C and C++ Strings

An Array Type for Strings • C-strings can be used to represent strings of characters – C-strings are stored as arrays of characters – C-strings use the null character '\0' to end a stri

C and C++ Strings

An Array Type for Strings

• C-strings can be used to represent strings of characters

– C-strings are stored as arrays of characters

– C-strings use the null character '\0' to end a string

• The Null character is a single character

– To declare a C-string variable, declare an array of characters:

• Declaring a C-string as char s[10] creates space for only nine characters

– The null character terminator requires one space

• A C-string variable does not need a size

variable

– The null character immediately follows the last

character of the string

• Example:

s[0] H i s[1] s[2] M s[3] s[4] o m s[5] ! s[6] \0 s[7] s[8] ? s[9] ?

C-string Details

Initializing a C-string

• To initialize a C-string during declaration:

char my_message[20] = "Hi there.";

– The null character '\0' is added for you

• Another alternative:

char short_string[ ] = "abc";

but not this:

char short_string[ ] = {'a', 'b', 'c'};

– because null character '\0' will NOT be added

Is called a string constant

or string literal

Length of C Strings

To calculate the length of a string use the strlen() function – returns the

number of characters between the start of the string to the terminating

Assignment With C-strings

• This statement is illegal:

char my_name[20], your_name[20]; my_name = "Thao";  ERROR!

– This is an assignment statement, not an

A Problem With strcpy

• strcpy can create problems if not used

strcpy (my_name, "Thao Thi Nguyen");

 this will overwrite memory!

A Solution for strcpy

• A safer version of strcpy is named strncpy

– strncpy uses a third argument representing the maximum number of characters to copy

– Example: char another_string[10];

strncpy(another_string,

another_string[9] = ‘\0’;

This code copies up to 9 characters into

another_string, leaving one space for '\0'

Comparing C-strings

• The = = operator does not work with C-strings

– The predefined function strcmp is used to compare C-string variables

– Example: #include <cstring>

More C-string Functions

– The second argument is added to the end of the first

– The result is placed in the first argument

– Example:

char string_var[20] = "The rain";

strcat(string_var, "in Spain");

Now string_var contains "The rainin Spain "

– A third parameter specifies a limit for the number

of characters to concatenate

C-string Output

• C-strings can be output with the insertion

operator <<

char news[] = "is good.";

cout << "The news " << news << endl;

Output: The news is good

Example

C-string Input

• The extraction operator >> can fill a C-string

– Whitespace ends reading of data

– Example: char a[80], b[80];

cout << "Enter input: " << endl;

Issues with cin >> for strings

cin >> (extraction operator) uses whitespace (spaces, tab,

newline) to delineate a string Therefore it will only read one word when it extracts input for a string (skipping all the

whitespace characters before reading each word)

char name[20], address[50];

cout << "Please enter your name:\n";

cin >> name;

user types "John Smith" and hits enter key

cout << "Please enter your address:\n";

Reading an Entire Line

• Predefined member function getline can read an entire line, including spaces

– getline is a member of all input streams (like cin)

– getline has two arguments

• The first is a C-string variable to receive input

• The second is an integer, specifying the size of the string variable

Using getline

spaces into a single C-string variable

– char a[80];

cout << "Enter input:\n";

cin.getline(a, 80);

cout << a << End Of Output\n";

and could produce:

Enter some input:

Do be do to you!

Do be do to you!End of Output

Mixing cin >> and getline

• Recall cin >> n skips whitespace (spaces, tabs, newline) to find what it needs to read, then stops reading when whitespace is found

• cin >> leaves the '\n' character in the input

getline after cin >>

double value;

cin >> value;

char str[80];

cin.getline(str, 80);

And you type: 3.14<return>

• 3.14 is read into value

• The newline following the 3.14 is still sitting in the input buffer

• cin.getline(str)immediately processes the newline that

is still on the input buffer

str becomes an empty string (" \0")

• It looks like that the application "skipped" the cin.getline()

statement

• Solution?

• Add a call to cin.get()/cin.ignore() after calling cin >> x

3 1 4 \n

cin.ignore

characters, including '\n' that remain in a line

– Ignore takes two arguments

• First, the maximum number of characters to discard

• Second, the character that stops reading and discarding – Example:

• cin.ignore(1000, '\n');

discards up to 1000 characters until it finds a '\n' cin.ignore();

discards the next character

• discard:sự chui bài, sự dập bài

The Standard string Class

• The string class allows the programmer to

treat strings as a basic data type

– No need to deal with the implementation as with C-strings (array of chars)

– No need to worry about size of string!

• The string class is defined in the string library and the names are in the standard namespace

– To use the string class you need these lines:

#include <string>

using namespace std;

Using + With strings

• Variables of type string can be concatenated with the + operator

string Constructors

• The default string constructor initializes a

string object to the empty string

• Another string constructor takes a C-string

Comparing strings

For C Strings use strcmp

if (str1 == "Danang"){ …

Member Function length

• The string class member function length return the number of characters in the string object:

string str1 = "ABC ";

cout << str1.length(); // prints 3

• Is exactly the same as member function size()

• string.empty() is the same as string.length() == 0

str1 is an object (or instance) of class string

length() is a member function of class string

• If you use int you will get compiler warnings:

for (int i = 0; i < str.length(); i++)

• Solution – use size_t instead:

for (size_t i = 0; i < str.length(); i++)

Operator[] and the string Class

Operator[] works the same as for C-strings char arrays

I/O With Class string

• The insertion operator << is used to output objects of type string

– Example: string s = "Hello Mom!";

cout << s;

• The extraction operator >> can be used to

input data for objects of type string

Example: string s1;

cin >> s1;

>> skips leading whitespaces and stops on

encountering more whitespace (reads one word)

getline and Type string

• A getline function exists to read entire lines into a string variable

– This version of getline is not a member of the istream class, it is a non-member function

– Syntax for using this getline is different than that used with cin: cin.getline(…)

getline(Istream_Object, String_Object);

cout << line << "END OF OUTPUT\n";

Output could be:

Enter some input:

Do be do to you!

Do be do to you!END OF OUTPUT

Another Version of getline

• The versions of getline we have seen, stop reading at the end of line marker '\n'

• getline can stop reading at any character specified in the argument list

• This code stops reading when a '?' is read

C++ string vs C-string

Old-style C-string

• pointer to character array, null-terminated ('\0')

• string functions are dangerous (strcpy, strncpy, )

• String literals are actually C-strings ("hello\n");

C++ string

• are objects of class string

• are much easier and safer to use than C-Strings

• have member functions such as:

length(); find(…); insert( ); replace( );

Converting C++ string to C-string

Converting C-string to C++ string

• Happens automatically in most cases

string s = "abc";

• Can force using string("abc")

string s = "abc" + "def"; // compile error

string s = string("abc") + "def";

Converting C++ string to C-string

• Some older functionality requires use of C-string

• Use member function c_str() :

printf("%s", last_name.c_str());

For more information on string class see:

http://www.cplusplus.com/reference/string/string/

Strings and Unicode

To store Unicode strings C++ defines the wstring type, which uses wchar_t as its character type

All wide string literals need to be prefixed with 'L'

string::find()

string class implements a find function:

size_t find(string pattern, size_t start_position);

returns position where pattern found,

or the constant string::npos if not found

string str = "My name is Pam";

size_t found;

found = str.find("name"); // returns 3

found = str.find("am", 4); // returns 12

found = str.find("john"); // returns string::npos

There is also a similar string::rfind() function which searches

from the end of the string (reverse find)

:: means member function of class

string::substr()

string substr(size_t pos, size_t n);

Returns the substring that starts at character position pos and has a length of n characters

If you omit the n parameter it means ‘to then end of the string’