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Problem Solving &Programming II

C++ ProgrammingStringC-string values and variables

Character manipulating functions

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- A STRING is a null terminated (\0 sentinel string character)

array of characters

- A new data type is now considered, namely, the character

string, which is used to represent a sequence of characters

regarded as a single data item. In C++ strings of characters are

held as an array of characters, one character held in each arrayelement. In addition a special null character, represented by `\0',

is appended to the end of the string to indicate the end of the

string.

- strings are defined as an array of characters or a pointer to a

portion of memory containing ASCII characters. A string in C is a

sequence of zero or more characters followed by a NULL (

)character:

- Header file :

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The Sentinel String Character '\0'

For example, the following two arrays both contain all the characters in the

string value ""Enter age: "", but only the array on the left contains a proper

string representation.

NOTE: although both "phrase" and "list" are arrays of characters, only "phrase" is big enough to

contain the string value ""Enter age: "". We don't care what characters are stored in the variables

"phrase[12]" and "phrase[13]", because all the string functions introduced below ignore characters

after the "'\0'".

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A string variable s1 could be declared as follows:

char s1[10];

The string variable s1 could hold strings of length up to nine characters since

space is needed for the final null character. Strings can be initialised at the

time of declaration just as other variables are initialised. For example:

char s1[] = "example";char s2[20] = "another example"

would store the two strings as follows:

s1 |e|x|a|m|p|l|e|\0|

s2 |a|n|o|t|h|e|r| |e|x|a|m|p|l|e|\0|?|?|?|?|NOTE:In the first case the array would be allocated space for eight characters, that is space for

the seven characters of the string and the null character. In the second case the string is set by

the declaration to be twenty characters long but only sixteen of these characters are set, i.e. the

fifteen characters of the string and the null character. Note that the length of a string does not

include the terminating null character.

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String Variable Declarations and Assignments

String variables can be declared just like other arrays:

char phrase[14];

String arrays can be initialised or partially initialised at the same time asbeing declared, using a list of values enclosed in "{}" braces (the same is

true of arrays of other data types). For example, the statement

char phrase[14] = {'E','n','t','e','r',' ','a','g','e',':',' ','\0'};

char phrase[14]=Enter Integer:

Or char phrase[ ] = {'E','n','t','e','r',' ','a','g','e',':',' ','\0'};

char phrase[ ]=Enter Integer:

Or char phrase[12] = {'E','n','t','e','r',' ','a','g','e',':',' ','\0'};

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putsoutput used for the strings/special output for

string

syntax: puts();

gets

input used for the strings/special input for strings

syntax: gets();

NOTE: header file for input/output for string is

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coutoutput used for the strings/special output for

string

syntax: cout

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C++ for Engineers and Scientists, Second Edition 8

The string Class

Class: a user-created data type that defines avalid set of data values and a set of operationsthat can be used on them

Object: storage area declared for a class

string class permits string literals String literal: any sequence of characters

enclosed in double quotation marks; also calledstring value, string, constant

Figure 7.1 The

storage of a string

as a sequence of

characters.

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C++ for Engineers and Scientists, Second Edition 9

The string Class (continued)

string class provides functions for declaring,

creating, and initializing a string

Methods: functions in a class

Constructor: the method which creates andinitializes a string object

string header file is required to use the

string class

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C++ for Engineers and Scientists, Second Edition 10

The string Class (continued)

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C++ for Engineers and Scientists, Second Edition 11

The string Class (continued)

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C++ for Engineers and Scientists, Second Edition 12

The string Class (continued)

Character positions within a string are

numbered starting with zero

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C++ for Engineers and Scientists, Second Edition 13

The string Class (continued)

Strings can be input from the keyboard anddisplayed on the screen using cout,cin, andgetline

getline continuously accepts and storescharacters from the keyboard until theterminating key is pressed

If the last argument is omitted, the Enter key

will terminate the inputSyntax:

getline(cin, strObj, terminatingChar)

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C++ for Engineers and Scientists, Second Edition 14

The string Class (continued)

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C++ for Engineers and Scientists, Second Edition 15

The string Class (continued)

Using cin and getline inputs together in thesame program may cause problems

cin accepts the input, but leaves the newline

code from the Enter key in the buffer, which willbe picked up by the getline as the end of its

input

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C++ for Engineers and Scientists, Second Edition 16

The string Class (continued)

Three possible solutions to this problem:

Do not use cin and getline inputs in the

same program

Follow the cin input with cin.ignore()

Accept the Enter key into a character variable

and then ignore it

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C++ for Engineers and Scientists, Second Edition 17

The string Class (continued)

string class provides many methods formanipulating strings, including

length: returns the length of the string

at: returns the character at the specified index

compare: compares two strings

empty: tests if the string is empty

erase: removes characters from the string

find: returns the index of the first occurrence ofa string in an object

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C++ for Engineers and Scientists, Second Edition 18

The string Class (continued)

string class methods (continued): insert: inserts one string into another string at

the specified index

replace: replaces characters in an object at

the specified index substr: returns a portion of a string

swap: swaps characters in a string with a

specified object

Concatenation operator (+)combines two

strings

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C++ for Engineers and Scientists, Second Edition 19

The string Class (continued)

All relational operators may be used to

compare strings; ASCII or UNICODE code

values are used for comparison

Figure 7.5 The initial strings used in Program 7.6.

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C++ for Engineers and Scientists, Second Edition 20

The string Class (continued)

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C++ for Engineers and Scientists, Second Edition 21

The string Class (continued)

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C++ for Engineers and Scientists, Second Edition 22

The string Class (continued)

Example: Inserting characters in a string

string str = This cannot be;

str.insert(4, I know);

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C++ for Engineers and Scientists, Second Edition 23

The string Class (continued)

Example: Replacing characters in a stringstr.replace(12, 6, to);

str += correct;

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C++ for Engineers and Scientists, Second Edition 24

The string Class (continued)

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strcpy( )- used to copy the content of string2 into string1

Syntax: strcpy(string1,string2)

Example: The following code fragment copies hello into string1

main()

{ char string1[8];

strcpy(string1,hello);

.

. }

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strcat( )

- concatenates a copy of string2 to string1 with a null. The

string is untouched by the operation

Syntax: strcat(string1,string2)

Example: this program appends the first string inputted to

the second string inputted. For example assuming the user

enters hello and there, the program will pring hellothere.

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main()

{

char string1[8],string2[8];

gets(string1);

gets(string2);

strcat(string1,string2);

puts(string1);

.

.

}

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strcmp( )

- lexicographically compares two null terminated strings.

Syntax: (strcmp(string1,string2)(,= =)0)

Related Function: strcmpi(),stricmp(),strcoll()

Value Meaning

< 0 string1 is less than string2>0 string1 is greater than string2

= =0 string1 is equal to string2

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strcspn( )- returns the index or the subscript of the first character in the

string pointed to by string1 that matches any of the characters in

the string pointed to by string2.

Syntax: strcspn(string1,string2)

Example: This program prints the number 8.

main()

{ int length;

length=strcspn(this is a test,a)

cprintf(%i,length);}

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strlen( )- returns the length of a null terminated string pointed to by strin

Syntax: strlen(string1)

Example: This program prints the number 4 if the entered valu

of string1 isRHEN.

main()

{ char string1[20];

gets(string1);

printf(%i,strlen(string1));}

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strlwr( )

- converts the string pointed to by string1 to lowercase

Related Function: strupr()

Syntax: strlwr(string1)

Example: This program prints merry christmas on the screen.

main()

{ char string1[50];

strcpy(string1,MERRY CHRISTMAS);

strlwr(string1);

printf(string1));}

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strncpy( )

- used to copy up to count characters from the string pointed to by

source into the string pointed to by destination, the source must be

a pointer to a null terminated string.

Syntax: strncpy(string1,string2)

Example: The following code fragment copies at most 79

characters of string1 into string2, thus ensuring that no array

boundaries overflow will occur.

Main()

{ char string1[120],string2[80];

gets(string1);

strncpy(string2,string1,79);}

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strrev( )

- reverses all the characters, except the null terminator, in the

string pointed to by str. It returns str.

Syntax: strrev(string)

Example: This programs prints hello backwards on the screen.

Main()

{ char s[ ]=hello;

strrev(s);

printf(s);

return 0;

}

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C++ for Engineers and Scientists, Second Edition 34

Character Manipulation Methods

Character manipulation methods require theheader file string orcctype

character class provides methods for

character manipulation, including

Character type detection: isalpha,isalnum,isdigit,isspace,isprint,isascii,isctrl,ispunct,isgraph,isupper,islower

Character case conversion: toupper,tolower

Character Manipulation Methods

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C++ for Engineers and Scientists, Second Edition 35

Character Manipulation Methods

(continued)

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C++ for Engineers and Scientists, Second Edition 36

Character Manipulation Methods

(continued)

Basic character I/O methods, requiring theheader file cctype, include

cout.put: put a character on the output stream

cin.get: extract a character from the inputstream

cin.peek: assign the next character from the

input stream to a variable without extracting it

cin.putback: pushes a character back onto

the input stream

cin.ignore: ignores input characters

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C++ for Engineers and Scientists, Second Edition 37

Character Manipulation Methods

(continued)

When a character is requested after the userhas already input some other data, get()method may pick up the Enter key instead of

the new data Two ways to solve this problem:

Use a cin.ignore() after the cin.get()

Accept Enter key and do not process it

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C++ for Engineers and Scientists, Second Edition 38

Character Manipulation Methods

(continued)

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C++ for Engineers and Scientists, Second Edition 39

Input Data Validation

User-input validation includes checking eachentered character to ensure it is the expected

data type

Can accept user input as a string, and thencheck each character for proper type

Example: input of an integer number

Data must contain one or more characters

If the first character is a + or -, there must be at

least one digit

Only digits from 0 to 9 are acceptable

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C++ for Engineers and Scientists, Second Edition 42

Common Programming Errors

Failure to include the string header file whenusing string class objects

Failure to remember that the newline

character, \n, is a valid data input character

Failure to convert a string class object usingthe c_str() method when converting to

numerical data types

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C++ for Engineers and Scientists, Second Edition 43

Summary

String literal: any sequence of characters

enclosed in double quotation marks

string class can be used to construct a string

string class objects are used when

comparing, searching, examining, or extracting

a substring, or replacing, inserting, or deleting

characters cin object terminates input when a blank is

encountered

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Summary (continued)

getline() method can be used forstring

class data input

cout object can be used to display string

class strings