Lecture 7

Strings

Strings are a concatenation of characters enclosed in quotation marks. String is a data type that is not a part of C++ but is programmer defined from the use of including the standard C++ library header file string.

Review string operations

string name;

ifstream indata;

indata.open(“data.txt”);

cin >> name; // reads in characters from keyboard until white space one

indata>> name;// reads in characters from data file, data.txt, until white space one

getline ( cin, name); // reads in from keyboard all characters of name including white space ones

getline (indata, name); // reads in from data file all characters of name including white space ones

relational operators between string variables continue until there is a mismatch. If none is found they are the same (==). If the first difference between the series of characters is found the character lower in the ascii numbering value is considered the smaller string

str1 = “the”;

str2 = “tag”;

(str2 < str1) evaluates to true

New data types

The string header file introduces some useful new data types

string::size_type ; // an unsigned integer data type

string:npos; // the maximum value of the string::size_type

New operations

Can add.strings using the + operator

e.g. str1=”Bob”;

str2=”Jones”

str3=stri + str2;

then str3=BobJones

well what if we want a space between the first and last name

solution is to have str3=str1 + “ “ + str2;

Warning For the operator + to work with string one of the operands must be a string variable. For example str3 = “Bob “ + “Jones” is illegal

Array subscript operator [] causes string replacement of a character at a string position

String str1 = “hello There”;

Str1[0]=’H’; // this replaces the first character position with ‘H’

Length function

string_Variable.length(); // returns the number characters in a string as an unsigned integer. The value can then be returned to a variable. Since string::size_type is associated with string the variable used for the returning value should be of this type.

e.g. string str1;

str1 = “Bob Jones”;

string::size_type len;

len = str1.length(); // value of len = ?

string_Variable.size(); // Does the same as the function length

Find function

string_Variable.find(string_expression); //searches a string for a substring and returns the position of the occurrence of the first character of the matching substring. If not successful returns the value of string::npos (not a position within the string). The position value can be returned to a variable of string::size_type.

String_Variable.find(string_expression, position); // As above function but starts searching at the position given in the parameter. Positions always begin with 0.

e.g. string str1;

str1 = “Bob Jones”;

string::size_type position;

position = str1.find(ob); // value of position = ?

Substring function

string_Variable.substr(position, length); //returns a substring of the length specified in the function parameter starting at the position indicated.

e.g. string str1, str2;

str1 = “Bob Jones”;

str2 = str1.substr(0, 5); // str2 = ?

Swap Function

string_Variable1.swap(string_Variable2); //After execution of this statement both string variables exchange their contents

Exercise

1. Write the code needed to concatenate two strings “abc” and “xyz”.
2. What do the following C++ statements display?

string message = “Hello world”;

cout << message.length() << endl;

3. What do the following C++ statements display?

string message = “Hello world”;

cout << message[3] << endl;

4. What do the following C ++ statements display?

string message = “Hello world”;

cout << message.substr(6,5);

5. What occurs when the following C++ statements are executed?

string message = “Hello world”;

cout << size() <<endl;

6. True or false. In ANSI standard C++, the data type string requires the iostream

header file.

7. True or false. The function length returns a value one less than the function size returns because length begins counting with position 0 and size begins counting with position 1.