double[ ] sales = new double[20]; //Array has 20 items

the first sales array element is sales[0], and

the last sales element is sales[19].

THE NUMBER IN THE [ ] IS THE ELEMENT POSITION. NOT THE VALUE. 

sales[0] = 2100.00; // The first array element assigned 2100.00

WriteLine(sales[19]); // Prints the 20th array element

Parallel arrays are a common programming technique where you use multiple arrays to store related data. Each array holds a specific type of data, and elements at the same index in each array are related to each other.

Ex.) // Parallel arrays: names, ages, and grades

        string[] names = { "Alice", "Bob", "Charlie", "Diana" };

        int[] ages = { 20, 21, 19, 22 };

        char[] grades = { 'A', 'B', 'A', 'C' };

BinarySearch( ) method - MUST BE ARRANGED IN ASCENDING ORDER TO WORK. A sorted list of objects is split in half repeatedly as the search gets closer and closer to a match. Starts with Is it less than 50? Less than 25? Less than 12? Etc. 

Array.Sort( ) method - arranges array items in ascending order. Often used before BinarySearch( ). 

Array.Reverse( ) method - The Reverse() method does not sort array elements; it only rearranges their values to the opposite order.

dataType[,] arrayName = new dataType[rows, columns];

double[,] sales = new double[3,3];

            { 1, 2, 3 },  // Row 0

            { 4, 5, 6 },  // Row 1

            { 7, 8, 9 }   // Row 2

sales [0, 2] refers to the element in Row 0, Column 2; which in this case is the value "3."

Similar to two-dimensional arrays in the way that it is written:

Ex.)

dataType[, , ] arrayName = new dataType[dimension1, dimension2, dimension3];

int[,,] threeDArray = new int[3, 3, 3];