CMSC 202

1. CMSC 202 Streams

2. Warmup • What happens in the following? vector<string> names; names.at(0) = “Fred”; names.at(1) = “Flintstone”; for (int i = 0; i <= names.size(); ++i) cout << names.at(i) << “ “; cout << endl;

3. What is a stream? • Flow of characters (or other data) • Input • Characters that come IN to your program • Output • Characters that come OUT of your program • Input/Output (Sources/Sinks) • Input: Keyboard, File, … • Output: Monitor, File, …

4. Standard In/Out • Standard in • cin • Standard out • cout • Why are these interesting? • Under any OS – these can be “redirect” • You can treat a file as cin • You can treat a file as cout

5. Error Stream • cerr • Standard error stream • NOT redirected when cout is redirected • Use exactly like cout • Example if (denominator == 0) { cerr << “Fatal error: denominator == 0” << endl; exit(1); }

6. File I/O • File streams work EXACTLY like cin/cout • More setup is required • Open a file • Read from/Write to a file • Close the file • Library • #include <fstream>

7. File Input • ifstream • Datatype for file input • Opening an input file ifstream fin(filename); Or… ifstream fin; fin.open(filename); • Reading from an input file (you can read any primitive!) char a; fin >> a; • Closing an input file fin.close();

8. File Input - Example • Read a list of last names from a file ifstream fin(“names.txt”); vector<string> names; string name; while (fin >> name) { names.push_back(name); } What’s happening here?

9. File Output • ofstream • Datatype for file output • Opening an output file ofstream fout(filename); Or… ofstream fout; fout.open(filename); • Writing a char to an output file char a = ‘A’; fout << a; • Closing an output file fout.close();

10. File Output - Example • Write a list of names to a file ofstream fout(“names.txt”); vector<string> names; string name; // Assume vector gets values… for (int i = 0; i < names.size(); ++i) { fout << names[i] << endl; }

11. File Streams - #1 Issue • Streams expect a C-string as its parameter • Example string inFilename = “input.txt”; ifstream fin( inFilename.c_str() ); string outFilename = “output.txt”; ofstream fout( outFilename.c_str() ); • What does c_str() do? • Look back at the string material!

12. Practice • Open a file for input called “phones.txt” • File has a name (string) followed by a number (int) • Read in the data from the file • Close the input file • Open a file for output called “reverse.txt” • Print the number followed by the name to the file • Close the output file

13. Input Streams – Pitfall! • Mixing getline() and >> can be bad! • >> • Skips all leading whitespace • Leaves trailing whitespace for next extraction • Leaves the \n character • getline • Retrieves until end of line character • Removes the \n character, but does not store it

14. Input Streams – Pitfall! • Example int age; string name; cout << "Input your age and first name"; cin >> age; getline( cin, name ); • User types 42 Bob Smith • Solution? • cin.ignore(); // discard a single character • cin.ignore(1000, ‘\n’); // discard up to 1000 chars, stopping at \n age == 42 name == “”

15. Checking for end-of-file • End of file (fin) or End of input (cin) • 3 strategies • while (!fin.eof()) // Best • while (fin >> variable) // Pretty good • while (fin) // NOT preferred… • .eof() • Returns “true” if EOF character has been seen

16. Formatting Output • Setf – set formatting flags • outStream.setf( ios::fixed) • Floating point values have a decimal point • outStream.setf( ios::showpoint) • Floating point values have trailing zeros • outStream.setf( ios::right) • Output is right-justified in output field • outStream.setf( ios::left) • Output is left-justified in output field • Setf stays in effect until reset

17. Formatting Output • outStream.precision( int places ) • Sets the number of places to the right of the decimal • Stays in effect until next call to precision() • outStream.width( int size ) • Sets minimum number of spaces to use to output the NEXT item • Only works on ONE item at a time… • HINT: great for aligning tabular output!

18. Manipulators • Library • #include <iomanip> • setprecision( int places ) • Same as outStream.precision( int places ) outStream << setprecision(2) << money << endl; • setw( int size ) • Same as outStream.width( int size ) outStream << setw(10) << Name << endl; • fixed • Same as setf( ios::fixed ); outStream << fixed << money << endl; • showpoint • Same as setf( ios::showpoint ); outStream << showpoint << money << endl;

19. Formatting Output – Pitfall! • Recall • Most manipulators stay “on” until reset • Issue? • Function that modifies these stream flags… • Solution? • Save the current state int savePrecision = outStream.precision(); int saveFlags = outStream.flags(); • Reset the current state outStream.precision( savePrecision ); outStream.flags( saveFlags );

20. String Streams • ostringstream • Format messages and treat like a string • Example string FormatMessage( int age ) { ostringstream msg; msg << "\nThis is my message\n"; msg << "John is " << setw(6) << age << " years old\n"; // use the str() function to access the // C++-string in the msg return msg.str( ); }

21. String Streams • istringstream • Parse messages based on whitespace • Example void PrintWords( string stringToPrint) { // create and initialize the istringstream // from the string istringstream inStream( stringToPrint ); string word; while ( inStream >> word ) cout << word << endl; }

22. Practice • Use setf, setw, fixed, showpoint, and setprecision to do the following: • Ask the user for 2 names and salaries • Print them formatted like this: Name Salary --------- ------------ John Doe $ 43523.00 Jane Donner $ 3129.97

23. Challenge • Use vectors, files and strings to • Part 1: • Read in an unknown number of paired values from a file (Name - string, Phone Number - integer) • Part 2: • Print the collection to another file • Align the output vertically • Format the phone number using dashes • HINT: think about using modulus % and integer division!