Find the vowels in a string with C# .NET

Here comes a classic developer interview question: write a function that accepts a string and returns the number of vowels that it contains. The vowels in the English language are the following: a, e, i, o u. We want the vowel search to be case-insensitive, i.e. AEIOU must also be counted. This should be a fairly simple problem with multiple solutions. We’ll look at three of them in this post but there are certainly more:

  • using a counter
  • using a regex
  • LINQ

Let’s start with the class skeleton:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace Algorithms
{
    public class CountVowels
    {
        public int FindVowels(string input)
        {
            return 0;
        }        
    }
}

Here comes a short set of unit tests:

using Microsoft.VisualStudio.TestTools.UnitTesting;
using Algorithms;

namespace Algorithms.Tests
{
    [TestClass]
    public class CountVowelsTests
    {
        [TestMethod]
        public void CountVowelsTest()
        {
            var findVowels = new CountVowels();
            Assert.AreEqual(0, findVowels.FindVowels(null));
            Assert.AreEqual(3, findVowels.FindVowels("Hello World!!!"));
            Assert.AreEqual(5, findVowels.FindVowels("afdfgehhiddfgOdfgdU"));
            Assert.AreEqual(3, findVowels.FindVowels("UOE"));
            Assert.AreEqual(0, findVowels.FindVowels("rtyplkjhgfdszxcvb"));
        }
    }
}

Let’s start with the most basic counter-based solution that most developers came across in an intro course:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace Algorithms
{
    public class CountVowels
    {
        public int FindVowels(string input)
        {
            return FindVowelsWithHelpArray(input);            
        }

        private int FindVowelsWithHelpArray(String inputString)
        {
            int vowelCounter = 0;

            if (inputString != null)
            {
                var vowels = new List<char>() { 'a', 'e', 'i', 'o', 'u' };
                foreach (char c in inputString.ToLower())
                {
                    if (vowels.Contains(c))
                    {
                        vowelCounter++;
                    }
                }
            }            

            return vowelCounter;
        }        
    }
}

We initialise the counter variable vowelCounter to 0. Then if the input string is not null then we create a list of chars to contain all the vowels we want to count. Then we iterate through the lower-cased string char by char and if the current char is part of the vowel list then we increase the counter. Finally we return the counter from the function.

Run the unit tests, they all pass.

Next, let’s take a look at the Regex solution:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace Algorithms
{
    public class CountVowels
    {
        public int FindVowels(string input)
        {
           //return FindVowelsWithHelpArray(input);
           return FindVowelsWithRegex(input);
        }

        private int FindVowelsWithHelpArray(String inputString)
        {
            abridged...
        }

        private int FindVowelsWithRegex(String inputString)
        {
            if (inputString == null) return 0;
            return Regex.Matches(inputString, "[aeiou]", RegexOptions.IgnoreCase).Count;
        }        
    }
}

We use the Regex.Matches (https://docs.microsoft.com/en-us/dotnet/api/system.text.regularexpressions.regex.matches?view=netcore-3.1) function to count all the matches in the incoming string. The match is based on a simple array that reflects the vowels that we want to find. I took a shortcut and added the built-in RegexOptions enumeration to make sure that the search is case-insensitive instead of expressing it directly in the Regex string.

Run the unit tests, they will all still pass.

Finally let’s see a LINQ-based solution. LINQ is very versatile and there are certainly multiple ways to solve this problem using LINQ expressions. We’ll go for the most straightforward one using the Count() function:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace Algorithms
{
    public class CountVowels
    {
        public int FindVowels(string input)
        {
           //return FindVowelsWithHelpArray(input);
           //return FindVowelsWithRegex(input);
           return FindVowelsWithLinq(input);
        }

        private int FindVowelsWithHelpArray(String inputString)
        {
            abridged
        }

        private int FindVowelsWithRegex(String inputString)
        {
            abridged
        }

        private int FindVowelsWithLinq(String inputString)
        {
            if (inputString == null) return 0;
            var vowels = new List<char>() { 'a', 'e', 'i', 'o', 'u' };
            return inputString.ToLower().Count(c => vowels.Contains(c));
        }
    }
}

We again use the list of vowels just like in the first solution. Then we use the Count() LINQ function to count the vowels. We do that by providing a lambda function to go through each character in the string and see if the vowel list contains the given character.

The unit tests will pass just like before.

Capitalise each word in a sentence using C# .NET

Here comes a classic developer interview problem: write a function that takes a string as a parameter and returns the same string with each word in that string capitalised. Examples:

InputOutput
hello worldHello World
a beautiful morningA Beautiful Morning
the show must go onThe Show Must Go On
i can’t help falling in love with youI Can’t Help Falling In Love With You

There are of course multiple solutions to this problem. In this post we’ll go through two of them. Since this blog is mostly dedicated to C# and .NET we’ll use C# but the solutions can easily be translated into other OOP languages.

Note that a third solution based on regex was posted by http://gravatar.com/wkinkeldei in the comment section below.

We can start with a skeleton class and a small set of unit tests.

Capitalise.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Algorithms
{
    public class Capitalise
    {
        public string CapitaliseWordsInSentence(string input)
        {
            return null;
        }        
    }
}

…and a corresponding set of unit tests:

using Algorithms;
using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace Algorithms.Tests
{
    [TestClass]
    public class CapitaliseTests
    {
        [TestMethod]
        public void CapitaliseWordsInSentenceTests()
        {
            Capitalise c = new Capitalise();
            Assert.AreEqual("Hello World", c.CapitaliseWordsInSentence("hello world"));
            Assert.AreEqual("A Beautiful Morning", c.CapitaliseWordsInSentence("a beautiful morning"));
            Assert.AreEqual("The Show Must Go On", c.CapitaliseWordsInSentence("the show must go on"));
            Assert.AreEqual("I Can't Help Falling In Love With You", c.CapitaliseWordsInSentence("i can't help falling in love with you"));
        }        
    }
}

If you run the unit tests then they will of course fail:

Assert.AreEqual failed. Expected:<Hello World>. Actual:<(null)>.

The first possible solution we’ll be looking at is based on a simple iteration. We iterate through each character in the input string. As soon as the character before the current one is equal to an empty space we capitalise the current char. Add the following function to the Capitalise class:

namespace Algorithms
{
    public class Capitalise
    {
        public string CapitaliseWordsInSentence(string input)
        {
            return CapitaliseWordsWithIteration(input);
        }

        private string CapitaliseWordsWithIteration(string input)
        {
            StringBuilder stringBuilder = new StringBuilder(input[0].ToString().ToUpper());
            for (int i = 1; i < input.Length; i++)
            {
                if (input[i - 1] == ' ')
                {
                    stringBuilder.Append(input[i].ToString().ToUpper());
                } else
                {
                    stringBuilder.Append(input[i]);
                }
            }
            return stringBuilder.ToString();
        }     
    }
}

We use a StringBuilder to build the return string character by character. By default we capitalise the first character to begin with. Then we start the iteration from the second character of the input string until its end. If the character before the current one, i.e. at position i - 1 is a whitespace then we capitalise the current char. Otherwise we just add the char to the StringBuilder as-is. Finally we return the final string.

Let’s re-run the unit tests:

OK, good.

Let’s try another approach based on substrings. We split the input string using a whitespace delimiter. Then we iterate through each element in the delimited array. We take the first character of each word, capitalise it, and push the rest of the word to it. We collect the capitalised words in a list which we then use to rebuild the sentence using string.Join. Here’s the function:

private string CapitaliseWordsWithSubstrings(string input)
        {
            List<string> elements = new List<string>();
            foreach(string s in input.Split(' '))
            {               
                elements.Add(s[0].ToString().ToUpper() + s.Substring(1));   
            }

            return string.Join(" ", elements);
        }

If you call this function from CapitaliseWordsInSentence then the tests should pass just like before:

public string CapitaliseWordsInSentence(string input)
{
      return CapitaliseWordsWithSubstrings(input);
     //return CapitaliseWordsWithIteration(input);
}

We can improve the CapitaliseWordsInSentence by adding guards against nulls and empty strings:

public string CapitaliseWordsInSentence(string input)
{
    if (string.IsNullOrEmpty(input))
    {
        return "";
    }
    return CapitaliseWordsWithSubstrings(input);
    //return CapitaliseWordsWithIteration(input);
}

That’s it really.

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