Arrays

Arrays#

Array Fundamentals#

One of the most important capabilities in programming comes from the ability to process some sequence of data. For example we might write programs to process grades of students in a class, information about respondents to the US Census, or many other applications. Most programming languages provide a first introduction to processing sequences of data with the concept of arrays. An array is a fixed-length structure that stores multiple values of the same type.

In Java, the array syntax is mostly the same to how arrays are used in other languages. The largest difference is how you specify the type of an array. If you want an array storing integer values, the type is int[] (read as “int array”). The following code snippet makes an int array storing 5 elements, and uses assignment statements to fill in the array. One thing to note is that Java starts arrays with index 0. So an array of length 5 has valid indices 0, 1, 2, 3, 4.

JAVA

int[] array = new int[5];
array[0] = 1;
array[1] = 14;
array[3] = array[1];
array[4] = 5;

System.out.println(Arrays.toString(array));

PYTHON

array = [0] * 5  # Short-hand to make a length 5 list
array[0] = 1
array[1] = 14
array[3] = array[1]
array[4] = 5

print(array)

JAVASCRIPT

let array = [0, 0, 0, 0, 0];

array[0] = 1;
array[1] = 14;
array[3] = array[1];
array[4] = 5;

console.log(array);

C

int array[5];
array[0] = 1;
array[1] = 14;
array[2] = 0; // in C, you need to explictly 0 initialize memory
array[3] = array[1];
array[4] = 5;

printf("[");
for (int i = 0; i < 4; i++) {
    printf("%d, ", array[i]);
}
printf("%d]\n", array[4]);

C++

int array[5];
array[0] = 1;
array[1] = 14;
array[2] = 0; // in C++, you need to explictly 0 initialize memory
array[3] = array[1];
array[4] = 5;

cout << "[";
for (int i = 0; i < 4; i++) {
    cout << array[i] << ", ";
}
cout << array[4] << "]" << endl;

The syntax of assigning or getting a value inside an array is much like using a variable, but we add a [x] at the end to indicate we want to use index x. So on the left-hand of the = for assignment we are assigning into the variable at that index, and on the right side we are getting the value at that index.

By default, an array is initialized with the zero-equivalent of whatever type the array stores. So after creating the array in the example above, its contents start [0, 0, 0, 0, 0]. By zero-equivalent, we mean there is a special value for each type that Java considers the most like 0 which will be the default contents for arrays of that type. So for a boolean[], it defaults to false and for a double[] it defaults to 0.0. You can also make arrays of object types such as String[] which defaults to a special value called null. Note that it is not expected that you understand the concept of null yet, so we do not explain what this special value means.

Two important notes about arrays in Java:

Because Java requires you to declare types of variables ahead of time, it’s not really possible to store an array of mixed types. You couldn’t make an array of boolean or int types in the same array, since int[] only allows int and boolean[] only allows boolean.
Arrays in Java are fixed-length. This means when you make an array, you have to specify how long it is in the line new int[10]. This is a limitation of arrays in that they are not dynamically sized. We will learn more complex data structures such as ArrayList later that dynamically resize based on what values you add.

Printing Arrays#

To print the contents of a Java array in a readable format, we first need to convert it to a String using the Arrays.toString method. We can pass our array as a parameter to this method, and it will return the formatted String version of the array.

The formatted String will look like a comma-separated list with square brackets on both ends (e.g. [8, 1, 7, 2, 5]).