1

Arrays and Strings

Lecture: 25

19.09.2012

2

Design Problem Consider a program to calculate class

average

Why??

?

3

Add to Design Problem Now your client says, I need to ALSO

calculate and display “deviations” from the average

Describe why this will or will NOT work

4

Enter in the scores again Use 100 separate variables

» and cout and cin commands Read (then re-read) from a file The real answer …

Possible Solutions

Use arrays!!

5Simple vs Structured Data Types

Simple data type => data element contains a single value

Structured data type => a data element contains a collection of data values

x : 15 avg : 84.35 ch : ‘A’

scores : 85 79 92 57 68 80

name : ‘C’ ‘L’ ‘Y’ ‘D’ ‘E’

6

Arrays Arrays are Structured Data Types They have a means of accessing

individual components Values can be retrieved from and stored

in the structure

scores : 85 79 92 57 68 800 1 2 3 4 5

cout << scores[2];scores[0] = 100;

7

One Dimensional Array Structured collection of components

» All of the same type Structure given a single name Individual elements accessed by index

indicating relative position in collection Type of elements stored in an array can be

“just about” anything Index of an array must be an integer

8Use of Array for Our Problem

Store elements in array as read in Go back and access for deviations

Note declaration

9

Declaring Arrays Syntax:

Data_type Array_name [constant]; Note declaration from our example

Tells how many elements set aside

10

Declaring Arrays Example specifies an array…

» each element is an integer» there is space for 100 elements» the are numbered 0 through 99

scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

11Accessing Individual Components

Use the name of the array Followed by an integer expression

inside the square brackets [ ]

scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

max = scores[0];for (x = 0; x < 100; x++) if (scores[x] > max) max = scores[x];

Index can be:- constant- variable- expressionMUST be an integer

12

Out of Bounds Index What happens if …

C++ does NOT check for index out of range Possible to walk off into “far reaches” of

memory -- clobbers ...» other variable locations» .exe code » the operating system (??)

float f_list [50];

f_list [100] = 123.456;

13Initializing Arrays in Declarations

Possible to declare the size & initialize

Possible to omit size at declaration» Compiler figures out size of array

int results [5] = {14, 6, 23, 8, 12 }

float prices [ ] = { 2.41, 85.06, 19.95, 3.91 }

14

Arrays as Parameters This is one task that CAN be done to the

WHOLE array C++ always passes arrays by reference

15

Arrays as Parameters The name of the array is a pointer

constant The address of the array is passed to the

function Size of the

array alsopassed tocontrol loop

16

Arrays as Parameters Note the empty brackets in parameter

list » A number can be placed here but it

will beignored

17

Multidimensional Arrays A collection of a fixed number of components

arranged in two dimensions» All components are of the same type

The syntax for declaring a two-dimensional array is:dataType arrayName[intexp1][intexp2];

where intexp1 and intexp2 are expressions yielding positive integer values.

18

Multidimensional Arrays The two expressions intexp1 and intexp2 specify

the number of rows and the number of columns, respectively, in the array

Two-dimensional arrays are sometimes called matrices or tables

19

Multidimensional Arrays

double sales[10][5];

20

Accessing Array Elements The syntax to access a component of a two-

dimensional array is:arrayName[indexexp1][indexexp2]

where indexexp1 and indexexp2 are expressions yielding nonnegative integer values

indexexp1 specifies the row position and indexexp2 specifies the column position

21

Accessing Array Elementssales[2][3] = 35.60;

35.60

22

2-DIM. Array Initialization Like one-dimensional arrays

» Two-dimensional arrays can be initialized when they are declared

To initialize a two-dimensional array when it is declared1. Elements of each row are enclosed within braces

and separated by commas2. All rows are enclosed within braces3. For number arrays, if all components of a row are

not specified, the unspecified components are initialized to zero

23

2-DIM. Array Initialization Example: int anArray[3][5] =

{{ 1, 2, 3, 4, 5, }, // row 0{ 6, 7, 8, 9, 10, }, // row 1{ 11, 12, 13, 14, 15 } // row 2

};

24

2-DIM. Array Initialization Accessing all of the elements of a two-dimensional

array requires two loops: one for the row, and one for the column.

Since two-dimensional arrays are typically accessed row by row, generally the row index is used as the outer loop.

for (int nRow = 0; nRow < nNumRows; nRow++)     for (int nCol = 0; nCol < nNumCols; nCol++)         cout << anArray[nRow][nCol];

25

Multidimensional Arrays A collection of a fixed number of elements (called

components) arranged in n dimensions (n >= 1) Also called an n-dimensional array General syntax of declaring an n-dimensional array is:

dataType arrayName[intExp1][intExp2]...[intExpn];

where intExp1, intExp2, … are constant expressions yielding positive integer values

Example: 3-Dimensional array:int anArray[5][4][3];

26C-Strings or Character Arrays

We have learned that the elements of an array can be just about anything

Consider an array whose elements are all characters» Called a C-String» Has a collection of special routines

27C-Strings or Character Arrays

Character array: An array whose components are of type char

String: A sequence of zero or more characters enclosed in double quote marks

C-stings are null terminated (‘\0’)

The last character in a string is the null character

28C-Strings or Character Arrays

char str[80] = “Amanuensis”

29C-Strings or Character Arrays

There is a difference between 'A' and "A"

» 'A' is the character A

» "A" is the string A

Because strings are null terminated, "A" represents two characters, 'A' and '\0‘

Similarly, "Hello" contains six characters, 'H', 'e', 'l', 'l', 'o', and '\0'

30C-Strings or Character Arrays

Consider the statementchar name[16];

Because C-strings are null terminated and name has sixteen components» the largest string that can be stored in name

is 15 If you store a string of length, say 10 in name

» the first 11 components of name are used and the last 5 are left unused

31

Declaration of C-Strings Similar to declaration of any arraychar name[30];

// no initializationchar title [20] = "Le Grande Fromage"; // initialized at declaration

// with a stringchar chList [10] = {'a', 'b', 'c', 'd'}; // initialized with list of char

// values

32

Initializing Strings When a character array is declared, it is legal

to use the assignment operator to initialize

Note : use of the = operator only legal for char array initialization

But : aggregate array assignment is NOT

greeting = “don’t do it;

C-Strings: Example-1#include<iostream>#include<conio.h>using namespace std;

int main(){ const int MAX = 80; //maximum characters in a stringchar str[MAX]; //string variable strcout<< “Enter a string \n”;cin>>str; //put string in str

cout<< “You entered: ” << str << endl; //display string from strgetch();return 0; }

33

Reading Embedded Blanks#include<iostream>#include<conio.h>using namespace std;

int main(){ const int MAX = 80;char str[MAX];cout<< “Enter a string \n”;cin.get(str,MAX); //member function get() of the stream class of

// which cin is an objectcout<< “You entered: ” << str << endl;getch();return 0; }

34

Reading Multiple Lines#include<iostream>#include<conio.h>using namespace std;

const int MAX = 2000;char str[MAX];

int main(){ cout<< “Enter a string \n”;cin.get(str, MAX, ‘$’); //terminate with $cout<< “You entered: ” << str << endl;getch();return 0; }

35

Copying a string: Hard-way#include<iostream>#include<cstring> // for copying a string#include<conio.h>using namespace std;

int main(){ char str1[] = “C++ is the best programming”

“language that I have ever used.”;const int MAX = 80;char str2[MAX];for(int j=0; j < strlen(str1); j++)

str2[j] = str1[j];cout<< str2 << endl;getch();return 0; }

36

Copying a string: Easy-way#include<iostream>#include<cstring> // for copying a string#include<conio.h>using namespace std;

int main(){ char str1[] = “C++ is the best programming”

“language that I have ever used.”;const int MAX = 80;char str2[MAX];strcpy(str2, str1);cout<< str2 << endl;getch();return 0; }

37

Standard C++ string Class38

#include<iostream>#include<string> //string class#include<conio.h>using namespace std;

int main(){ string s1(“Man”); string s2 = “Beast”; string s3;s3 = s1; //assigncout << “s3 = “ << s3 << endl;

s3 = “Neither “ + s1 + “ nor ”; //concatenates3 += s2; //concatenatecout << “s3 = ” << s3 << endl;s1.swap(s2); //swap s1 and s2cout << s1 << ” nor “ << s2 << endl;getch(); return 0; }

39Contrast/Compare Strings and C-Strings

Assignment is OKstring s;s = "hi mom";

Comparison OKif (s < "geek") …

I/O allowedcin >> s;cin.getline(s,'\n');cout << s;

Assignment is illegalchar cs[30];cs = "don't do it";

Comparisons not allowed I/O allowed much the

same way

40

Working with Strings Functions provided in #include <cstring>

Used instead of assignment

Used for comparisons

Working with Strings C-strings are compared character by character

using the collating sequence of the system If we are using the ASCII character set

1. The string "Air" is smaller than the string "Boat"

2. The string "Air" is smaller than the string "An"

3. The string "Bill" is smaller than the string "Billy"

4. The string "Hello" is smaller than "hello"

41

Working with Strings42

Passing Arrays to Functions

Arrays can be used as arguments to functions.

43

Passing Arguments to Arrays

#include<iostream>#include<conio.h>using namespace std;

int sum(int list[], int listSize){int index, sum = 0;for(index=0; index<listSize; index++) sum = sum + list[index];return sum; }

int main(){ int myArray[] = {2, 3, 5}; cout<< "The is: " << sum(myArray, 3); getch(); return 0; }

44