Input/Output Sujana Jyothi C++ Workshop Day 2

C++ I/O Basics 2 I/O - Input/Output is one of the first aspects of programming that needs to be mastered: formatting whitespace structured inputs - getting data into the correct internal form However, do not fall into the beginner’s traps: brilliant I/O with incorrect functionality thinking functionality is wrong because I/O is too complicated to get right forgetting that random tests are often better than user dependent I/O

Scientific Notation #include Using namespace std; main() { float z = ; cout << z << endl; }

Outline Simple input/output (iostream) cin and cout output insertion operator (<<) extraction operator (>>) Advanced input/output object flags (setf, unsetf) input status bits manipulators (iomanip.h) file input/output (fstream.h) opening/closing files

Setting Format Flags The object cout has flags that determine how objects are printed. To change how things are printed we access and change these flags To set a flag(s) we use the setf() function which is associated with objects such as cout and cin To call setf() we say cout.setf(flags) –the setf function is a field of the object cout –Q: But what flags? A: C++ predefines them

Setting Format Flags (cont) But in order to be able to set flags we often have to unset other flags first, to do so we use the unsetf() function: cout.unsetf(flags) C++ also provides a short-hand to combine both operations: cout.setf(OnFlags,OffFlags) –First turns off the flags OffFlags –Then turns on the flags OnFlags

Explicit precision manipulation // Example 10 #include Using namespace std; int main ( void){ float myNumber = ; cout.setf ( ios::fixed, ios::floatfield );// decimal format cout.setf ( ios::showpoint ) ; // print decimal point //ios is c++ standard library reference cout << "Number is " << setprecision ( 3 ) << myNumber << endl ; return 0 ; } 7 Number is Output:

setw(n) requires #include and appears in an expression using insertion operator (<<) affects only the very next item displayed “set width” specifies n as the number of total columns to display a number. The number of columns used is expanded if n is too narrow. Useful to align columns of output

Setw() Example //Example 11 #include Using namespace std; int main ( void) { float myNumber = ; float yourNumber = ; cout.setf ( ios::fixed, ios::floatfield ) ; cout.setf ( ios::showpoint ) ; cout << "Numbers are: " << setprecision (4) << endl << setw ( 10 ) << myNumber << endl << setw ( 10 ) << yourNumber << endl ; return 0 ; } 9 Numbers are: Output:

Whitespace Characters Include... blanks tabs end-of-line (newline) characters The newline character is created by hitting Enter or Return at the keyboard, or by using the manipulator endl or “\n” in a program.

Another way to read char data The get( ) function can be used to read a single character. It obtains the very next character from the input stream without skipping any leading white space characters. 11

char first ; char middle ; char last ; cin.get ( first ) ; cin.get ( middle ) ; cin.get ( last ) ; NOTE: The file reading marker is left pointing to the space after the ‘B’ in the input stream. firstmiddlelast At keyboard you type: A [ space]B [ space ] C [ Enter] firstmiddlelast ‘A’‘ ’‘B’ 12

Example program – I/O Formatting #include int main() { int n; float f; double d; char s[100]; cin >> n; // input an integer cout << n << endl; // print an integer, no formatting cout << setw(6) << n << endl; // print an integer, padded on left with spaces to total 6 chars cout << setw(-6) << n << endl; // print an integer, padded on right with spaces to total 6 chars cin >> s; // input a string (whitespace delineated) cout << s << endl; // print a string, no formatting cout << setw(20) << s << endl; // print a string, padded with spaces on left to 20 chars cout << setiosflags(ios::left) << setw(20) << s << endl; // print a string, padded with spaces on right to 20 chars cin >> f; // input a single precision floating point number cout << setiosflags(ios::fixed) << f << endl; // print a float, default precision is 6 places cin >> d; // input a double precision floating point number cout << d << endl; // print a double, default precision is 6 places cout << setprecision(2) << d << endl; // print a double, 2 places of precision cout << setw(10) << setprecision(2) << d << endl; // print a double, 2 places of precision, padded with space to 10 }