In this chapter, you will deepen your understanding of string processing by taking on a practical challenge: counting the number of times a specific letter appears in a sentence, regardless of whether the letter is upper or lower case. This exercise builds on your experience with searching and counting occurrences in strings, but now requires you to handle case-insensitive comparisons. Being able to perform such operations is essential for many real-world text processing tasks, such as analyzing user input, validating data, or even building search features in applications.


using Xunit;
namespace ConsoleApp {
    public class ProgramTests {
        [Fact(DisplayName = "Returns correct count for multiple occurrences in different cases")]
        public void CountLetterOccurrences_MultipleOccurrencesDifferentCases()
        {
            Assert.Equal(2, Program.CountLetterOccurrences("Programming in C# is fun!", 'g'));
            Assert.Equal(2, Program.CountLetterOccurrences("Programming in C# is fun!", 'G'));
            Assert.Equal(2, Program.CountLetterOccurrences("ProGramminG in C# is fun!", 'g'));
        }

        [Fact(DisplayName = "Returns zero when letter does not appear")]
        public void CountLetterOccurrences_LetterDoesNotAppear()
        {
            Assert.Equal(0, Program.CountLetterOccurrences("No matches here.", 'z'));
            Assert.Equal(0, Program.CountLetterOccurrences("", 'a'));
        }

        [Fact(DisplayName = "Handles both upper and lower case letters in both sentence and target letter")]
        public void CountLetterOccurrences_HandlesUpperLowerCase()
        {
            Assert.Equal(3, Program.CountLetterOccurrences("Hello World", 'l'));
            Assert.Equal(3, Program.CountLetterOccurrences("Hello World", 'L'));
            Assert.Equal(3, Program.CountLetterOccurrences("Case Insensitive", 'i'));
            Assert.Equal(3, Program.CountLetterOccurrences("Case Insensitive", 'I'));
        }

        [Fact(DisplayName = "Works correctly when sentence contains no alphabetic characters")]
        public void CountLetterOccurrences_NoAlphabeticCharacters()
        {
            Assert.Equal(0, Program.CountLetterOccurrences("1234567890!@#$%", 'a'));
            Assert.Equal(0, Program.CountLetterOccurrences(" ", 'b'));
        }
    }
}

ProgramTests.cs

Master the essentials of string manipulation, text parsing, and efficient text processing in C#. This course guides you from the basics of working with strings to advanced text analysis and formatting, using engaging real-world examples and hands-on challenges.

Explore the basics of strings in C#, including creation, concatenation, and essential properties. Practice manipulating and analyzing text with beginner-friendly challenges.

Advance your skills with string formatting, splitting, trimming, and replacing text. Tackle real-world parsing and formatting tasks.

Apply your string skills to analyze text, check for palindromes, perform frequency analysis, and implement basic ciphers.

Challenge: Letter Counter

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Challenge: Letter Counter

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