# 18.1 Array index operator

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An introduction to the array index operator as used within the C++ programming language.

## Array index operator in c++

Example:

`int ages[5] = {49,48,26,19,16};`

`int my_age;`

`my_age = ages[2]`

This second usage of the square brackets is as the array notation of dereference or more commonly called the index operator . As an operator it either provides the value held by the member of the array (Rvalue) or changes 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 variable named my_age. 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. The array members (or elements) are referenced starting at zero. The more common way for people to reference a list is by starting with one. Many programming languages reference array members starting at one, however for some languages (and C++ is one of them) you will need to change your thinking . Consider:

 Position C++ Miss America Other Contests zero offsets from the front ages [0] Winner 1 st Place one offsets 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.

The C++ operator name is called the array index or simply the index operator and it uses the square brackets as the operator symbols.

## Definitions

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.

#### Questions & Answers

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Source:  OpenStax, Programming fundamentals - a modular structured approach using c++. OpenStax CNX. Jan 10, 2013 Download for free at http://cnx.org/content/col10621/1.22
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