Arrays
Multi-dimensional
initialize & display
Sample programs
Sorting
Searching
Part II
Multidimensional Arrays
a
everything about one dimensional arrays
applies *
a
all elements of the same data type
a
just need additional sets of [ ]
a
a 3-D array has rows, columns, and rank
* Except leaving the size out of the formal parameter
Parallel Arrays
Used when related data is of different data types.
grade
% of class
A
28
B
40
C
29
D
9
F
14
parallel arrays = two or more arrays in which elements
with corresponding indexes are related
We WILL do this in the next lab
*
Parallel Arrays
for(row…
for(col…
{
cout << “Enter id#”;
cin >> id[row][col];
cout << “Enter grade”;
cin >> grade[row][col];
}
OR
for(row…
for(col…
{
}
cout << “Enter id and grade”;
cin >> id[row][col] >> grade[row][col];
Sum a Row
void main(void)
{
double nums [3][4] = {1, 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12};
double sumRow(double [3] [4]); // prototype
cout << “The sum of row 3 is “
<< sumRow(nums)<<endl; //function call
}
Sum a Row
double sumRow(double ary[3][4])
{
int col;
double total=0;
for(col = 0; col < 4; col++)
ary[ ][col];
total += ary[2][col];
//enter row # -1
return total;
}
output
The sum of row 3 is 42
Sum a Column
void main(void)
{
double nums [3][4] = {1, 2, 3, 4, 5, 6, 7, 8,
9, 10, 11, 12};
double sumCol(double [3] [4]); // prototype
cout << “The sum of column 3 is “
<< sumCol(nums) << endl; //function call
}
Sum a Column
double sumCol(double ary[3][4])
{
int row;
double total=0;
for(row = 0; row<3; row++)
ary[row][ ]; //enter col # -1
total += ary[row][2];
return total;
}
output
The sum of column 3 is 21
Array Review -1
Write a C++ program that adds equivalent
elements of the two-dimensional arrays named
first and second. Both arrays should have two
rows and three columns. For example, element
[1][2] of the resulting array should be the sum of
first[1][2] and second[1][2].
first
16 18 23
54 91 11
sum
second
40 70 100 24 52 77
70 110
70 16 19 59
*
Sorting
Internal Sorts [for small data sets]
bubble (exchange)
selection
External Sorts [for large data sets]
Bubble Sort
21 13 9 25 17
Put smaller first
13 21 9 25 17
Put smaller first
13 9 21 25 17
No change
13 9 21 25 17
Put smaller first
Bubble Sort
13 9 21 17 25
Begin again and put
smaller first
9 13 21 17 25
No change
9 13 21 17 25
Put smaller first
9 13 17 21 25
A Bubble Sort Function
void bubble_sort(int array[ ], int length)
{
int j, k, flag=1, temp;
for(j=1; j<=length && flag; j++)
{
flag=0;
// false
for(k=0; k < (length-j); k++)
{
if (array[k+1] > array[k]) // > low to high
{
temp=array[k+1];
// swap
array[k+1]= array[k];
array[k]=temp;
flag=1;
// indicates a swap
} } } }
// has occurred
Selection Sort
index (k) sm_index
21 13 9 15 17
0
2
swap 21, 9
9 13 21 15 17
1
1
swap 13, 13
9 13 21 15 17
2
3
swap 21, 15
9 13 15 21 17
3
4
swap 21, 17
9 13 15 17 21
Selection Sort
void sort(double [5]);
void swap(double [5], int, int);
// prototypes
void main(void)
{ int index;
double my_list[ ] = {21, 13, 9, 15, 17};
cout << "\nThe unsorted array is: \n";
for(index=0; index<5; index++)
cout << “ “ << my_list[index] << endl;
sort(my_list);
// function call
cout << "\nThe sorted array is: \n";
for(index=0; index<5; index++)
cout << “ “ << my_list[index] << endl;
}
Selection Sort
void sort(double testArray[5])
{ int n, k, sm_index, pass=0;
double smallest;
for(k=0; k<4; k++)
// size-1 = number of passes
{ smallest = testArray[k];
sm_index = k;
for(n=k+1; n<5; n++) // size = # elem. to look at
if(testArray[n] < smallest)
{
smallest = testArray[n];
sm_index = n;
}
swap(testArray, sm_index, k); // call to swap()
}
}
Selection Sort
void swap(double testArray[5], int smaller, int
position)
{
// position = current position: k
double temp;
temp = testArray[position];
testArray[position] = testArray[smaller];
testArray[smaller] = temp;
}
Linear Search Pseudocode
For all the items in the list
Compare the item with the desired item
If the item was found
Return the index value of the current item
(the position of the element in the array)
End If
End For
Return -1 because the item was not found
The “Classic” Linear Search Function
int LinearSearch(int list[], int size, int key)
{
// or “Sequential search”
int i;
for (i = 0; i < size; i++)
{
if (list[i] = = key)
return i; // return location of element
//this will terminate the loop
}
return -1;
// element not in list
}
Main for linear search
int main() {
int LinearSearch(int [], int, int); // prototype
const int NUMEL = 10;
int nums[NUMEL] = {22,5,67,98,45,32,101,99,73,10};
int item, location;
cout << "\nEnter the item you are searching for: ";
cin >> item;
location = LinearSearch(nums, NUMEL, item);
if (location > -1)
cout << "The item was found at index location "
<< location << endl;
else cout << "The item was not found in the list\n";
return 0; }
Binary Search
List must be in sorted order
The function looks at the midpoint
If the midpoint is the item, return the location
Else, determine if the item is less than or greater
than the midpoint
Eliminate the side where the item cannot be, and
declare a new midpoint
Go again. (A recursive function)
Half the list is eliminated on each pass
int BinarySearch(int list[], int size, int key)
{ int left, right, midpt;
// list must be in sorted order
left = 0;
// left end is the first element
right = size - 1;
// right is the last element
while (left <= right){
midpt = (int) ((left + right) / 2); // why integer division?
if (key == list[midpt])
return midpt; // if found, the key will be the midpoint
else if (key > list[midpt])
// eliminate the left half
left = midpt + 1;
else right = midpt - 1; // else - eliminate the right half
}
// end while
return -1;}
// the key was never found
Main for binary search
int main() {
int BinarySearch(int [], int, int); // prototype
const int NUMEL = 10;
int nums[NUMEL] = {5,10,22,32,45,67,73,98,99,101};
int item, location;
cout << "\nEnter the item you are searching for: ";
cin >> item;
location = BinarySearch(nums, NUMEL, item);
if (location > -1)
cout << "The item was found at index location "
<< location << endl;
else cout << "The item was not found in the list\n";
return 0; }
Array Review
a
is an ordered sequence of data
of the same type
a
can be of any valid data type
a
can be 1-, 2-, or multi- dimensional
a
must be declared before used
a
can be assigned and initialized
a
element numbering starts at zero
Array Review
a
use for loops to access
(nested for multidimentional)
a
can be passed back and forth between
functions
when sent to functions the actual
values are manipulated - not a copy
(passed by reference)
a
Array Review - 6a
There is an array of three students each with
four exam scores. Assume the scores are
known and are: {77, 68, 86, 73}, {96, 87,
89, 78}, {70, 90, 86, 81}. Create a program
which will display the lowest grade, the
highest grade and the average of the grades
to two decimal places.
Array Review - 6b
#include <iostream>
#include <iomanip.>
using namespace std;
const int STUDENTS = 3; // global, so main and all
const int EXAMS = 4;
// functions can access
int mini (int [][EXAMS], int, int);
int maxi (int [][EXAMS], int, int);
float average(int [], int);
void printArray(int [][EXAMS], int, int);
Array Review - 6c
void main(void)
{ int studentGrades[STUDENTS][EXAMS] =
{{77, 68, 86, 73}, {96, 87, 89, 78}, {70, 90, 86, 81}};
cout << "The array is:" << endl;
printArray(studentGrades, STUDENTS, EXAMS);
cout<<endl<<endl<<"Lowest grade: "
<< mini (studentGrades, STUDENTS, EXAMS)
<< endl << "Highest grade: "
<< maxi (studentGrades, STUDENTS, EXAMS)<<endl;
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;
}
Array Review - 6d
int mini(int grades[][EXAMS],
int maxi(int grades[][EXAMS],
int pupils, int tests)
int pupils, int tests)
{
{
int lowGrade = 100;
int highGrade = 0;
for (int i = 0; i < pupils; i++)
for (int i = 0; i < pupils; i++)
for (int j = 0; j < tests; j++)
for (int j = 0; j < tests; j++)
if (grades[i][j] <
if (grades[i][j] > highGrade)
lowGrade)
highGrade = grades[i][j];
lowGrade = grades[i][j];
return highGrade;
return lowGrade;
}
}
Array Review - 6e
float average(int setOfGrades[],void printArray(int grades[][EXAMS],
int tests)
int pupils, int tests)
{
{
int total = 0;
cout << "
[0] [1] [2] [3]";
for (int i = 0; i < tests; i++)
for (int i = 0; i < pupils; i++) {
cout << endl<< "studentGrades["
total += setOfGrades[i];
<< i << "] ";
return (float) total / tests;
}
for (int j = 0; j < tests; j++)
cout << setiosflags(ios::left)
<< setw(5)
<< grades[i][j];
}
Array Review - 6f
function call:
average(studentGrades[person], EXAMS)
float average(int setOfGrades[], int tests)
{ int total = 0;
for (int i = 0; i < tests; i++)
total += setOfGrades[i];
}
return total / tests;
Common Errors

Not declaring the array

First element is called zero; last element
is one less than the number of elements

Out of range subscripts - no warning

Error in the for loop - check the counter

Not initializing the array
Common Errors

Aggregate operations not allowed

Omitting array size - permitted only
when declared as a formal parameter
initialized in the declaration

If array is /* in */ only, declare the
formal parameter as const to prevent
accidental modification
Debugging
array subscripts
recheck array size in declaration,
initialization, and for loops
Prevention - plan first!
Valuation tables
Display values with cout
C++ Debugger
End Note
I really hate this darn machine,
I wish they would sell it.
It never does quite what I want,
But only what I tell it.