1. C++ Programming Lecture 16 Arrays – Part IIIBy
Ghada Al-Mashaqbeh
The Hashemite University
Computer Engineering Department

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Outline
Introduction.
Searching arrays:
Linear search.
Binary search.
Multisubscripted arrays.
Passing Multisubscripted arrays to functions.
Examples.
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Introduction
In this lecture we will continue with specific topics related to arrays.
We will explore how to search an array for some value.
The result of the search is the location (i.e. index or subscript) of the element that has an equal value to the one that we are searching for.
The second part of our lecture introduce multi-subscripted arrays which correspond to multidimensional matrices.
We will se how to define, initialize, and pass such arrays to functions.
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Searching Arrays I
Search array for a key value and return the subscript of the location in the array.
Two types:
Linear search.
Binary search.
Linear search
Compare each element of array with key value till you find the key.
Useful for small and unsorted arrays.
The best case is to find the key in the first element of the array. So, you need 1 comparison or one step.
The worst case is to find the key in the last element of the array. So, you need arraySize comparisons or steps.
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Searching Arrays II
Binary search
Can only be used on sorted arrays.
Compares middle element with key
If equal, match found
If key < middle, repeat search through the first half of the array
If key > middle, repeat search through the last half of the array
Very fast; at most n steps, where 2n is the smallest number larger than the number of elements in the array.
E.g.: 30 element array takes at most 5 steps
25 > 30
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6. Linear and Binary Search Code
On board.
Found in Figure 4.20 in the textbook.
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7. Multiple-Subscripted Arrays I
Column subscript
Row 0
Row 1
Row 2
Column 0
Column 1
Column 2
Column 3
a[ 0 ][ 0 ]
a[ 1 ][ 0 ]
a[ 2 ][ 0 ]
a[ 0 ][ 1 ]
a[ 1 ][ 1 ]
a[ 2 ][ 1 ]
a[ 0 ][ 2 ]
a[ 1 ][ 2 ]
a[ 2 ][ 2 ]
a[ 0 ][ 3 ]
a[ 1 ][ 3 ]
a[ 2 ][ 3 ]
Row subscript
Array name
Multiple subscripts - tables with rows, columns
Like matrices: specify row, then column.
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8. Multiple-Subscripted Arrays II
Referenced like normal
cout << b[ 0 ][ 1 ];
Will output the value of 0
Cannot reference with commas
cout << b( 0, 1 );
Will try to call function b, causing a syntax error
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9. Multiple-Subscripted Arrays III
Initialization:
Using two nested for loops.
Using cin to get values from the keyboard.
Using initializers list: Initializers grouped by row in braces
int b[ 2 ][ 2 ] = { { 1, 2 }, { 3, 4 } };
int b[ 2 ][ 2 ] = { { 1 }, { 3, 4 } };
1 2
3 4
1 0
3 4
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10. Multiple-Subscripted Arrays IV
Try the following on your computer and see what is the results of the following initializations and whether there is a syntax error or not.
int b[ 2 ][ 2 ] = { 1, 2, 5 }; //Correct
int b[][] = {{1,3},{5},{6,7,8}}; //Syntax Error
int b[ 2 ][] = { 1, 2, 5 };//Syntax Error
int b[][ 2 ] = { 1, 2, 5 }; //Correct
int b[][ 2 ][4] = { 1, 2, 5 }; //Correct
int b[][ 2 ][] = { 1, 2, 5 }; // Syntax Error
int b[][] = { 1, 2, 5 };//Syntax Error
Note that only the first subscript (or dimension size) is allowed to be empty.
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11. Passing Multiple Subscripted Arrays to Functions
The same as one-dimensional array with the exception that you are allowed to have the first dimension only empty, the rest of the dimensions must have their actual size in its square brackets in both the prototype and the function definition.
If you use a variable to denote the size of a dimension in a prototype, you must have this variable as global.
Again, arrays are passed as call-by-reference by default.
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1 // Fig. 4.23: fig04_23.cpp
2 // Double-subscripted array example
3 #include <iostream> 4
5 using std::cout;
6 using std::endl;
7 using std::ios; 8
9 #include <iomanip> 10
11 using std::setw;
12 using std::setiosflags;
13 using std::setprecision; 14
15 const int students = 3; // number of students
16 const int exams = 4; // number of exams 17
18 int minimum( int [][ exams ], int, int );
19 int maximum( int [][ exams ], int, int );
20 double average( int [], int );
21 void printArray( int [][ exams ], int, int ); 22
23 int main()
24 {
25 int studentGrades[ students ][ exams ] =
{ { 77, 68, 86, 73 },
{ 96, 87, 89, 78 },
{ 70, 90, 86, 81 } }; 29
30 cout << "The array is:\n";
31 printArray( studentGrades, students, exams );
32 cout << "\n\nLowest grade: "
<< minimum( studentGrades, students, exams )
Each row is a particular student, each column is the grades on the exam.
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<< "\nHighest grade: "
<< maximum( studentGrades, students, exams ) << '\n'; 36
37 for ( int person = 0; person < students; person++ )
cout << "The average grade for student " << person << " is "
<< setiosflags( ios::fixed | ios::showpoint )
<< setprecision( 2 )
<< average( studentGrades[ person ], exams ) << endl; 42
43 return 0;
44 } 45
46 // Find the minimum grade
47 int minimum( int grades[][ exams ], int pupils, int tests )
48 {
49 int lowGrade = 100; 50
51 for ( int i = 0; i < pupils; i++ ) 52
for ( int j = 0; j < tests; j++ ) 54
if ( grades[ i ][ j ] < lowGrade )
lowGrade = grades[ i ][ j ]; 57
58 return lowGrade;
59 } 60
61 // Find the maximum grade
62 int maximum( int grades[][ exams ], int pupils, int tests )
63 {
64 int highGrade = 0; 65
66 for ( int i = 0; i < pupils; i++ )
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for ( int j = 0; j < tests; j++ ) 69
if ( grades[ i ][ j ] > highGrade )
highGrade = grades[ i ][ j ]; 72
73 return highGrade;
74 } 75
76 // Determine the average grade for a particular student
77 double average( int setOfGrades[], int tests )
78 {
79 int total = 0; 80
81 for ( int i = 0; i < tests; i++ )
total += setOfGrades[ i ]; 83
84 return static_cast< double >( total ) / tests;
85 } 86
87 // Print the array
88 void printArray( int grades[][ exams ], int pupils, int tests )
89 {
90 cout << " [0] [1] [2] [3]"; 91
92 for ( int i = 0; i < pupils; i++ ) {
cout << "\nstudentGrades[" << i << "] "; 94
for ( int j = 0; j < tests; j++ )
cout << setiosflags( ios::left ) << setw( 5 )
<< grades[ i ][ j ];
98 }
99 }
3. Define functions
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The array is:
[0] [1] [2] [3]
studentGrades[0]
studentGrades[1]
studentGrades[2]
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
Program Output
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Additional Notes
This lecture covers the following material from the textbook:
Fourth Edition:
Chapter 4: Sections 4.8 and 4.9
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