1. Chapter 11 Standard C++ Strings and File I/O Dept of Computer Engineering Khon Kaen University

2. 178110: Computer Programming (II/2546) 2 Outline Reading inputs Formatted input Unformatted input The Standard C++ String Type Files String Streams

3. 178110: Computer Programming (II/2546) 3 The Input Operator Input passes through an istream object and output passes through an ostream object The istream class defines the behavior of objects like cin The most common behavior is the use of the extraction operator >> (also called the input operator)

4. 178110: Computer Programming (II/2546) 4 Formatted Input It has two operands The istream object from which it is extracting characters The object to which it copies the corresponding value formed from those characters This process of forming a typed value from raw input characters is called formatting

5. 178110: Computer Programming (II/2546) 5 Example: Formatted Input int main() { int n; cin >> n; cout << "n ='" << n << "'" << endl; } What is the output when you enter “ 46”? n=’46’

6. 178110: Computer Programming (II/2546) 6 The Extraction Operator >> The extraction operator >> formats the data that it receives through its input stream It extracts characters from the stream and uses them to form a value of the same type as its second operand It ignores all whitespace characters that precede the characters it uses Can you use the extraction operator to read whitespace characters? No, we must use an unformatted input operation

7. 178110: Computer Programming (II/2546) 7 The Expression cin >> x The operator expression cin >> x has a value that can be interpreted in a condition as boolean (true or false) When the expression will be true? When the reading of the input is successful This allows such an expression to be used to control a loop

8. 178110: Computer Programming (II/2546) 8 Using the Extraction Operator int n; while (cin >> n) cout << "n= " << n << endl; What are the outputs of these inputs? 46 22 44 66 88 33, 55, 77, 99

9. 178110: Computer Programming (II/2546) 9 Unformatted Input The file defines several functions inputting characters and C- strings that do not skip over whitespace The most common are the cin.get() function for reading individual characters and the cin.getline() function for reading C-strings

10. 178110: Computer Programming (II/2546) 10 Inputting Characters with cin.get() Function while (cin.get(c)) { if (c >= 'a' && c <= 'z') c += 'A' - 'a'; // capitalize c cout.put(c); if (c == '\n') break; } What are the outputs of these inputs? 123,456 Hello kku

11. 178110: Computer Programming (II/2546) 11 Inputting C-Strings with cin.getline() Function const int LEN=32; // maximum word length const int SIZE=10; // array size typedef char Name[LEN]; // defines Name to be a C-string type int main() { Name king[SIZE]; int n = 0; while (cin.getline(king[n++], LEN) && n<= SIZE) ; --n;// now n = the number of names read for (int i=0; i < n; i++) cout << "\t" << i+1 << ". " << king[i] << endl; }

12. 178110: Computer Programming (II/2546) 12 The Standard C++ string Type Standard C++ defines its string type in the header file Objects of type string can be declared and initialized in several ways string s1; // s1 contains 0 characters string s2 = “New York”; // s2 has 8 characters string s3(60, ‘*’); // s3 has 60 ‘*’ characters string s4 = s3; // s4 has 60 ‘*’ characters string s5(s2, 4, 2); // s5 = ‘Yo’

13. 178110: Computer Programming (II/2546) 13 C++ string Type C++ strings have a getline() function that works almost the same way as the cin.getline() function for C-strings string s6 = “ABCDEFG”; string s7; getline(cin,s7); // read an entire line of characters into s7 They also use the subscript operator the same way that C-strings do: char c = s6[2]; // assign ‘C’ to character c s6[4] = ‘*’; // s6 = ABCD*FG

14. 178110: Computer Programming (II/2546) 14 C++ string Type C++ strings can be converted to C-strings like this: const char* cs = s6.c_str(); The C++ string class also defines a length() function that can be used like this to determine how many characters are stored in a string cout << s6.length() << endl;

15. 178110: Computer Programming (II/2546) 15 C++ string Type Operators C++ strings can be compared using the relational operators like fundamental types: string s8 = “hello”; string s9 = “waddee”; if (s8 < s9) cout << “hello precedes waddee”; else cout << “hello does not precede waddee”;

16. 178110: Computer Programming (II/2546) 16 C++ string Type Operator You can also concatenate and append strings using the + and += operators; s6 = s6 + “HIJK”; // s6 = “ABCD*FGHIJK” s2 += s5; // s2 = “New YorkYo” The substring() function is used like this s4 = s6.substr(5,3); // s4 = “FGH” The erase() function is used like this s6.erase(4, 2); // s6 = “ABCDGHIJK”

17. 178110: Computer Programming (II/2546) 17 C++ string Type Operators The replace() function is used like this s6.replace(5,2, “xyz”); // s6 = “ABCDGxyzJK” The find() function returns the index of the first occurrence of a given substring string s10 = “Khon Kaen U” cout << s10.find(“on”) << endl; // print 2 cout << s10.find(“en”) << endl; // print 7

18. 178110: Computer Programming (II/2546) 18 Using the C++ string Type int main() { string word; while (cin >> word) { cout << "before: " << word << endl; int numChars = word.length(); for (int i = 0; i < numChars; i++) { if (is_vowel(word[i])) word.replace(i,1,"*"); } cout << "after: " << word << endl; } bool is_vowel(char c) { return (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' || c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U'); }

19. 178110: Computer Programming (II/2546) 19 Files File processing in C++ is very similar to ordinary interactive input and output because the same kind of stream objects are used Input from a file is managed by an ifstream object the same way that input from the keyboard is managed by the istream object cin Output to a file is managed by an ofstream object the same way that output to the monitor or printer is managed by the ostream object cout

20. 178110: Computer Programming (II/2546) 20 Files Unlike istream and ostream, ifstream and ofstream must be declared explicitly and initialized with the external name of the file which they manage You also have to #include the header file that defines ifstream and ofstream

21. 178110: Computer Programming (II/2546) 21 Capitalizing All the Words in a Text File #include … int main() { ifstream infile("input.txt"); ofstream outfile("output.txt"); string word; char c; while (infile >> word) { if (word[0] >= 'a' && word[0] <= 'z') { word[0] = word[0] + 'A' - 'a';} outfile << word << " "; }

22. 178110: Computer Programming (II/2546) 22 Merging Two Sorted Data Files bool more(ifstream& fin, int& n) { if (fin >> n) return true; else return false; } bool copy(ofstream& fout, ifstream& fin, int& n) { fout << " " << n; return more(fin, n); }

23. 178110: Computer Programming (II/2546) 23 Merging Two Sorted Data Files int main() { ifstream fin1("north.dat"); ifstream fin2("south.dat"); ofstream fout("combined.dat"); int n1, n2; bool more1 = more(fin1, n1); bool more2 = more(fin2, n2); while (more1 && more2) {if (n1 < n2) more1 = copy(fout, fin1, n1); else more2 = copy(fout, fin2, n2);} while (more1) more1 = copy(fout, fin1, n1); while (more2) more2 = copy(fout, fin2, n2); fout << endl; }

24. 178110: Computer Programming (II/2546) 24 String Streams A string stream is a stream object that allows a string to be used as an internal text file This is also called in-memory I/O String streams are quite useful for buffering input and output Their types isstringstream and ostringstream are defined in the header file

25. 178110: Computer Programming (II/2546) 25 Using an Output String Stream int main() { string s; istringstream iss("ABCDEFG 44"); string s; iss >> s; int n; iss >> n; cout << “s is “ << s << “ n is “ << n << endl;}