BITG 1113: Array (Part 1) LECTURE 8

Transcription

1 BITG 1113: Array (Part 1) LECTURE 8 1 1

2 LEARNING OUTCOMES At the end of this lecture, you should be able to: 1. Describe the fundamentals of arrays 2. Describe the types of array: One Dimensional (1 D) and Two Dimensional (2 D) 3. Write declaration, initialization and assignment 4. Manipulate data in arrays (operations on arrays) 5. Explore the applicability of arrays 2 2

3 WHAT IS AN ARRAY? 3 Array of bugs An array is a derived data type (derived from fundamental data type) It is a group of a fixed number of elements (values or variables) of the same data type, placed in consecutive memory locations that are acessed or referenced using the array name and a subscript (or also known as index). If the name of an array of 10 elements is ARY, then its elements will be referenced as : ARY[0], ARY[1], ARY[2],, ARY[9] WHY ARRAY? Array is used to store a large number of values of the same data type under a single variable. That means, for example, using an array we can store 5 different values of the same type (int for example) with ONE unique identifier without having to declare 5 different variables. E.g.: To store the Marks of 50 students. To record the Sales of 100 salesman.

4 BASIC OPERATIONS ON ARRAY Some basic operations performed on arrays are: Initializing elements of array Input data to an array Output data stored in an array Finding the largest and/or smallest element Computations Each operation requires ability to step through the elements of the array Easily accomplished by a for loop. 44

5 TYPES OF ARRAY : THE MEANING OF DIMENSION Types of array: One dimensional (1-D), Two dimensional (2-D), etc. The dimension of an array is the number of indices (subscripts) needed to select an element. Thus a one-dimensional array is a list of data, a twodimensional array a rectangle of data, a threedimensional array a block of data, etc. Like a regular variable, an array must be declared before it is used. Array is declared using [ ] operator. 5

6 THE DIMENSIONS OF ARRAY 1-D array: with 10 elements. 2-D array: with 20 elements (4 x 5) 3-D array with: 24 elements(2x3x4) 3-Dimensional 6

7 1-D ARRAY : DECLARATION AND MEMORY LAYOUT Declaration Syntax: data_type array_name[size]; (Note :number of elements in the array) Example: (Note: size declarator - is an integer constant > 0) int number[5]; allocates the following memory to the elements of array number: SUBSCRIPT number [0] [1] [2] [3] [4]????? first element second element third element fourth element fifth element 7 In C++ the subscript of the first element is always 0.

8 8 1-D ARRAY : DECLARATION AND MEMORY LAYOUT

9 1 D ARRAY DECLARATION IN C++ PROGRAM : EXAMPLES #include <iostream> a) #define SIZE 20 b) using namespace std; int main() { int Marks[SIZE]; char Names[20]; } Execution statements #include <iostream> using namespace std; int main() { const int SIZE = 20; } int Marks[SIZE]; char Names[20]; Execution statements 9 * SIZE : is a size declarator; it must be a constant. ** It is advisable to use named constant as size declarator. This eases program maintenance when the size of the array needs to be changed.

10 1 D ARRAY INITIALIZATION Initial values are given to the array elements during array declaration. Examples on array initialization: char vowels[5] = {'a', 'e', 'i', 'o', 'u'}; bool anskey[] ={true, true, false, true, false, false}; char word[] = "Hello"; vowels 'a' 'e' 'i' 'o' 'u' anskey true true false true false false word 'H' 'e' 'l' 'l' 'o' '\0' 10

11 1 D ARRAY INITIALIZATION int scores[9] = {23,45,12,67,95,45,56,34,83}; 11

12 12 1 D ARRAY INITIALIZATION : SOME VARIATIONS

13 1-D ARRAY : ACCESSING ARRAY ELEMENTS Subscripts are used to access individual elements of an array. General format to access an array element: array_name[subscript] Example: const int size = 5; double number[size]; for (int i=0; i<=4; ++i) number[i] = double(i) + 0.5; From the above statements, each element of the array is accessed and assigned with a value as follows : 13 number

14 1-D ARRAY : ACCESSING ARRAY ELEMENTS Array elements can be accessed and used as regular variables, such as the following: numbers[0] = 79.5; cout << numbers[0]; cin >> numbers[1]; numbers[4] = numbers[0] + numbers[1]; Arrays must be accessed via individual elements: cout << numbers; // not legal We can copy an array to another array with the same size and type but it is not legal to assign one array to another as shown below : Example: 14 int num1[5]={1,2,3,4,5}; int num2[5]; num2 = num1; //error

15 CONT ASSIGNING VALUES TO 1-D ARRAY Example : to copy elements of num1 into num2. int num1[5] = {1,2,3,4,5}, num2[5]; for(int i =0 ; i<5; i++) { num2[i]= num1[i]; //correct } 15

16 MORE EXAMPLES : ASSIGNING VALUES TO 1-D ARRAY int num[5]; int list[10]; 16

17 INPUT DATA TO 1-D ARRAY Beside Assignment or Initialization, another way to fill the array is by inputting values (read) Can be done using a loop the most appropriate is the for loop int scores[9]; for (i=0;i<9;i++) { cin >> scores[i]; } The input are: <enter> 17 17

18 DISPLAY DATA STORED IN 1-D ARRAY for (i=0; i<9; i++) { cout <<scores[i]<< \n ; } Output:

19 EXAMPLE OF 1-D ARRAY FLOWCHART int main() { for ( int i = 0; i < 20; i++) cin >> numbers[i]; for ( int i = 0; i < 20; i++) cout << numbers[i]; 19 }

20 #include <iostream> #include <iomanip> using namespace std; EXAMPLE : INPUT AND OUTPUT OF 1-D ARRAY int main() { int i, a[5]={2,4,6,8,10}, b[5], c[5]; cout<< Enter 5 numbers:\n ; for(i=0; i<5; i++) cin >> b[i]; cout<< setw(15)<< a << setw(10) << b << setw(10) << c << endl; 20 } for(i=0; i<5; i++) { c[i]= a[i] + b[i]; cout<< setw(15) << a[i] << setw(10) << b[i] << setw(10) << c[i] <<endl; } return 0; 20

21 Output : Enter 5 numbers: ary1 ary2 ary

22 EXAMPLE : DETERMINE THE MINIMUM VALUE #include <iostream> using namespace std; void main() { int i, score[5], min; } 22 cout<< Enter 5 scores:\n ; for(i=0; i<5; i++) cin >> score[i]; min = score[0]; Output : for(i=1; i<5; i++) { if(score[i] < min ) min = score[i]; } cout<< The lowest score is << min; Enter 5 scores: The minimum score is 55 22

23 OPERATIONS ON ARRAY : MORE EXAMPLES 23

24 EXCHANGING VALUES IN 1-D ARRAY : THE INCORRECT WAY Problem: To exchange the values between number[1] and number[3] 24 24

25 THE CORRECT WAY - USING TEMPORARY VARIABLE 25 25

26 EXAMPLE : EXCHANGING VALUES #include <iostream> using namespace std; int main() { int temp; int numbers[5] = {3,7,12,24,45}; cout << "Before exchange the value :"<< endl; for(int i=0; i<5; i++) cout << numbers[i] << ' '; temp = numbers[3]; numbers[3] = numbers[1]; numbers[1] = temp; cout<<"\nafter exchange the value :"<<endl; for(int i=0; i<5; i++) cout << numbers[i] << ' '; } cout << endl; return 0;

27 TWO-DIMENSIONAL ARRAYS A two dimensional array stores a fixed number of data as a collection of rows and columns. Sometimes called matrices or tables. Each element of a two-dimensional array has a row position and a column position. The declaration of a two-dimensional array requires a row size and a column size. Declaration syntax: data_type array_name[row_size][column_size]; o where row_size and column_size are expressions with positive integer values, and specify the number of rows and the number of columns, respectively, in the array. o Note :number of elements in the array = row_size X column_size

28 DECLARATION OF 2-D ARRAY First dimension specifies the number of rows in the array Second dimension specifies the number of columns in each row A consecutive block of 50 (row size X column size) memory locations are allocated

29 2-D ARRAY : DECLARATION AND MEMORY LAYOUT The statement int table [5][4]; means array table is an array of one-dimensional array (two-dimensional array) COLUMN SUBSCRIPT [0] [0] [1] [2] [3] [1] ROW SUBSCRIPT [2] [3] [4] Array_name

30 30 30 MEMORY LAYOUT OF 2-D ARRAY

31 EXAMPLE: DECLARATION OF 2-D ARRAY int data[2][3]; row column [ 0 ] [ 1 ] [ 0 ] [ 1 ] [ 2 ]?????? data 31

32 2-D ARRAY: DECLARATION AND INITIALIZATION Two-dimensional arrays can be initialized when they are declared: Elements of each row are enclosed within braces and separated by commas. All rows are enclosed within braces. For number arrays, if all elements of a row aren t specified, unspecified ones are set to

33 2-D ARRAY: DECLARATION AND INITIALIZATION double t[2][2] = { {0.1,0.2}, {1.1, 1.2} }; [ 0 ] [ 1 ] [ 0 ] [ 1 ] t 33 33

34 2-D ARRAY: DECLARATION AND INITIALIZATION Initialization : Some variations int table [5][4] = {0,1,2,3,10,11,12,13,20,21,22,23,30,31,32, 31,40,41,42,43}; int table [5][4] = {{0,1,2,3},{10,11,12,13},{20,21,22,23}, {30,31,32,31},{40,41,42,43}}; int table [5][4] = {{0,1},{10},{20,21,22},{30},{40,41,42,43}}; 34 34

35 2-D ARRAY : ACCESSING ARRAY ELEMENTS The name of the array holds the address of the first element of the array. To access an element in a two-dimensional array, you must specify the name of the array followed by: a row subscript, and a column subscript General format to access a 2-D array element: array_name[row_subscript][column_subscript] Nested for loops are often used when accessing elements of a two-dimensional array

36 EXAMPLE : ASSIGNING DATA VALUE TO ARRAY ELEMENT. 36

37 EXAMPLE : ASSIGNING DATA VALUE TO ARRAY ELEMENTS. Let us assume that we want to assign our 5 x 4 array as shown below The following code will perform the assignment : int table[5][4]; for(int r=0;r<5;r++) for(int c=0;c<4;c++) table[r][c]= r * 10 + c ; 37 37

38 EXAMPLE : ASSIGNING DATA VALUE TO ARRAY ELEMENTS. To assign the fifth row to 0: To assign the entire matrix to 0: 38 38

39 EXAMPLE : ASSIGNING DATA VALUE TO ARRAY ELEMENTS. int v[rsize][csize]; for (int i=0; i<rsize; ++i) //every row for (int j=0; j<csize; ++j )//every col v[i][j] = i+j; [ 1 ] [ 2 ] [ 0 ] [ 1 ] [ 2 ] V

40 EXAMPLE : INPUT DATA VALUES TO 2-D ARRAY ELEMENTS. double table[rsize][csize]; for (int i=0; i<rsize; ++i) //every row for (int j=0; j<csize; ++j )//every col cin >> table[i][j]; for(int r=0;r<5;r++) for(int c=0;c<4;c++) cin >> table[r][c]; 40 40

41 COMPUTATION AND OUTPUT The for statement is useful when performing computations on arrays. The index variable of the for statement can be used as an array subscript. Computations can be easily performed on an entire two-dimensional array, or on a sub set of the array

42 COMPUTATIONS Compute the average of an array with n rows and m columns. for(int i=0; i<n; ++i) { for(int j=0; j<m; ++j) { sum += array[i][j]; } } average = sum/(n*m); 42 42

43 COMPUTATIONS Compute the average of the nth row of a twodimensional array with r rows and c columns. for(int col=0; col < c; ++col) { sum += array[n][col]; } rowaverage = sum/c; 43 43

44 OUTPUT Two dimensional arrays are often printed in a row by row format, using nested for statements. When printing the row values of an array, be sure to print: whitespace (blank space) between the values in a row. a newline character at the end of each row

45 DISPLAYING DATA VALUE OF ARRAY ELEMENTS. int table [5][4] = {{0,1,2,3},{10,11,12,13},{20,21,22,23}, {30,31,32,31},{40,41,42,43}}; for(int r=0;r<5;r++) { for(int c=0;c<4;c++) cout << table[r][c] << ; cout << endl; } 45 45

46 #include <iostream> #include <iomanip> using namespace std; EXAMPLE: TWO DIMENSIONAL ARRAY int main() { int i,j, a[4][3]; for(i = 0; i < 4; i++) { cout << "Enter 3 numbers for row " << i+1 << " : "; for(j = 0; j < 3; j++) cin >> a[i][j]; cout << endl; } cout<<"elements of array are : "<<endl; 46 } for(i = 0; i < 4; i++) { for(j = 0; j < 3; j++) cout << setw(5) << a[i][j] ; cout << endl; } return 0; 46

47 EXERCISES 1) Find the output of the following codes: int row = 3; int col = 3; int mark[row][col] = {{1,2,3}, {4,5,6}, {7,8,9}}; cout<<mark[1][1]<<endl; cout<<mark[2][1]<<endl; cout<<mark[3][3]<<endl;

48 2) Find the output of these codes: 48 #define Row 3 #define Col 3 int main() { int i, j; int xyz[row][col] = {{1,2,3}, {4,5,6},{7,8,9}}; for (i = 0; i < Row; i++) for (j = 0; j < Col; j++) cout << xyz[i][j]<<endl; return 0; } // end main

49 3) Problem : Write a program to enter five names with the maximum length of each name is 25 characters. The program then displays the name in the reverse order of entry. This means that the last name will be displayed first, followed by the fourth, and so on. - END - 49