6.2: Index Notation

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Overview

Index notation is used to specify the elements of an array.^[1] Most current programming languages use square brackets [] as the array index operator. Older programming languages, such as FORTRAN, COBOL, and BASIC, often use parentheses () as the array index operator.

Discussion

Example:

Language	Example
C++	`int ages[] = {49, 48, 26, 19, 16}; int myAge = ages[2];`
C#	`int[] ages = {49, 48, 26, 19, 16}; int myAge = ages[2];`
Java	`int[] ages = {49, 48, 26, 19, 16}; int myAge = ages[2];`
JavaScript	`var ages = [49, 48, 26, 19, 16]; int myAge = ages[2];`
Python	`ages = [49, 48, 26, 19, 16] my_age = ages[2]`
Swift	`var ages:[Int] = [49, 48, 26, 19, 16] var my_age = ages[2]`

As an operator, square brackets either provide the value held by the member of the array (Rvalue) or change the value of member (Lvalue). In the above example, the member that is two offsets from the front of the array (the value 26) is assigned to the variable named myAge. The dereference operator of [2] means to go the 2^nd offset from the front of the ages array and get the value stored there. In this case, the value would be 26. In most current programming languages, the array members (or elements) are referenced starting at zero. The more common way for people to reference a list is by starting with position one. Consider:

Position	Index	Miss America	Other Contests
zero offsets from the front	`ages[0]`	Winner	1^st Place
one offset from the front	`ages[1]`	1^st Runner Up	2^nd Place
two offsets from the front	`ages[2]`	2^nd Runner Up	3^rd Place
three offsets from the front	`ages[3]`	3^rd Runner Up	4^th Place
four offsets from the front	`ages[4]`	4^th Runner Up	5^th Place

Saying that my cousin is the 2^nd Runner-Up in the Miss America contest sounds so much better than saying that she was in 3^rd Place. We would be talking about the same position in the array of the five finalists.

ages[3] = 20;

This is an example of changing an array’s value by assigning 20 to the 4^th member of the array and replacing the value 19 with 20. This is an Lvalue context because the array is on the left side of the assignment operator.

Key Terms

array member: An element or value in an array.

index: An operator that allows us to reference a member of an array.

offset: The method of referencing array members by starting at zero.

References

cnx.org: Programming Fundamentals – A Modular Structured Approach using C++

Wikipedia: Index notation ↵