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Engineering LibreTexts

19.2: Array Index Operator

  • Page ID
    10360
  • Array Index Operator

    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 2nd 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 1st Place
    one offsets 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.

    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.