5.1 - Arrays

An array is a container object that holds a fixed number of values from a single type. In previous lessons, we had to create a single variable for each piece of data we wanted to store. An array is a single object that can store multiple values.

Up until now, if I wanted to store 5 different inputs from a user, I would need to create 5 separate variables. Now we have a single array of length 5, where we can refer to each data type by its index. An index is the location of an element stored in an array.

The index is the location of the element. It starts at 0. So if the length is 10, the index ranges from zero to nine.

How to create & then fill an array.

To create an array, we must do several things.

1./2. declare the type and put [].

3. give the array a name to refer to it by.

4. assignment operator

5. new keyword

6./7. same data type & length of array in []

This creates an empty array, where we then have to fill the array by its index.

8. name_of_array[index] = value;

and you’ll have to do this for each index.

How to create & fill an array at one time.

This way is called an Array Literal. Where we create and fill the array at the same time.

Another way to create an array and fill it at the same time would be:

1./2. declare the type and put [] brackets.

3. give the array a name to refer to it by.

4. assignment operator

5./6. {} braces & the values you want to store inside the braces, separated by commas.

In this example, the creation and filling is done at the same exact time! and the length will depend on how many values you give.

Raw Code & Video Example

Youtube Video

Example II - Methods for Printing an Array

Printing Individually.

In order to print out an array, we have to call the array by name and in [] refer to the index we want to print. All inside of a system.out.println();

But what if we want to print out everything in the array at the same time. If we had 1000 indexes, printing each index individually would really.... stink.

Printing An Entire Array With A Loop

In order to print out an entire array, we have to use a for loop (or while loop) to iterate through each index and print it out. The .length variable of an array becomes very handy here.

If we use the array_name.length; It'll return the length of the array for our use!

Raw Code & Video Example

Youtube Video

Example III - Can We Do that?

Can we change the value at a specific index?

Yes!

In order to change the value at a specific index, we just refer to that index and assign it a new value.

Can we increase the length of an array?

Short Answer: No

Once we give an array the length of 5, that cannot be changed, so we cannot add a 6th element.

If you try to refer to an index that is greater than or equal to the length, you'll receive an ArrayIndexOutOfBounds Exception.

Long Answer: With some trickery.

If we create a new array with a bigger length, fill it with all the old values, we technically increased the size.

Raw Code & Video Example

Youtube Video

Example IV - Using a Loop to Fill an Array

A For Loop w/o User Input

Filling an array with a for loop is going to look very similar to what we already have seen. In the gist to the right, we are iterating through each index of the array and giving it a value. This value also uses the for loop variable.

A For Loop w/ User Input

The gist to the right assumes you have already imported a scanner.

Inside the for loop, we can ask the user for input and store it in the array. This works because we use the for loop variable as the index for the array which changes every loop so we are not saving over the same exact index.

Raw Code & Video Example

Youtube Video

Example V - For Each Loop

A Better For Loop For Iterating Arrays

There is another type of for loop, so why haven't I shown it yet? This loop does not work without a type of collection or list. Simply, we hadn't worked with the structures necessary for it to be used. Now that we know arrays, this loop is a better option for iterating through them.

In order to do a For Each loop:

Use the for keyword & then ().
inside the parenthesis we create a variable that is the same data type we are listing through.
a colon (not a semi-colon)
the list or array you want to loop through
Then in {} we can use the variable we created to get access to each element in the array.

Pro Tip: The for loop does not have a variable to access each index and that makes it very hard to use this loop when wanting to get input and resave it. Just because it looks easier, doesn't mean it's good for all use cases.

Raw Code & Video Example

Youtube Video

Example VI - Arrays as Arguments/Returns

Simple Example

Arrays can be used as arguments all the same, with one stipulation. When we pass an array, we are passing the access to it. Therefore, any changes we make to the array are changed everywhere. It is not a copy like with any other variable.

Raw Code & Video Example

Youtube Video

Check Your Understanding

Quick Code

Pick 2 out of 3.

Option 1 - Find the Max Number of an Int Array

You should ask the user to input the numbers for an integer array and then you print out which number was the biggest!

W/O using array shortcut methods!

Option 2 - Find the Average of a Double Array

You should ask the user to input the numbers for a double array and then you print out the average number!

W/O using array shortcut methods!

Challenge Option - Sorting An Integer Array From Least to Greatest

Think the class is too easy for you? Well... Try this.

Ask the user for an integer array, and then sort the array from least to greatest. Do this without the sorting method. Manually sort it yourself! :]

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