Moving Data Within a C++ Program

1. Moving Data Within a C++ Program • Input • Getting data from the command line (we’ve looked at this) • Getting data from the standard input stream • Getting data from files • Output • Sending data to standard output (we’ve looked at this) • Sending data to files • Transfer within the program • Moving data into and out of different types of variables • Moving data into and out of different data structures

2. Overview of Today’s Session • Using the on-line C++ reference pages (throughout) • Basic input and output stream features • Basic file input and output stream features • Moving data into and out of variables • Moving character data into and out of strings • Moving data into and out of a vector container

3. C++ Reference Link on CSE232 Web Page

4. Main C++ Reference Page

5. C++ I/O Page

6. C++ I/O Examples Page

7. File I/O Examples • Exercise: try out examples from C++ I/O reference • Do they work as written? • What files do you need to include to make them work? • What happens if you try to open a file that doesn’t exist? • What other ways can you explore the behaviors of the features those examples are using/illustrating?

8. C++ I/O Display All

9. #include <iostream> using namespace std; int main (int, char*[]) { int i; // cout == std ostream cout << “how many?” << endl; // cin == std istream cin >> i; cout << “You said ” << i << “.” << endl; return 0; } <iostream> header file Use istream for input Use ostream for output Overloaded operators << ostream insertion operator >> istream extraction operator Other methods ostream: write, put istream: get, eof, good, clear Stream manipulators ostream: flush, endl, setwidth, setprecision, hex, boolalpha Review: C++ Input/Output Stream Classes

10. #include <fstream> using namespace std; int main () { ifstream ifs; ifs.open (“in.txt”); ofstream ofs (“out.txt”); if (ifs.is_open () && ofs.is_open ()) { int i; ifs >> i; ofs << i; } ifs.close (); ofs.close (); return 0; } <fstream> header file Use ifstream for input Use ofstream for output Other methods open, is_open, close getline seekg, seekp File modes in, out, ate, app, trunc, binary Review: C++ File I/O Stream Classes

11. Redirecting File Output • Exercise: printing to a file vs. to stdout • Use the standard syntax for main that we used last week • Program always writes out “hello, world!” • If argc > 1 writes to file whose name is given by argv[1] • Otherwise writes to standard output

12. #include <iostream> #include <string> using namespace std; int main (int, char*[]) { string s; // empty s = “”; // empty s = “hello”; s += “, ”; s = s + “world!”; cout << s << endl; return 0; } <string> header file Various constructors Assignment operator Overloaded operators += + < >= == [] The last one is really useful: indexes string if (s[0] == ‘h’) … Review: C++ string Class

13. #include <iostream> #include <fstream> #include <sstream> using namespace std; int main () { ifstream ifs (“in.txt”); if (ifs.is_open ()) { string line_1, word_1; getline (ifs, line_1); istringstream iss (line_1); iss >> word_1; cout << word_1 << endl; } return 0; } <sstream> header file Use istringstream for input Use ostringstream for output Useful for scanning input Get a line from file into string Wrap string in a stream Pull words off the stream Useful for formatting output Use string as format buffer Wrap string in a stream Push formatted values into stream Output formatted string to file Review: C++ String Stream Classes

14. #include <string> #include <cstring> #include <sstream> using namespace std; int main (int argc, char *argv[]) { if (argc < 3) return 1; ostringstream argsout; argsout << argv[1] << “ ” << argv[2]; istringstream argsin (argsout.str()); float f,g; argsin >> f; argsin >> g; cout << f << “ / ” << g << “ is ” << f/g << endl; return 0; } Program gets arguments as C-style strings But let’s say we wanted to input floating point values from the command line Formatting is tedious and error-prone in C-style strings (sprintf etc.) iostream formatting is friendly Exercise: check whether any of the strings passed by argv are unsigned decimal integers (leading zeroes still ok) print their sum if there are any otherwise print the value 0 Using C++ String Stream Classes

15. In-Memory “(in core)” Formatting • Exercise: read and translate a file • Read integers and booleans (“true”, “false”) from a file • Test by writing your own test file with different combinations • Use a string to get one line of data at a time • Use a string stream to extract space separated tokens into another string variable • Check whether each token is a boolean (if not treat as int) • Convert to local variable of that type using another string stream • Printout whether it’s a boolean or an integer, and print out the value of the local variable, making sure to preserve formatting

16. A Couple More Things to Try • Exercise: printing out text from a named file • Open a text file whose name is given in argv • Print out the contents of the file to standard output • Detect the end of the file • Stop reading text, close the named file, and end the program • Exercise: typing text into a named file • Read text from the standard input stream • Put the text into a file whose name is given by argv[1] • Detect when the user types in the character sequence q! • Stop reading text, close the named file, and end the program