Programming Fundamentals/Index Notation

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] tells the computer, go to the 2nd 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 1st Place one offset from the front `ages[1]` 1st Runner Up 2nd Place two offsets from the front `ages[2]` 2nd Runner Up 3rd Place three offsets from the front `ages[3]` 3rd Runner Up 4th Place four offsets from the front `ages[4]` 4th Runner Up 5th Place

Saying that my cousin is the 2nd Runner-Up in the Miss America contest sounds so much better than saying that she was in 3rd 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 4th 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.