2003 Prentice Hall, Inc. All rights reserved. 1 Arrays Outline Introduction Arrays Declaring Arrays...
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Transcript of 2003 Prentice Hall, Inc. All rights reserved. 1 Arrays Outline Introduction Arrays Declaring Arrays...
2003 Prentice Hall, Inc. All rights reserved.
1
Arrays
OutlineIntroductionArraysDeclaring ArraysExamples Using Arrays
2003 Prentice Hall, Inc. All rights reserved.
2
Introduction
• Arrays– Structures (collection) of related data items
(same type data items)– Static entity (maintain the same size throughout
program execution)
2003 Prentice Hall, Inc. All rights reserved.
3
Arrays
• Array– Consecutive group of memory locations – Same name and type (int, char, etc.)
• To refer to an element– Specify array name and position number (index)– Format: arrayname[ position_number ]– An Array index must be an integer– First element at position 0
• N-element array cc[ 0 ], c[ 1 ] … c[ n - 1 ]
– Nth element as position N-1
2003 Prentice Hall, Inc. All rights reserved.
4
Arrays
• Array elements like other variables– Assignment, printing for an integer array c
c[ 0 ] = 3;
cout << c[ 0 ];
• Can perform operations inside subscriptc[ 5 – 2 ] same as c[3]
int a = 6, b= 5;
c[ a + b ] += 2;
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5
Arrays
c[6]
-45
6
0
72
1543
-89
0
62
-3
1
6453
78
Name of array (Note that all elements of this array have the same name, c)
c[0]
c[1]
c[2]
c[3]
c[11]
c[10]
c[9]
c[8]
c[7]
c[5]
c[4]
Position number of the element within array c
Value
2003 Prentice Hall, Inc. All rights reserved.
6
Declaring Arrays
• When declaring arrays, specify– Name– Type of array
• Any data type
– Number of elements– type arrayName[ arraySize ];
int c[ 10 ]; // array of 10 integers
float d[ 3284 ]; // array of 3284 floats
– arraySize: MUST be a positive integer
• Declaring multiple arrays of same type– Use comma separated list, like regular variables
int b[ 100 ], x[ 27 ];
2003 Prentice Hall, Inc. All rights reserved.
7
Declaring Arrays
• Array of type char can be used to store a character string– string objects have so far been used to store
character strings– Arrays can do the same thing (more later)
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8
Examples Using Arrays
• Initializing arrays – For loop
• Set each element
– Initializer list• Specify each element when array declaredint n[ 5 ] = { 1, 2, 3, 4, 5 }; • If not enough initializers rightmost elements 0• If too many syntax error
– To set every element to same valueint n[ 5 ] = { 0 };
– If array size omitted initializers determine sizeint n[] = { 1, 2, 3, 4, 5 };
• 5 initializers, therefore 5 element array
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Outline9
1 // Fig. 7.3: fig07_03.cpp2 // Initializing an array.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 int main()13 {14 int n[ 10 ]; // n is an array of 10 integers15 16 // initialize elements of array n to 0 17 for ( int i = 0; i < 10; i++ ) 18 n[ i ] = 0; // set element at location i to 019 20 cout << "Element" << setw( 13 ) << "Value" << endl;21 22 // output contents of array n in tabular format 23 for ( int j = 0; j < 10; j++ ) 24 cout << setw( 7 ) << j << setw( 13 ) << n[ j ] << endl;25
Declare a 10-element array of integers.
Initialize array to 0 using a for loop. Note that the array has elements n[0] to n[9].
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Outline10
26 return 0; // indicates successful termination27 28 } // end main
Element Value
0 0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0
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Outline11
1 // Fig. 7.4: fig07_04.cpp2 // Initializing an array with a declaration.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 int main()13 {14 // use initializer list to initialize array n 15 int n[ 10 ] = { 32, 27, 64, 18, 95, 14, 90, 70, 60, 37 };16 17 cout << "Element" << setw( 13 ) << "Value" << endl;18 19 // output contents of array n in tabular format20 for ( int i = 0; i < 10; i++ )21 cout << setw( 7 ) << i << setw( 13 ) << n[ i ] << endl;22 23 return 0; // indicates successful termination24 25 } // end main
Note the use of the initializer list.
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Outline12
Element Value
0 32
1 27
2 64
3 18
4 95
5 14
6 90
7 70
8 60
9 37
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13
Examples Using Arrays
• Array size– Can be specified with constant variable (const)
• const int size = 20;
– Constants cannot be changed– Constants must be initialized when declared– Also called named constants or read-only
variables
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Outline14
1 // Fig. 7.5: fig07_05.cpp2 // Initialize array s to the even integers from 2 to 20.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 int main()13 {14 // constant variable can be used to specify array size15 const int arraySize = 10;16 17 int s[ arraySize ]; // array s has 10 elements18 19 for ( int i = 0; i < arraySize; i++ ) // set the values20 s[ i ] = 2 + 2 * i; 21 22 cout << "Element" << setw( 13 ) << "Value" << endl;23
Note use of const keyword. Only const variables can specify array sizes.
The program becomes more scalable when we set the array size using a const variable. We can change arraySize, and all the loops will still work (otherwise, we’d have to update every loop in the program).
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Outline15
24 // output contents of array s in tabular format25 for ( int j = 0; j < arraySize; j++ ) 26 cout << setw( 7 ) << j << setw( 13 ) << s[ j ] << endl;27 28 return 0; // indicates successful termination29 30 } // end main
Element Value
0 2
1 4
2 6
3 8
4 10
5 12
6 14
7 16
8 18
9 20
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Outline16
1 // Fig. 7.6: fig07_06.cpp2 // Using a properly initialized constant variable.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 int main()9 {10 const int x = 7; // initialized constant variable11 12 cout << "The value of constant variable x is: "13 << x << endl;14 15 return 0; // indicates successful termination16 17 } // end main
The value of constant variable x is: 7
Proper initialization of const variable. (cannot be modified later)
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Outline17
1 // Fig. 7.7: fig07_07.cpp2 // A const object must be initialized.3 4 int main()5 {6 const int x; // Error: x must be initialized7 8 x = 7; // Error: cannot modify a const variable9 10 return 0; // indicates successful termination11 12 } // end main
d:\cpphtp7_examples\ch04\Fig07_07.cpp(8) : error C2166:
l-value specifies const object
Uninitialized const results in a syntax error. Attempting to modify the const is another error.
2003 Prentice Hall, Inc.All rights reserved.
Outline18
1 // Fig. 7.8: fig07_08.cpp2 // Compute the sum of the elements of the array.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 int main()9 {10 const int arraySize = 10;11 12 int a[ arraySize ] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };13 14 int total = 0;15 16 // sum contents of array a 17 for ( int i = 0; i < arraySize; i++ )18 total += a[ i ]; 19 20 cout << "Total of array element values is " << total << endl;21 22 return 0; // indicates successful termination23 24 } // end main
Total of array element values is 55
Summing the elements of an array
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19
Showing array data graphically
– Example: show distribution of grades of a specific class
– Grade distribution is stored in an array of 11 elements, each corresponds to a category of grade
• Example: – category 1: [0..9]– category 2: [10..19]– etc…
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Outline20
1 // Fig. 7.9: fig07_09.cpp2 // Histogram printing program.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 int main()13 {14 const int arraySize = 11;15 int n[ arraySize ] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 };16 17 cout << “Grade Distribution:" << endl; 18 19 // for each element of array n, output a bar in histogram20 for ( int i = 0; i < arraySize; i++ ) {21 // output bar labels (“0-9:”,…“90-99:”, “100:”) 22 if (i == 0)
23 cout << “0-9:”;24 else if (i == 10)25 cout << “100:”;26 else27 cout << i * 10 << “-” << (i * 10) + 9<< “: ”;
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Outline21
28 // print bar of asterisks29 for ( int stars = 0; stars < n[ i ]; stars++ )30 cout << '*'; 31 32 cout << endl; // start next line of output33 34 } // end outer for structure35 36 return 0; // indicates successful termination 37 } // end main
Grade distribution
0-9:
10-19:
20-29:
30-39:
40-49:
50-59:
60-69: *
70-79: * *
80-89: * * * *
90-99: * *
100: *
Number of students with grades in the range: 10*i--10*i+9.
2003 Prentice Hall, Inc.All rights reserved.
Outline22
1 // Fig. 7.10: fig07_10.cpp2 // Roll a six-sided die 6000000 times.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 #include <cstdlib>13 #include <ctime>14 15 int main()16 {17 const int arraySize = 7; // ignore element zero18 int frequency[ arraySize ] = { 0 };19 20 srand( time( 0 ) ); // seed random-number generator21 22 // roll die 6000000 times23 for ( int roll = 1; roll <= 6000000; roll++ ) 24 frequency[ 1 + rand() % 6 ]++; 25
Remake of old program to roll dice. An array is used instead of 6 regular variables, and the proper element can be updated easily (without needing a switch).
This creates a number between 1 and 6, which determines the index of frequency[] that should be incremented.
Using Elements of array as counters
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Outline23
26 27 cout << "Face" << setw( 13 ) << "Frequency" << endl;28 29 // output frequency elements 1-6 in tabular format30 for ( int face = 1; face < arraySize; face++ ) 31 cout << setw( 4 ) << face32 << setw( 13 ) << frequency[ face ] << endl;33 34 return 0; // indicates successful termination35 36 } // end main
Face Frequency
1 1000167
2 1000149
3 1000152
4 998748
5 999626
6 1001158
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24
Using arrays to summarize survey results
Forty students were asked to rate the quality of the food in the student cafeteria on a scale of 1 to 10 (1 meaning awful and 10 meaning excellent). Place the 40 responses in an integer array and summarize the results of the poll
2003 Prentice Hall, Inc.All rights reserved.
Outline25
1 // Fig. 7.11: fig07_11.cpp2 // Student poll program.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 #include <iomanip>9 10 using std::setw;11 12 int main()13 {14 // define array sizes15 const int responseSize = 40; // size of array responses16 const int frequencySize = 11; // size of array frequency17 18 // place survey responses in array responses19 const int responses[ responseSize ] = { 1, 2, 6, 4, 8, 5, 9, 7, 8,20 10, 1, 6, 3, 8, 6, 10, 3, 8, 2, 7, 6, 5, 7, 6, 8, 6, 7,21 5, 6, 6, 5, 6, 7, 5, 6, 4, 8, 6, 8, 10 };22 23 // initialize frequency counters to 024 int frequency[ frequencySize ] = { 0 };25
Array declared as const int this means its value cannot and should not change
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Outline26
26 // for each answer, select value of an element of array27 // responses and use that value as subscript in array28 // frequency to determine element to increment29 for ( int answer = 0; answer < responseSize; answer++ )30 frequency[ responses[answer] ]++;31 32 // display results33 cout << "Rating" << setw( 17 ) << "Frequency" << endl;34 35 // output frequencies in tabular format36 for ( int rating = 1; rating < frequencySize; rating++ )37 cout << setw( 6 ) << rating38 << setw( 17 ) << frequency[ rating ] << endl;39 40 return 0; // indicates successful termination41 42 } // end main
responses[answer] is the rating (from 1 to 10). This determines the index in frequency[] to increment.
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Outline27
Rating Frequency
1 2
2 2
3 2
4 2
5 5
6 11
7 5
8 7
9 1
10 3
NOTE: what happens when the data in responses contain an invalid value (e.g., 13)… C++ does not do a boundary check logic error happens (NOT SYNTAX ERROR)
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28
Using character arrays to store and manipulate strings
• string has been used to store character strings– A string such as “Hello” is an array of characters
• A character array can be declared and initialized as:– char string1[] = “first”;– The size of array string1 is not explicitly defined, but
it is determined by the compiler based on the length of the string.
– NOTE: • all strings represented by a character array ends with a
special character (the null character ‘\0’)• A character array representing a string should be
defined large enough to hold the number of characters in the string + the termination character
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29
Using character arrays to store and manipulate strings
• One could also use this method to declare strings:
char string1[] = {‘f’, ‘i’, ‘r’, ‘s’, ‘t’, ‘\0’};
If you don’t use ‘\0’, the array would simply represent an array of characters, not a string!
if you specify the size of string1 and the size of “list of initializers” is larger syntax error
• char arrays can be manipulated as other arrays, example:– string[0] = ‘f’;
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30
Using character arrays to store and manipulate strings
• Input from keyboardchar string2[ 20 ];
cin >> string2;
– Puts user input in string• Stops at first whitespace characterAdds null character
– If too much text entered, data written beyond array
• We want to avoid this
• Printing strings– cout << string2 << endl;
• Does not work for other array types
– Characters printed until null found
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Outline31
1 // Fig. 7.12: fig07_12.cpp2 // treating character arrays as strings.3 #include <iostream>4 using std::cout;5 using std::cin;6 using std::endl;78 int main()9 {10 char string1 [20]; // reserve 20 characters11 char string2 [] = “string literal”; // reserve 15 characters12 13 // read string from user into array string114 cout << “Enter the string “\hello there\”: ”;15 cin >> string1; // read “hello” [space terminates input]1617 // output strings18 cout << “string1 is: ” << string1 << “\nstring2 is: ” << string2; 19 cout << “\nstring1 with spaces between characters is:\n”;2021 // output characters until null character is reached22 for ( int i = 0; string1[ i ]!= ‘\0’; i++ )23 cout << string1[ i ] << ‘ ’;2425 cin >> string1; // reads “there”26 cout << “\nstring1 is: ” << string1 <<endl; 2728 return 0; // indicates successful termination 29 } // end main
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Outline32
Enter the string “hello there”: hello there
string1 is: hello
string2 is: string literal
string1 with spaces between characters is:
h e l l o
string1 is: there
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33
Static and automatic local variables
• Recall static storage – If static, local variables save values between
function calls– Visible only in function body– Can declare local arrays to be static
• Initialized to zerostatic int array[3];
• If not static– Created (and destroyed) in every function call
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Outline34
1 // Fig. 7.13: fig07_13.cpp2 // Static arrays are initialized to zero.3 #include <iostream>4 5 using std::cout;6 using std::endl;7 8 void staticArrayInit( void ); // function prototype9 void automaticArrayInit( void ); // function prototype10 11 int main()12 {13 cout << "First call to each function:\n";14 staticArrayInit();15 automaticArrayInit();16 17 cout << "\n\nSecond call to each function:\n";18 staticArrayInit();19 automaticArrayInit();20 cout << endl;21 22 return 0; // indicates successful termination23 24 } // end main25
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Outline35
26 // function to demonstrate a static local array27 void staticArrayInit( void )28 {29 // initializes elements to 0 first time function is called30 static int array1[ 3 ]; 31 32 cout << "\nValues on entering staticArrayInit:\n";33 34 // output contents of array135 for ( int i = 0; i < 3; i++ )36 cout << "array1[" << i << "] = " << array1[ i ] << " ";37 38 cout << "\nValues on exiting staticArrayInit:\n";39 40 // modify and output contents of array141 for ( int j = 0; j < 3; j++ )42 cout << "array1[" << j << "] = " 43 << ( array1[ j ] += 5 ) << " ";44 45 } // end function staticArrayInit46
Static array, initialized to zero on first function call.
Array data is changed; the modified values stay.
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Outline36
47 // function to demonstrate an automatic local array48 void automaticArrayInit( void )49 {50 // initializes elements each time function is called51 int array2[ 3 ] = { 1, 2, 3 };52 53 cout << "\n\nValues on entering automaticArrayInit:\n";54 55 // output contents of array256 for ( int i = 0; i < 3; i++ )57 cout << "array2[" << i << "] = " << array2[ i ] << " ";58 59 cout << "\nValues on exiting automaticArrayInit:\n";60 61 // modify and output contents of array262 for ( int j = 0; j < 3; j++ )63 cout << "array2[" << j << "] = " 64 << ( array2[ j ] += 5 ) << " ";65 66 } // end function automaticArrayInit
Automatic array, recreated with every function call.
Although the array is changed, it will be destroyed when the function exits and the changes will be lost.
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Outline37
First call to each function:
Values on entering staticArrayInit:
array1[0] = 0 array1[1] = 0 array1[2] = 0
Values on exiting staticArrayInit:
array1[0] = 5 array1[1] = 5 array1[2] = 5
Values on entering automaticArrayInit:
array2[0] = 1 array2[1] = 2 array2[2] = 3
Values on exiting automaticArrayInit:
array2[0] = 6 array2[1] = 7 array2[2] = 8
Second call to each function:
Values on entering staticArrayInit:
array1[0] = 5 array1[1] = 5 array1[2] = 5
Values on exiting staticArrayInit:
array1[0] = 10 array1[1] = 10 array1[2] = 10
Values on entering automaticArrayInit:
array2[0] = 1 array2[1] = 2 array2[2] = 3
Values on exiting automaticArrayInit:
array2[0] = 6 array2[1] = 7 array2[2] = 8