Lecture_05

Multiple-Subscripted Arrays
Multiple subscripts
a[ i ][ j ]
Tables with rows and columns
Specify row, then column
“Array of arrays”
a[0] is an array of 4 elements
a[0][0] is the first element of that array
Row subscript
Array name
Column subscript

Multiple-Subscripted Arrays
To initialize
Default of 0
Initializers grouped by row in braces
int b[ 2 ][ 2 ] = { { 1, 2 }, { 3, 4 } };
int b[ 2 ][ 2 ] = { { 1 }, { 3, 4 } };
Row 0
Row 1

Multiple-Subscripted Arrays
Referenced like normal
cout << b[ 0 ][ 1 ];
Outputs 0
Cannot reference using commas
cout << b[ 0, 1 ];
Syntax error

1 // Fig. 3.22: fig04_22.cpp
2 // Initializing multidimensional arrays.
3 #include
4
5 using std::cout;
6 using std::endl;
7
8 void printArray( int [][ 3 ] );
9
10 int main()
11 {
12 int array1[ 2 ][ 3 ] = { { 1, 2, 3 }, { 4, 5, 6 } };
13 int array2[ 2 ][ 3 ] = { 1, 2, 3, 4, 5 };
14 int array3[ 2 ][ 3 ] = { { 1, 2 }, { 4 } };
15
16 cout << "Values in array1 by row are:" << endl;
17 printArray( array1 );
18
19 cout << "Values in array2 by row are:" << endl;
20 printArray( array2 );
21
22 cout << "Values in array3 by row are:" << endl;
23 printArray( array3 );
24
25 return 0; // indicates successful termination
26
27 } // end main

28
29 // function to output array with two rows and three columns
30 void printArray( int a[][ 3 ] )
31 {
32 for ( int i = 0; i < 2; i++ ) { // for each row
33
34 for ( int j = 0; j < 3; j++ ) // output column values
35 cout << a[ i ][ j ] << ' ';
36
37 cout << endl; // start new line of output
38
39 } // end outer for structure
40
41 } // end function printArray
Values in array1 by row are:
1 2 3
4 5 6
Values in array2 by row are:
1 2 3
4 5 0
Values in array3 by row are:
1 2 0
4 0 0

Multiple-Subscripted Arrays
Next: program showing initialization
After, program to keep track of students grades
Multiple-subscripted array (table)
Rows are students
Columns are grades

1 // Fig. 3.23: fig04_23.cpp
2 // Double-subscripted array example.
3 #include
4
5 using std::cout;
6 using std::endl;
7 using std::fixed;
8 using std::left;
9
10 #include
11
12 using std::setw;
13 using std::setprecision;
14
15 const int students = 3; // number of students
16 const int exams = 4; // number of exams
17
18 // function prototypes
19 int minimum( int [][ exams ], int, int );
20 int maximum( int [][ exams ], int, int );
21 double average( int [], int );
22 void printArray( int [][ exams ], int, int );
23

24 int main()
25 {
26 // initialize student grades for three students (rows)
27 int studentGrades[ students ][ exams ] =
28 { { 77, 68, 86, 73 },
29 { 96, 87, 89, 78 },
30 { 70, 90, 86, 81 } };
31
32 // output array studentGrades
33 cout << "The array is:\n";
34 printArray( studentGrades, students, exams );
35
36 // determine smallest and largest grade values
37 cout << "\n\nLowest grade: "
38 << minimum( studentGrades, students, exams )
39 << "\nHighest grade: "
40 << maximum( studentGrades, students, exams ) << '\n';
41
42 cout << fixed << setprecision( 2 );
43

44 // calculate average grade for each student
45 for ( int person = 0; person < students; person++ )
46 cout << "The average grade for student " << person
47 << " is "
48 << average( studentGrades[ person ], exams )
49 << endl;
50
51 return 0; // indicates successful termination
52
53 } // end main
54
55 // find minimum grade
56 int minimum( int grades[][ exams ], int pupils, int tests )
57 {
58 int lowGrade = 100; // initialize to highest possible grade
59
60 for ( int i = 0; i < pupils; i++ )
61
62 for ( int j = 0; j < tests; j++ )
63
64 if ( grades[ i ][ j ] < lowGrade )
65 lowGrade = grades[ i ][ j ];
66
67 return lowGrade;
68
69 } // end function minimum

70
71 // find maximum grade
72 int maximum( int grades[][ exams ], int pupils, int tests )
73 {
74 int highGrade = 0; // initialize to lowest possible grade
75
76 for ( int i = 0; i < pupils; i++ )
77
78 for ( int j = 0; j < tests; j++ )
79
80 if ( grades[ i ][ j ] > highGrade )
81 highGrade = grades[ i ][ j ];
82
83 return highGrade;
84
85 } // end function maximum
86

87 // determine average grade for particular student
88 double average( int setOfGrades[], int tests )
89 {
90 int total = 0;
91
92 // total all grades for one student
93 for ( int i = 0; i < tests; i++ )
94 total += setOfGrades[ i ];
95
96 return static_cast< double >( total ) / tests; // average
97
98 } // end function maximum

99
100 // Print the array
101 void printArray( int grades[][ exams ], int pupils, int tests )
102 {
103 // set left justification and output column heads
104 cout << left << " [0] [1] [2] [3]";
105
106 // output grades in tabular format
107 for ( int i = 0; i < pupils; i++ ) {
108
109 // output label for row
110 cout << "\nstudentGrades[" << i << "] ";
111
112 // output one grades for one student
113 for ( int j = 0; j < tests; j++ )
114 cout << setw( 5 ) << grades[ i ][ j ];
115
116 } // end outer for
117
118 } // end function printArray

The array is:
[0] [1] [2] [3]
studentGrades[0] 77 68 86 73
studentGrades[1] 96 87 89 78
studentGrades[2] 70 90 86 81
Lowest grade: 68
Highest grade: 96
The average grade for student 0 is 76.00
The average grade for student 1 is 87.50
The average grade for student 2 is 81.75

“Well, I’ll eat it,” said Alice, “and if it makes me grow larger, I
can reach the key; and if it makes me grow smaller, I can creep
under the door; so either way I’ll get into the garden.”
Lewis Carroll, Alice’s Adventures in Wonderland

VECTORS
Array
cannot change the length
Vector
the same purpose as arrays
except can change length while the program is running
Like an array, a vector has a base type, and like an array, a vector stores a collection of values of its base type.

Vectors
Library:
#include
Declaration:
vector
name;
Example:
vector
v;
vector
v(10);

Vectors
To add an element to a vector for the first time, you normally use the member function push_back.
Example:

Vectors
The number of elements in a vector is called the size of the vector.
The member function size can be used to determine how many elements are in a vector.
Example:

// Demonstrating C++ Standard Library class template vector.

Two / Three / Multi Dimensioned arrays using vector
A two dimensional array is a vector of vectors.
The vector contructor can initialize the length of the array and set the initial value.
Example of a vector of vectors to represent a two dimensional array:
vector< vector
> vI2Matrix(3, vector
(2,0));

Loop by index:
0
1
10
11
20
21

Two / Three / Multi Dimensioned arrays using vector
A three dimensional vector would be declared as:

Quiz
1. What is vector?
Benefits of vector over array.
Difference between array and vector.
Which one is better for what purpose?
2. Write a program that create a 2-dimensional vector vec1.
initialize all elements of a vector to 1.
then output content of the vector vec1.