# 3.2 Data Abstraction
## Enduring Understanding
To find specific solutions to generalizable problems, programmers represent and organize data in multiple ways.
## Learning Objective
Represent a list or string using a variable.
## Essential Knowledge
A list is an ordered sequence of elements. For example,
\[value1, value2, value3, ...]
describes a list where value1 is the first element, value2 is the second element, value3 is the third element, and so on.
An element is an individual value in a list that is assigned a unique index.
An index is a common method for referencing the elements in a list or string using natural numbers.
A string is an ordered sequence of characters.
## Learning Objective
For data abstraction:
a. Develop data abstraction using lists to store multiple elements.
b. Explain how the use of data abstraction manages complexity in program code.
## Essential Knowledge
Data abstraction provides a separation between the abstract properties of a data type and the concrete details of its representation.
Data abstractions manage complexity in programs by giving a collection of data a name without referencing the specific details of the representation.
Data abstractions can be created using lists.
Developing a data abstraction to implement in a program can result in a program that is easier to develop and maintain.
Data abstractions often contain different types of elements.
The use of lists allows multiple related items to be treated as a single value. Lists are referred to by different names, such as array, depending on the programming language.
{% hint style="info" %}
Exclusion Statement: The use of linked lists is outside of the scope of this course and the AP Exam.
{% endhint %}
The exam reference sheet provides the notation
\[value1, value2, value3, ...]
Text:
aList <-- \[value1, value2, value3, ...]
Block:
creates a new list that contains the values value1, value2, value3, and ... at indices 1, 2, 3, and ... respectively and assigns it to aList.
Text:
aList <-- \[]
Block:
creates a new empty list and assigns it to aList.
Text:
aList <-- bList
Block:
assigns a copy of the list bList to the list aList. For example, if bList contains \[20, 40, 60], then aList will also contain \[20, 40, 60] after the assignment.
The exam reference sheet describes a list structure whose index values are 1 through the number of elements in the list, inclusive. For all list operations, if a list index is less than 1 or greater than the length of the list, an error message is produced and the program will terminate.
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