1. 計算機概論實習 2007-03-09

2. 2 Stream Stream: sequence of bytes Input: from device (keyboard, disk drive) to memory Output: from memory to device (monitor, printer, etc.) in c scanf("%c", &ch); in c++ cin >> ch; in c printf("%c", ch); in c++ cout << ch; byte stream Computer

3. 3 iostream Library Header Files iostream library Has header files with hundreds of I/O capabilities Standard input (cin) Standard output (cout) Unbuffered error (cerr) Buffered error (clog) Formatted I/O with parameterized stream manipulators File processing operations

4. 4 Standard input (cin) Format input cin >> grade Need to include iostream and std:cin Using >> operator Normally skips whitespace (blanks, tabs, newlines) Can change this Returns 0 when EOF encountered Otherwise, returns reference to object State bits set if errors occur Discussed in 12.7 and 12.8

5. 5 Standard output (cout) Format input cout << grade Need to include iostream and std:cout Using << operator

6. 6 Exmaple Code #include using namespace std; void main(){ int i; cin >> i; cout << i; }

7. 7 Multi-Input/Output int main(){ int a, b; cout << “Input two number: ”; cin >> a >> b; cout << “The inputs are ” << a << “ and ” << b; return 0; } Input two number: 4 7 The inputs are 4 and 7 space

8. 8 Member Function: cin.get() get(charRef) With character reference argument Gets only one character, stores in charRef Returns reference to istream If EOF, returns -1 get(charArray, size) get(charArray, size, delimiter) Reads until size-1 characters read, or delimiter encountered Default delimiter '\n' Delimiter stays in input stream Can remove with cin.get() or cin.ignore() Makes array null-terminated

9. 9 Member Function: cin.getline() getline(array, size) getline(array, size, delimiter) Reads size-1 characters, or until delimiter found Default delimiter '\n' Removes delimiter from input stream Puts null character at end of array

10. 10 What Difference Among cin, cin.get(), and cin.getline() (1/3) int i; cin >> i; cout << “i = ” << i; 456\n 456 i = 456 \n skip due to >> \n

11. 11 What Difference Among cin, cin.get(), and cin.getline() (2/3) char ch[10]; cin.get(ch, 10); cout << ch; 456\n 456 ch[0] = ‘4’ ch[1] = ‘5’ ch[2] = ‘6’ ch[3] = 0 ch[0] = 0 56\n 6 Delimiter stays in input stream Nothing is read

12. 12 What Difference Among cin, get(), and getline() (3/3) char ch[10]; cin.getline(ch, 10); cout << ch; 456\n 456 ch[0] = ‘4’ ch[1] = ‘5’ ch[2] = ‘6’ ch[3] = 0 ch[0] = 0 56\n 6 456 Removes delimiter from input stream

13. 13 Throw Away Delimiter If use cin.get(), the delimiter will stay in the stream. Using the following method to Throw it Away. int main() { char buf[100]; char trash; while(cin.get(buf,100)) { // Get chars until ‘\n’ cin.get(trash); // Throw away the terminator cout << buf <<'\n'; // Add the ‘\n’ at the end } return 0; } Input : This is a test Output: This is a test

14. 14 Stream Error States Test state of stream using bits eofbit set when EOF encountered Function eof returns true if eofbit set cin.eof() failbit set when error occurs in stream Data not lost, error recoverable Function fail() returns true if set

15. 15 Stream Error States badbit set when data lost Usually nonrecoverable Function bad() goodbit set when badbit, failbit and eofbit off Function good()

16. 16 Stream Error States Member functions rdstate() Returns error state of stream Can test for goodbit, badbit, etc. Better to test using good(), bad() clear() Default argument goodbit Sets stream to "good" state, so I/O can continue Can pass other values cin.clear( ios::failbit ) Sets failbit Name "clear" seems strange, but correct

17. 17 int main(){ int integerValue; // display results of cin functions cout << "Before a bad input operation:" << "\ncin.rdstate(): " << cin.rdstate() << "\n cin.eof(): " << cin.eof() << "\n cin.fail(): " << cin.fail() << "\n cin.bad(): " << cin.bad() << "\n cin.good(): " << cin.good() << "\n\nExpects an integer, but enter a character: "; cin >> integerValue; // enter character value cout << endl;

18. 18 // display results of cin functions after bad input cout << "After a bad input operation:" << "\ncin.rdstate(): " << cin.rdstate() << "\n cin.eof(): " << cin.eof() << "\n cin.fail(): " << cin.fail() << "\n cin.bad(): " << cin.bad() << "\n cin.good(): " << cin.good() << endl << endl; cin.clear(); // clear stream // display results of cin functions after clearing cin cout << "After cin.clear()" << "\ncin.fail(): " << cin.fail() << "\ncin.good(): " << cin.good() << endl; return 0; }

19. 19 Before a bad input operation: cin.rdstate(): 0 cin.eof(): 0 cin.fail(): 0 cin.bad(): 0 cin.good(): 1 Expects an integer, but enter a character: c After a bad input operation: cin.rdstate(): 2 cin.eof(): 0 cin.fail(): 1 cin.bad(): 0 cin.good(): 0 After cin.clear() cin.fail(): 0 cin.good(): 1 Different error bit set leads to different value of rdstate()

20. 20 Error Input Format – Simple Example int main () { int a; while (cin>>a){ cout<<"The input is:" <<a<<endl; } cout << "fail?" << cin.fail(); } 123 The input is:123 abc fail? 1

21. 21 Error Input Format – Complex Example 123 The input is:123 ab fail?1 ab1 fail?1 123b The input is:123 fail?1 1 2 3 4

22. 22 Explanation of Example - 1 If there are not any error input, then program can get value from cin stream. while (cin>>a) 123 a=123

23. 23 Explanation of Example - 2 When the input appears format error, the failbit set. In other words, the return value of cin.fail() is 1. But, the cin stream still keep the input value. For example, when you input ab, it will be kept in the stream. while (cin>>a) ab ab a = -87361838 cin.fail() = 1

24. 24 Explanation of Example - 4 When you input 1ab, a will be 1 but the ab will be kept in the cin stream. When you try to get from cin stream again, the failbit set. while (cin>>a) ab 1ab a=1 cin.fail() = 0 a=-8193787 cin.fail() = 1 First time Second time

25. 25 Error Input Format – Another Example Reset cin state and clear the stream. Then to request user to re-input with correct format (ex. Integer).

26. 26 Example Program // in order to prevent user from inputting incorrect format, clear stream void clear2eol(){ cin.ignore(numeric_limits ::max(), '\n'); } int main(){ int a; while (cout > a)) { cin.clear(); // reset cin iostate as goodbit clear2eol(); // clear the stream cout << "Wrong input. "; } clear2eol(); return 0; } Need include limits Note that this code can not check the error such as “23abc”

27. 27 Practice 1 (P1) Try to write a program that request user to input the student’s information including name and score and output the information. Please check that if the input of score is correct. If the input is not correct, then to request the user to re-input until the program gets a correct one.