string Class

1. string Class

2. C-style and C++ string Classes • C-style strings, called C-strings, consist of characters stored in an array ( we’ll look at them later) • C++ standard library strings are usable in a manner similar to the built-in types • Remember, a variable's/object's type determines what operations may be performed on it. • Below are some ways C++ strings (variables/objects of type string) can be used • Note: • #include <string> is required • There are functions for operations such as insert, erase, replace, clear, etc.

3. String Declaration • String declaration: string mystr; //mystr is empty • Value assignment at declaration: string mystr(”Hello”); //mystr is initialized to string mystr2=”Hello”; //the string ”Hello” string mystr3(mystr2); string mystr4(5,’n’); //mystr = “nnnnn” • Assignments from other types are not permitted: string error1 = ’c’; // error string error2 = 22; // error • Can assign a character with assignment operator: mystr=’n’;

4. String I/O • String can be output as any other type: string s=”hello world”; cout << s << endl; • Two ways to input strings: • using extraction operator - strips white space and assigns the first “word” to the string: cin >> s; hello world\n – input assigns only hello to s • using getline() function - assigns all characters to string up to newline (not included): getline(cin, s); hello world\n - input assigns hello world to s • do not mix one and the other way of input!

5. Assignment and Concatenation • Can use assignment operator as with other types: string s1, s2, s3; s1=”Intro”; s2=”OOP”; • Plus “+” is used for string concatenation: s3=s1 + ”to” + s2; • at least one operand has to be string variable! • Compound concatenation allowed: s1 += ”duction”; • Characters can be concatenated with strings: s2= s1 + ’o’; s2+=’o’; • No other types can be assigned to strings or concatenated with strings: s2= 42.54; // error s2=”Catch” + 22; // error

6. Comparing Strings • Comparison operators (>, <, >=, <=, ==, !=) are applicable to strings • Strings are compared lexicographically: string s1=”accept”, s2=”access”, s3=”acceptance”; s1 is less than s2 and s1 is less than s3 • Order of symbols is determined by the symbol table (ASCII) • letters in the alphabet are in the increasing order • longer word (with the same characters) is greater than shorter word • relying on other ASCII properties (e.g. capital letters are less than small letters) is bad style

7. Comparing Strings • Comparison to literal string constants and named constants is also legal: const string myname=”John Doe”; string hername=”Jane Doe”; if ((myname==hername)||(myname==”Jake Doe”)) cout << ”found him\n”;

8. String Functions String Characteristics • Number of standard functions are defined for strings. Usual syntax: string_name.function_name(arguments) • Useful functions return string parameters: • size() - current string size (number of characters currently stored in string • length()- same as size() • max_size() - maximum number of characters in string allowed on this computer • empty() - true if string is empty • Example: string s=”Hello”; cout << s.size(); // outputs 5

9. Accessing Elements of Strings • Similar to arrays a character in a string can be accessed and assigned to using its index (start from 0) cout << str[3]; • Could be an error to access an element beyond the size of the string: string s=”Hello”; // size is 5 cout << s[6]; • Type of the element of the string is character, assigning integers, strings and other types are not allowed s[3] = ”hi”; // compilation error s[3] = 22; // depends on compiler

10. Substrings • substr - function that returns a substring of a string: substr(start, numb), where start - index of the first character,numb - number of characters • Example: string s=”Hello”; // size is 5 cout << s.substr(3,2); // outputs ”lo”

11. Searching • find family of functions return position of substring found, if not found return global constant string::npos defined in string header • find(substring) - returns the position of the first character of substring in the string • rfind(substring) - same as find, search in reverse • find_first_of(substring) - finds first occurrence of any character of substring in the string • find_last_of(substring) - finds last occurrence of any character of substr in the string • All functions work with individual characters as well: cout << s.find(“l”);

12. Inserting • insert(start, substring)- inserts substring starting from position start string s=”place”; cout << s.insert(1, ”a”); // s=”palace” • Variant insert(start, number, character) – inserts number of character starting from position start string s=”place”; cout << s.insert(4, 2, ’X’);//s= ”placXXe” • note: ‘x’ is a character not a string

13. Replacing • replace (start, number, substring)- replaces number of characters starting from start with substring • the number of characters replaced need not be the same • Example string s=”Hello”; s.replace(1,4, ”i, there”); //s is ”Hi, there”

14. Appending, Erasing • append(string2)- appends string2 to the end of the string string s=”Hello”; cout << s.append(”, World!”); cout << s; // outputs ”Hello, World!” • erase(start, number)- removes number of characters starting from start string s=”Hello”; s.erase(1,2); cout << s; // outputs ”Hlo” • clear()- removes all characters s.clear(); // s becomes empty

15. Passing Strings as Parameters • Strings can be passed as parameters: • by value: void myfunc(string s); • by reference: void myfunc(string &s); • If string is not modified by a function, then use const: void myfunc(const string &s);

16. Returning Strings • Strings (unlike arrays) can be returned: • string myfunc(int, int); • Note: passing strings by value and returning strings is less efficient than passing them by reference • However, efficiency should in most cases be sacrificed for clarity

17. C-Style string • C-strings are the C language version of strings • A string may be comprised of 0 or more characters • A string of 1000 characters requires much more storage than a string of 2 characters • Thus strings are more complicated than ints • The C++ standard library provides a string type that hides the details of how the characters are stored • The C programming language uses arrays of characters • Note: #include <cstring> is required

18. Number of Elements • There are two ways to keep track of the number of items in an array: • Keep a count of how many items are in the array • Use a marker after the last valid item in the array • C-strings use a marker • What is a C-string? • C-string is a null-terminated char array • The null character is denoted by '\0’ • A null occurs after the last character of the string in the array

19. C-string Example • For an initialization using double quotes, "...", the compiler will insert the null • Except for str2 having more room, the following are all equivalent char str1[] = “hi!”; //compiler can determine array’s size char str2[20] = “hi!”; //plenty of room char str3[4] = “hi!”; //min size but allows room for null char str4[] = {‘h’,i’,’!’,’\0’};

20. C-string Example • Example of a length function for C-string intstrlength(char mystr[]) { int count = 0; while (mystr[count] ! = ‘\0’) ++count; return count; }

21. C-string Problems • C-strings problems stem from the underlying array • With C-style strings the programmer must be careful not to access the arrays out of bounds • Consider string concatenation, combining two strings to form a single string • Is there room in the destination array for all the characters and the null? • The C++ string class manages the storage automatically and does not have these problems

22. String Use Examples • string_0.cpp • string.cpp • compare.cpp