C# Academy · Lesson

Iterating Over Strings

Strings as char sequences.

Iterating Over Strings is a free C# Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Strings Are Sequences

A string is a sequence of char values in order.

This means you can loop over a string and look at one character at a time. Iterating is how you inspect, count, or transform the characters in text.

string text = "Hello";
// 'H', 'e', 'l', 'l', 'o'

The Length Property

Every string has a Length property that tells you how many characters it holds.

You will use Length to control loops and to know the last valid index, which is always Length - 1.

class Program
{
    static void Main()
    {
        string text = "Hello";
        System.Console.WriteLine(text.Length);
    }
}

Looping With foreach

The simplest way to visit each character is a foreach loop.

It gives you one char per iteration without needing an index. This is the cleanest option when you do not need to know the position.

class Program
{
    static void Main()
    {
        string text = "Hi!";
        foreach (char c in text)
            System.Console.WriteLine(c);
    }
}

Looping With for and Index

A for loop gives you the index, which you need when position matters.

Run the index from 0 up to Length - 1 and read each character with text[i].

class Program
{
    static void Main()
    {
        string text = "Hi!";
        for (int i = 0; i < text.Length; i++)
            System.Console.WriteLine(text[i]);
    }
}

Indexing Starts at Zero

Characters are numbered from 0, so the first character is at index 0.

For "Code", text[0] is 'C' and text[3] is 'e'. Reading an index that is too large throws an error.

string text = "Code";
char first = text[0]; // 'C'
char last = text[3];  // 'e'

Counting Specific Characters

Iteration lets you count characters that match a condition.

This program counts how many vowels are in a word by checking each character against a small set.

class Program
{
    static void Main()
    {
        string text = "banana";
        int count = 0;
        foreach (char c in text)
            if (c == 'a') count++;
        System.Console.WriteLine(count);
    }
}

Combining With char Helpers

Inside a loop you can call the char helpers you learned earlier.

For example, count only the digits in a string by testing each character with char.IsDigit.

class Program
{
    static void Main()
    {
        string text = "a1b2c3";
        int digits = 0;
        foreach (char c in text)
            if (char.IsDigit(c)) digits++;
        System.Console.WriteLine(digits);
    }
}

Building a Transformed Result

You can iterate and collect a new result as you go.

This loop reads each character, uppercases it with char.ToUpper, and prints it. The original string is never changed because strings are immutable.

class Program
{
    static void Main()
    {
        string text = "loud";
        foreach (char c in text)
            System.Console.Write(char.ToUpper(c));
    }
}

Strings Are Immutable

You cannot change a character in place with text[0] = 'X'; that is a compile error.

Strings are immutable, meaning their characters never change. To get different text, you create a new string instead of editing the old one.

string text = "cat";
// text[0] = 'b'; // not allowed

Reversing With Iteration

By iterating from the last index down to 0, you can print characters in reverse order.

Start the index at Length - 1 and decrement until it reaches 0.

class Program
{
    static void Main()
    {
        string text = "abc";
        for (int i = text.Length - 1; i >= 0; i--)
            System.Console.Write(text[i]);
    }
}

foreach vs for

Use foreach when you only need each character and not its position.

Use a for loop when you need the index, want to skip characters, or loop in a custom order like reverse. Both visit the same characters.

Quick Check

Test your understanding of iterating over strings.

Recap

A string is a sequence of characters you can loop over.

Use foreach for simple iteration and a for loop with an index when position matters. The Length property gives the count, indexes start at 0, and strings are immutable so transforming text always creates a new string.

Frequently asked questions

Is the “Iterating Over Strings” lesson free?

Yes — the full text of “Iterating Over Strings” is free to read here on the web, and the C# Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.

What will I learn in “Iterating Over Strings”?

Strings as char sequences. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Iterating Over Strings” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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