19 @@
+﻿using System.Linq;
+using System.Numerics;
+using Algorithms.Sequences;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Sequences
+{
+    public class NumberOfPrimesByPowersOf10SequenceTests
+    {
+        [Test]
+        public void First5ElementsCorrect()
+        {
+            var sequence = new NumberOfPrimesByPowersOf10Sequence().Sequence.Take(5);
+            sequence.SequenceEqual(new BigInteger[] { 0, 4, 25, 168, 1229 })
+                .Should().BeTrue();
+        }
+    }
+}
diff --git a/Algorithms/Sequences/NumberOfPrimesByPowersOf10Sequence.cs b/Algorithms/Sequences/NumberOfPrimesByPowersOf10Sequence.cs
@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Sequence of number of primes less than 10^n (with at most n digits).
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://wikipedia.org/wiki/Prime-counting_function.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: https://oeis.org/A006880.
+    ///     </para>
+    /// </summary>
+    public class NumberOfPrimesByPowersOf10Sequence : ISequence
+    {
+        /// <summary>
+        /// Gets sequence of numbers of primes.
+        /// </summary>
+        public IEnumerable<BigInteger> Sequence
+        {
+            get
+            {
+                ISequence primes = new PrimesSequence();
+                var powerOf10 = new BigInteger(1);
+                var counter = new BigInteger(0);
+
+                foreach (var p in primes.Sequence)
+                {
+                    if (p > powerOf10)
+                    {
+                        yield return counter;
+                        powerOf10 *= 10;
+                    }
+
+                    counter++;
+                }
+            }
+        }
+    }
+}
diff --git a/README.md b/README.md
@@ -107,6 +107,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [A001462 Golomb's](./Algorithms/Sequences/GolombsSequence.cs)
     * [A001478 Negative Integers](./Algorithms/Sequences/NegativeIntegersSequence.cs)
     * [A005132 Recaman's](./Algorithms/Sequences/RecamansSequence.cs)
+    * [A006880 Number of Primes by Powers of 10](./Algorithms/Sequences/NumberOfPrimesByPowersOf10Sequence.cs)
     * [A007318 Binomial](./Algorithms/Sequences/BinomialSequence.cs)
     * [A181391 Van Eck's](./Algorithms/Sequences/VanEcksSequence.cs)
   * [String](./Algorithms/Strings)
