C# Adventure 09: Arrays and Loops

We can handle larger amounts of data by using arrays. They are just one way to handle text with C#, but they are common in many programming languages so you should know what they are and how to use them.

We'll also discuss loops and use them to cycle through the content stored in our arrays.

Arrays

If we wanted to create an application that printed out a grocery list, we could have a separate variable for each item. Our variable declarations might look like this:

using System;

namespace GroceryList
{
    class Program
    {
        static void Main()
        {
           string item1 = "banana";
           string item2 = "orange";
           string item3 = "avocado";
        }

    }
}

Because all of these variables are related, we can bundle them together as an array.

The separate variables from above written as an array could look like this:

using System;

namespace GroceryList
{
    class Program
    {
        static void Main()
        {
           string[] items = {"banana", "orange", "avocado" };
        }

    }
}

Array Declaration

Creating a string array is similar to creating a string variable. You declare the data type and an identifier.

Initializing and assigning values is different, however. Array values are enclosed within curly braces. Each value is separated by a comma.

For strings, each of the values must be enclosed in double quote marks.

string[] items = {"banana", "orange", "avocado" };

Values on Separate Lines

You can have all the elements on the same line, or each on a separate line. The next two code snippets produce the same result. You can declare an array either way.

string[] items = {"banana", "orange", "avocado" };

string[] items = {
"banana", 
"orange", 
"avocado" 
};

If you have a lot of items or text, breaking up the space sometimes makes it easier to read and update. The compiler doesn't mind if you make your array more "human readable" by having values on different lines.

string[] items = new string[3];

string[] items = new string[]{"banana", "orange", "avocado" };

Array Index Numbers

An array stores multiple elements in one entity. Each of the elements within the array has an index number. The value stored in each element can be accessed by the index number.

Arrays in C# are zero based. That means when we want to access the first element, we access element zero.

This statement would print out the value stored at the index 0 (banana):

Console.WriteLine(items[0]);

If we wanted to write out the value of the second element (orange) to the console window, we could use this:

Console.WriteLine(items[1]);

Assigning Array Values

To change an element's value, use the array's index number like this:

items[0] = "avocado";

Array Review and Summary

Review Array Structures

An array declaration starts with the data type.

string[] items = {"banana", "orange", "avocado" };

In this case we are going to hold strings. The square brackets indicate this is an array.

Next is an identifier - items. The identifier is a name you as a programmer decide; you can name your array what you think makes sense.

Then the assignment operator (the equal sign).

Inside the curly braces {} are the values. Each is a string and must be enclosed in quote marks. Commas separate each of the values. All of the elements within an array must be the same data type.

At the end of the statement is a semicolon.

Our string array starts at index 0. The value stored at 0 is "banana".

Because the array is zero based, and we have three elements, the index numbers are 0, 1, and 2.

(If you tried to access a value at index 3 you would see an error something like "An unhandled exception of type 'System.IndexOutOfRangeException' occurred ".)

• C# arrays are zero based. The index starts at 0. To access the first element, you would refer to it at index 0.
• If you create an array without providing any data, the array will be populated with default values for the type of data that you will be storing.
• Each array can only hold one type (i.e., you can't mix integers and strings in the same array).
• If you don't initialize the values, you need to specify the size of the array.

Arrays are a fixed size. In a few sections we'll look at an alternative to using arrays (Lists).

Quick Quiz

string[] items = {"banana", "orange", "avocado" };

string[] items = {"banana", "orange", "avocado" };

Loops

If you have statements that need to repeat, you will probably want to use a loop structure. Loops make it easier to cycle through an array - especially if it holds a lot of data.

for

A for loop will cycle through statements until a condition is met. The basic form is:

for (initializer; condition; iterator)
body

The initializer sets the initial value that will be incremented or decremented. It acts as a counter. It is common in loops to have the counter variable named i or j. To cycle through our array, we will set ours to start at 0 with: i=0;.

Next is a condition. This is what is being tested to see if the loop continues, or stops. We want our loop to stop after it cycles through all the elements in our array. In the case of our GroceryList, which had 3 elements, the condition could look like: i < 3;.

Finally there is an iterator. This is what increments or decrements the variable that is acting as a counter. We could write i = i+1, but there is a short form for adding +1 to a variable. To cycle through our array we will increment the counter using: i++.

Example

Make a new project in Visual Studio and add the following example code to it. The first for loop will increment the counter with the shortcut i++ (the same as i = i+1). The second for loop decrements the counter with i--.

using System;

namespace Count
{
    class Program
    {
        static void Main()
        {
            Console.Title = "Examples of for loops";

            //increment 
            Console.WriteLine("Count 0 to 9!");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine(i);
            }

            //decrement
            Console.WriteLine("Count down 9 to 0!");
            for (int j = 9; j > -1; j--)
            {
                Console.WriteLine(j);
            }

            Console.ReadKey();
        }
    }
}

You should see something that looks like this:

In the first loop, i++ adds one to the counter (counting up). In the second example, j-- decrements the counter (counting down).

Array Length

Remember when we discussed the Length property of the String class? Instead of manually writing how many times the loop should run, we can say the equivalent of "run the following statements for each element in the array". We can find out how many elements are in the array by using the Length property:

lunch.Length

Try out this example in Visual Studio.

using System;

namespace Lunch
{
    class Program
    {
        static void Main()
        {
           string[] lunch = new string[3];
           lunch[0] = "sandwich";


            for (int i=0; i<lunch.Length; i++)
            {
                Console.WriteLine("The value at " + i + " is " + lunch[i]);
            }
            Console.WriteLine("The value of lunch.Length is: " + lunch.Length);
            Console.ReadKey();
        }
    }
}

The for loop will cycle through all the items in the array, writing the values to the console window.

The value written to the console window for lunch.Length is 3 because we declared the array would hold 3 elements. Even without values assigned, the elements are still there.

Replace the above example with the one below and run it again.

using System;

namespace Lunch
{
    class Program
    {
        static void Main()
        {
           string[] lunch = new string[3];
           lunch[0] = "sandwich";
           lunch[1] = "drink";
           lunch[2] = "apple";

            for (int i=0; i<lunch.Length; i++)
            {
                Console.WriteLine("The value at " + i + " is " + lunch[i]);
            }
            Console.ReadKey();
        }
    }
}

Instead of just seeing sandwich, we now also see drink and apple.

The foreach Statement

There are other ways of accessing elements in an array. Using foreach we can also write out array values to the console window.

Create a new project in Visual Studio and test out the example code below. The foreach statement makes it easy to print all the elements with just a few lines of code.

using System;

namespace GroceryList
{
    class Program
    {
        static void Main()
        {
            Console.Title = "Write Each Element in Array";
            string[] items = {
                "banana",
                "orange",
                "avocado",
                "pretzels",
                "lemon",
                "pecans"
            };

            foreach (string element in items)
            {
                Console.WriteLine(element);
            }
            Console.ReadLine();
        }

    }
}

The foreach has a set of parentheses that hold:

• a declaration of the data type that will be used
• an identifier
• the keyword "in"
• the name of the array to be accessed - in this case, "items"

Quick Quiz

 string[] books = new string[9];

string[] books = new string[9];
books[0] = 4133587;

string[] books = new string[9];
books[0] = "4133587";

Code Progress

At this point your project should have the branching narrative framework in the Choice() function, and from the walk-throughs you should have added design elements (color and/or ASCII art).

Consider having a "title screen" that displays the title and asks the player to press enter to start the game. After enter is pressed, the console window is cleared.

To Do: Practice Arrays and Loops

Assignments

1. Back up your project.
2. Add another method to your project that will show the end of the story. We'll also use it later to summarize what the player accomplished. Name it something like EndGame(). Call the new method at the end of StartGame(). Write out an instruction to press enter or return to exit out of the game, too. Code example below.
3. Create a small application that prints out words to a public domain poem or song. Use arrays. Example below.

Example StartGame() and EndGame() Methods

Try writing the code on your own before looking at the example.

Poem Example

An example poem that is a modified version of "One and One are Two" by Christina Georgina Rossetti.

• Can you programmatically add the numbers?
• Is it possible to only use one variable to write out all the numbers?
• How would you chart the flow of the program?

Related example: C# Loops: Ten Bears Song.

Code Files

If you have errors, or just want to check how your code compares in terms of progress, the code examples for this chapter are linked below.

• 9.1 For Loops
• 9.2 For Loops
• 9.3 For Loops
• 9.4 For Each
• 9.5 Framework Progress
• 9.6 Framework Progress

Next Step

In the next section we'll use arrays to manage our content and a switch statement to show content depending on which choice the player has made.