CSCI 3328 Object Oriented Programming in C# Chapter 7: Arrays 1 Xiang Lian The University of Texas Rio Grande Valley Edinburg, TX

Objectives In this chapter, you will: –Learn how arrays are used to store elements –Get familiar with the declaration, initialization, and reference to elements of the array –Know how to pass arrays to methods –Learn how to use multidimensional arrays –Learn how to use foreach statement to iterate through elements in the array 2

Arrays An array is a group of variables (called elements) containing values that all have the same data type For most languages, the number of elements are static and defined at declaration Single dim, two dim and multiple dim 3

Example of an Array The first element is the zeroth element The highest position number is array’s upper bound –Array C on RHS: 11 Access array elements –Use index (or subscript) –E.g., C[8] C[0]-45 C[1]6 C[2]0 C[3]72 C[4]34 C[5]39 C[6]98 C[7]-1345 C[8]939 C[9]10 C[10]40 C[11]33 4

Arrays (cont'd) Data structure consisting of related data items of the same type One variable name, multiple elements Arrays are passed as reference type, not value To access an element we use name of the array and index in square bracket 5

Declaring and Creating Arrays Declaration –int[] array = new int[12]; (creates indices 0 to 11) Or do it this way: –int[] array; –array = new int[12]; Or this way: –int[] array; –const int size = 12; –array = new int[size]; Since array is an object, we can resize the array as follows: –Array.Resize(ref array, 10); 6

Arrays of Other Data Types string [] str = new string [100]; char [] ch = new char [ 50]; … 7

Initializing an Array // creates 5-element array Initializing arrays while declaring –int [] arr = new int [] {10, 20, 30, 40, 50}; –int [] array = {10,20,30,40,50}; 8

Sum in an Array int[] array = {10,20,30,40,50}; // creates 5-element array Finding the sum double sum=0; double average; for (int counter =0; counter < array.Length; counter++) sum = sum+array[counter]; size of the array 9

Average in an Array int[] array = {10,20,30,40,50}; // creates 5-element array Finding the sum double sum=0; double average; for (int counter =0; counter < array.Length; counter++) sum = sum+array[counter]; average = sum/array.Length; 10

Standard Deviation Explained In your assignment, assuming array has length n, the standard deviation  is given by: 11

foreach Statements In addition to for loops, you can also work with: Syntax –foreach (type identifier in arrayName) REMEMBER: YOU HAVE TO START WITH ELEMENT 0! 12

Syntax of foreach Statements Example of computing summation of elements in an array called scores: foreach (int score in scores) { sum = sum + score; } 13

Passing Arrays by Value Declaration –int[] firstArray={1, 2, 3}; –int[] firstArrayCopy = firstArray; –public static void FirstDouble (int [] array) { for (int i = 0; i<array.Length; i++) array[i]*=2; array = new int [] {11, 12, 13}; } Call –FirstDouble(firstArray); 14

Passing Arrays by Reference Declaration –int[] secondArray={1, 2, 3}; –int[] secondArrayCopy = firstArray; –public static void SecondDouble (ref int [] array) { for (int i = 0; i<array.Length; i++) array[i]*=2; array = new int [] {11, 12, 13}; } Call –SecondDouble(ref secondArray); 15

Rectangular Arrays Rectangular array is a two dimensional array Similar to a table with rows and columns –arrayName[row, column] int [, ] a = new int [3, 4]; 16 a[0, 0]a[0, 1]a[0, 2]a[0, 3] a[1, 0]a[1, 1]a[1, 2]a[1, 3] a[2, 0]a[2, 1]a[2, 2]a[2, 3] Column 0Column 1Column 2Column 3 Row 0 Row 1 Row 2

Nested Array Initializer int [, ] arr = {{1, 2}, {3, 4}};

Example of Rectangular Arrays public int [,] hands = new int[5, 14]; for (i=1; i<=52; i++) { hands[player, hand] = sdeck[i]; } 18

Jagged Arrays Tables that have rows of unequal columns –Somewhat like an array of arrays –Fixed students and exams not everyone taking it.. int [] [] scores = new int [students][]; scores[0] = new int [5]; // student 0 takes 5 exams scores[1] = new int [4]; // student 1 takes 4 exams 19

Sort an Array C[0]-45 C[1]6 C[2]0 C[3]72 C[4]34 C[5]39 C[6]98 C[7]-1345 C[8]939 C[9]10 C[10]40 C[11]33 20 C[0]-1345 C[1]-45 C[2]0 C[3]6 C[4]10 C[5]33 C[6]34 C[7]39 C[8]40 C[9]72 C[10]98 C[11]939

Sort Method of Array You need to write your own sort methods later own For now you can use: –Array.Sort(scores); The identifier, scores, is the array you created 21

Search Method Searching: to determine whether a value (called search key) is in the array –Linear search Compare each element of the array with the search key For either sorted or unsorted array Efficient for small array –Binary search For sorted array 22

Linear Search 23 C[0]-45 C[1]6 C[2]0 C[3]72 C[4]34 C[5]39 C[6]98 C[7]-1345 C[8]939 C[9]10 C[10]40 C[11]33 search key

Binary Search You need to write your own binary search method later But now you can use: –int index = Array.BinarySearch(array, value); –If index < 0, then value is not found in the array –Otherwise, index stores the index of value in the array 24

Binary Search 25 C[0]-1345 C[1]-45 C[2]0 C[3]6 C[4]10 C[5]33 C[6]34 C[7]39 C[8]40 C[9]72 C[10]98 C[11]939 search key: 10 first last middle

Binary Search 26 C[0]-1345 C[1]-45 C[2]0 C[3]6 C[4]10 C[5]33 C[6]34 C[7]39 C[8]40 C[9]72 C[10]98 C[11]939 search key: 10 first last middle

Binary Search 27 C[0]-1345 C[1]-45 C[2]0 C[3]6 C[4]10 C[5]33 C[6]34 C[7]39 C[8]40 C[9]72 C[10]98 C[11]939 search key: 10 first last middle

Binary Search 28 C[0]-1345 C[1]-45 C[2]0 C[3]6 C[4]10 C[5]33 C[6]34 C[7]39 C[8]40 C[9]72 C[10]98 C[11]939 search key: 10 first last middle

Copying an Array You can copy one element at a time or the entire array –Array.Copy(fromArray, fromIndex, toArray, to Index, length); Note: the following statement does not copy, but creates another reference to the array –string[] copyDeck = oDeck; 29

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