Lec 25 - arrays-strings

Arrays and Strings

Lecture: 25

19.09.2012

1

2

Design Problem

Consider a program to calculate class
average

Why??
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
Describe why this will or will NOT work

4

Possible Solutions

Enter in the scores again 
Use 100 separate variables 
» and cout and cin commands
Read (then re-read) from a file 
The real answer …

Use arrays!!

Simple vs Structured Data 5

Types

Simple data type => data element
contains a single value
x ::15
x 15 avg ::84.35
avg 84.35 ch ::‘A’
ch ‘A’

Structured data type => a data element
contains a collection of data values
scores :: 85 79 92 57 68 80
scores 85 79 92 57 68 80

name :: ‘C’ ‘L’ ‘Y’ ‘D’ ‘E’
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 80
scores 85 79 92 57 68 80
0 1 2 3 4 5
cout << scores[2];
cout << scores[2];
scores[0] = 100;
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

8

Use of Array for Our Problem

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

Note declaration
Note declaration

9

Declaring Arrays

Syntax:
Data_type Array_name [constant];
Note declaration from our example

Tells how many elements set aside
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 ......
scores 85 79 92 57 68 80
0 1 2 3 4 5 98 99

Accessing Individual 11

Components

Use the name of the array
Followed by an integer expression
inside the square brackets [ ]
scores :: 85 79 92 57 68 80 ......
scores 85 79 92 57 68 80
0 1 2 3 4 5 98 99
Index can be:
Index can be: max = scores[0];
max = scores[0];
--constant
constant for (x = 0; x < 100; x++)
for (x = 0; x < 100; x++)
--variable
variable if (scores[x] > max)
if (scores[x] > max)
--expression
expression max = scores[x];
max = scores[x];
MUST be an integer
MUST be an integer

12

Out of Bounds Index
What happens if … float
float f_list [50];
f_list [50];
f_list [100] = 123.456;
f_list [100] = 123.456;

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 (??)

Initializing Arrays in 13

Declarations

Possible to declare the size & initialize
int results [5] = {14, 6, 23, 8, 12 }

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

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


The name of the array is a pointer
constant
The address of the array is passed to
the function
Size of the
array also
passed to
control loop

16


Note the empty brackets in parameter
list
» A number can be placed here but it
will be
ignored

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


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


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 Elements
sales[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
declared
1. Elements of each row are enclosed within braces
and separated by commas
2. All rows are enclosed within braces
3. For number arrays, if all components of a row are
not specified, the unspecified components are
initialized to zero

23

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

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

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];

26

C-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

27

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

28

char str[80] = “Amanuensis”

29

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'

30

Consider the statement
char 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 array
char name[30];
// no initialization
char title [20] = "Le Grande Fromage";
// initialized at declaration
// with a string
char 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;

33

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

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

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

34

Reading Embedded Blanks
#include<iostream>
#include<conio.h>

int main()
{
const int MAX = 80;
char str[MAX];
cin.get(str,MAX); //member function get() of the stream class of
// which cin is an object
cout<< “You entered: ” << str << endl;
getch();
return 0; }

35

Reading Multiple Lines
#include<iostream>
#include<conio.h>

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

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

36

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

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; }

37

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

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; }

38

Standard C++ string Class
#include<iostream>
#include<string> //string class
#include<conio.h>

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

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

39

Contrast/Compare Strings and C-Strings

Assignment is OK Assignment is illegal
string s; char cs[30];
s = "hi mom"; cs = "don't do it";
Comparison OK Comparisons not allowed
if (s < "geek") …
I/O allowed much the
I/O allowed
cin >> s; same way
cin.getline(s,'n');
cout << s;

40

Working with Strings

Functions provided in #include <cstring>

Used instead of assignment

Used for comparisons

41

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"

43

Passing Arrays to Functions
Arrays can be used as arguments to functions.

44

Passing Arguments to Arrays
#include<iostream>
#include<conio.h>

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; }

Lec 25 - arrays-strings

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