Add A006879 Number of Primes by Number of Digits sequence (TheAlgorithms#287)

OleksiiHe · web-flow · commit 4748ad0470e8 · 2022-02-15T11:00:40.000+02:00
diff --git a/Algorithms.Tests/Sequences/NumberOfPrimesByNumberOfDigitsSequenceTests.cs b/Algorithms.Tests/Sequences/NumberOfPrimesByNumberOfDigitsSequenceTests.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 NumberOfPrimesByNumberOfDigitsSequenceTests
+    {
+        [Test]
+        public void First5ElementsCorrect()
+        {
+            var sequence = new NumberOfPrimesByNumberOfDigitsSequence().Sequence.Take(5);
+            sequence.SequenceEqual(new BigInteger[] { 0, 4, 21, 143, 1061 })
+                .Should().BeTrue();
+        }
+    }
+}
diff --git a/Algorithms/Sequences/NumberOfPrimesByNumberOfDigitsSequence.cs b/Algorithms/Sequences/NumberOfPrimesByNumberOfDigitsSequence.cs
@@ -0,0 +1,45 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Number of primes with n digits
+    ///         (The number of primes between 10^(n-1) and 10^n).
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://wikipedia.org/wiki/Prime-counting_function.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: https://oeis.org/A006879.
+    ///     </para>
+    /// </summary>
+    public class NumberOfPrimesByNumberOfDigitsSequence : ISequence
+    {
+        /// <summary>
+        /// Gets sequence of number 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;
+                        counter = 0;
+                        powerOf10 *= 10;
+                    }
+
+                    counter++;
+                }
+            }
+        }
+    }
+}
diff --git a/README.md b/README.md
@@ -108,6 +108,7 @@ 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)
+    * [A006879 Number of Primes by Number of Digits](./Algorithms/Sequences/NumberOfPrimesByNumberOfDigitsSequence.cs)
     * [A006880 Number of Primes by Powers of 10](./Algorithms/Sequences/NumberOfPrimesByPowersOf10Sequence.cs)
     * [A007318 Binomial](./Algorithms/Sequences/BinomialSequence.cs)
     * [A011557 Powers of 10](./Algorithms/Sequences/PowersOf10Sequence.cs)
