Arrays

Note: This lesson covers JavaScript and Python.

Definition

The basic structure to store several values in a sequence is called an array in JavaScript and a list in Python. Both are used to hold multiple elements under a single name.

Syntax

arrays/list

There are various ways of creating an array/list.

Literal - syntax takes a list of elements separated by a comma and enclosed in square brackets.
General - you can create an empty array/list and add the elements individually.

JavaScript
Python

Literal

var array_name = [element0, element1, element2,... elementN];

General

var array_name = [];
array_name[0] = element0;
array_name[1] = element1;
array_name[2] = element2;
... 
array_name[N] = elementN;

Literal

list_name = [element0, element1, element2,... elementN]

General

list_name = []
list_name[0] = element0
list_name[1] = element1
list_name[2] = element2
... 
list_name[N] = elementN

Note: Reminder that this lesson covers similar structures - an array in JavaScript and a list in Python.

Access / Change value by index

marks	Value	76	34	65	25	49	57
marks	Index	0	1	2	3	4	5

Create array/list - In our example below, we create an array/list named marks which has six elements.
Access value by index - We can access a value by referring to its index number. Indexes start at zero:
- marks[0] is the first value - 76.
- marks[1]is the second value - 34.
- marks[5]is the last value - 57.
Change value by index - We can change a value by specifying the index number and the new value. Our example selects index zero, marks[0], to modify the first value in the list.

JavaScript
Python

Create Array
```
var marks = [76, 34, 65, 25, 49, 57];
```

Access value by index

alert(marks[0]);    // 76
alert(marks[1]);    // 34

Change value by index

marks[0] = 80;    // [80, 34, 65, 25, 49, 57]

Create List
```
marks = [76, 34, 65, 25, 49, 57]
```

Access value by index

print(marks[0])    # 76
print(marks[1])    # 34

Change value by index

marks[0] = 80    # [80, 34, 65, 25, 49, 57]

Adding end element

Each language has a built in method to add an element to the end of the array/list.

JavaScript uses push() and Python uses append().

JavaScript
Python

var marks = [76, 34, 65, 25, 49, 57];
marks.push(18);       // [76, 34, 65, 25, 49, 57, 18];
marks.push(79, 31);   // [76, 34, 65, 25, 49, 57, 18, 79, 31];

JavaScript Note:

The JavaScript push() method adds new items to the end of an array and returns the new length.
The push() function can be used to append more than one value to an array in a single call: marks.push(79, 31);

marks = [76, 34, 65, 25, 49, 57]
marks.append(18)    
# [76, 34, 65, 25, 49, 57, 18]

Python Note:

The Python append() method adds an item to the end of a list. The method takes a single argument.
It differs from the JavaScript push() method as it does not return the new length and does not allow for adding multiple elements.

Removing end element

Each language has a built in method to remove an element from the end of the array/list.

JavaScript
Python

var marks = [76, 34, 65, 25, 49, 57]
// remove last value
marks.pop();   // [76, 34, 65, 25, 49]

JavaScript Note:

The pop() method removes the last value of an array and returns that value.
The JavaScript pop() method does not have any parameters and you cannot pass it an index (unlike Python pop()).

marks = [76, 34, 65, 25, 49, 57]
# remove last value
marks.pop()    # [76, 34, 65, 25, 49] 
#remove value at index 1
marks.pop(1)   # [76, 65, 25, 49]

Python Note:

The Python pop() with an empty parameter will remove the last value.
Returns the element removed.
The Python pop() method can take an optional single argument to specify the index of the value to remove.

Length of array/list

The example code below shows how the number of elements in the array/list can be determined.

This example also shows that a JavaScript array and Python list can contain elements of different types.

JavaScript
Python

var results = ["Exam", 76, 34, 65, 25, 49, 57];
alert(results.length);     // 7

results = ["Exam", 76, 34, 65, 25, 49, 57]
print(len(results))         # 7

Python Note: The Python len() function can be used to determine the number of items found in the list. The list is passed as the argument.