Writing Array Elements

In this lesson, we will look at how to change array element values, what happens when writing by index, and how to protect against errors.

Writing array elements

To change an array element, you need to access it by index and assign a new value. Let's continue the movie theater row analogy. We have an array of seats:

Seat 1 (index 0) - Ivan
Seat 2 (index 1) - Kate
Seat 3 (index 2) - Pete
Seat 4 (index 3) - empty

If a new person sits in seat 4, we can write to the array element at index 3:


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string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
people[3] = "Olga";
Console.WriteLine(people[3]);
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Now the array is: ["Ivan", "Kate", "Pete", "Olga"]

Changing array elements

Suppose Kate and Ivan decided to swap seats


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string[] people = new string[] { "Ivan", "Kate", "Pete", "Olga" };
Console.WriteLine($"Seat 1 is occupied by {people[0]}");
Console.WriteLine($"Seat 2 is occupied by {people[1]}");
Console.WriteLine("Kate and Ivan are swapping seats");
people[0] = "Kate";
people[1] = "Ivan";
Console.WriteLine($"Seat 1 is occupied by {people[0]}");
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Writing to a non-existent index

As with reading, if you try to write a value to an index that does not exist in the array, an IndexOutOfRangeException will occur.


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string[] people = new string[] { "Ivan", "Kate", "Pete", "Olga" };
people[4] = "Kate";
Console.WriteLine(people[4]);
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The array does not expand when writing — its size is fixed.

How to check an index before writing

To avoid an error, you can check whether the index falls within the valid bounds.


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string[] people = new string[] { "Ivan", "Kate", "Pete", "empty" };
WriteName(2, "Sergey");
WriteName(4, "Anna");
WriteName(-1, "Marina");
void WriteName(int index, string name)
{
    if (index >= 0 && index < people.Length)
    {
        people[index] = name;
        Console.WriteLine($"Value written at index {index}: {name}");
    }
    else
    {
        Console.WriteLine($"Invalid index {index}! Cannot write.");
    }
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Summary

In this lesson you learned:

how to write values to array elements
that writing happens by index, just like reading
what happens when trying to write to an invalid index
how to check the index to avoid errors

Exercises

Change the third element of the array to "Orange"


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string[] fruits = new string[] { "Apple", "Banana", "Cherry", "Pear" };
Console.WriteLine(fruits[2]);
// Your code
Console.WriteLine(fruits[2]);
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Swap the first and last elements of the array.


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string[] names = new string[] { "Anna", "Boris", "Victor", "Galina" };
// Your code
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Try to change elements with index 4 and index -1. What will happen? Why?


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string[] names = new string[] { "Anna", "Boris", "Victor", "Galina" };
// Your code
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Write a method WriteFruit(int index, string value) that writes a value to the array only if the index is valid; otherwise prints the message "Invalid index!"


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string[] fruits = new string[] { "Apple", "Banana", "Cherry", "Pear" };
WriteFruit(1, "Mango");
WriteFruit(5, "Kiwi");
void WriteFruit(int index, string fruit)
{
    // Your code
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