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IOStreams revisited

IOStreams revisited. Streams, strings, and files. We’ve already seen these. cin – input from keyboard First keypress is read in first cout – output to screen First data out is displayed first. Buffering of cout. Data sent to cout is not always displayed right away

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IOStreams revisited

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  1. IOStreams revisited Streams, strings, and files CS-1030 Dr. Mark L. Hornick

  2. We’ve already seen these • cin – input from keyboard • First keypress is read in first • cout – output to screen • First data out is displayed first CS-1030 Dr. Mark L. Hornick

  3. Buffering of cout • Data sent to cout is not always displayed right away • It is buffered until flushed • Three options • cout << endl; // Send newline & flush • cout << flush; // Flush only • cout << ‘\n’; // Send newline only • cout is automatically flushed when cin is accessed CS-1030 Dr. Mark L. Hornick

  4. output streams • cout • The “normal” output, sometimes redirected to a file • You may wish to handle certain types of output differently… • There are two extra ostreams for doing this • cerr • Sent to user, automatically flushed • clog • Sent to user, uses normal buffering CS-1030 Dr. Mark L. Hornick

  5. Manipulating an ostream • The appearance of output can be controlled with manipulators • Iostream manipulators don’t print • Instead, they change the behavior of the output stream • Found in iomanip library #include <iomanip>using namespace ios; CS-1030 Dr. Mark L. Hornick

  6. Manipulating an ostream • cout << 20; • Outputs “20” in decimal (default) • cout << dec << 20 • Also outputs “20” in decimal • cout << oct << 20; • outputs “24” (octal representation of 20) • cout << hex << 20; • outputs “14” (hexadecimal rep. of 20) CS-1030 Dr. Mark L. Hornick

  7. More Manipulators Output streams default to right-aligned text. • To left-align the output: cout << setiosflags( left ) • What about other defaults? • setw(int w) • Control width of display field • Non-persistent (next item only) • fixed, scientific • Style of output CS-1030 Dr. Mark L. Hornick

  8. More Manipulators • setprecision(int d) • Number of digits of precision • setfill(char c) • Padding character (only with setw) • showbase, noshowbase • Displays/hides leading 0 or 0x for octal and hex • boolalpha, noboolalpha • Display true/false or 1/0 CS-1030 Dr. Mark L. Hornick

  9. Streams and Input/Output • <fstream> • <ios> • <iostream> // #includes istream • <iosfwd> • <iomanip> • <istream> // #includes ostream • <ostream> • <sstream> • <streambuf> CS-1030 Dr. Mark L. Hornick

  10. Reading and Writing to and from strings • Insertion (writing) to string: << • Display string, subject to setw(int)cout << myString; • Extraction (reading) from string: >> • Reads only to first whitespace; not to end of line!cin >> myString; • Extract entire line • Reads to specified delimiter (‘\n’ is newline) getline(cin, myString, ‘\n'); CS-1030 Dr. Mark L. Hornick

  11. Accessing Files • Input can be read from files • Output can be sent to files • Use a special stream – fstream • Or ifstream – for input only • Or ofstream – for output only • Behavior is similar to cin and cout • Described by the fstream library • #include <fstream> CS-1030 Dr. Mark L. Hornick

  12. First associate a file with a file stream • Done during fstream declaration • Input file: • ifstream Name(<filepath>); • Output file: • ofstream Name(<filepath>); • Example: • ifstream infile(“mydata.txt”); • Use \\ or / for DOS/Windows paths CS-1030 Dr. Mark L. Hornick

  13. Reading from a File to a string • Open a file and read some data: string fileName = “test.txt”; // filename string strText; // holds text from file ifstream infile(fileName.c_str()); if( infile ) { // write only if open // read from file to string getline( infile, strText ); infile.close(); // close the file } CS-1030 Dr. Mark L. Hornick

  14. Writing to a File • Open a file and output some data: string fileName = “test.txt”; // a file ofstream outfile(fileName.c_str()); if( outfile ) { // write only if open outfile << “Outputting to file” << endl; outfile << fileName; // write filename outfile.close(); // close the file } CS-1030 Dr. Mark L. Hornick

  15. Handling File Errors • What if there is an error with the file? (i.e. reading a non-existent file) • Simply ask the file stream: ifstream myfile(“baadname.txt”); if (!myfile) cout << “Error accessing file ”; • Or use the is_open() method: if (!myfile.is_open()) cout << “Error accessing file ”; CS-1030 Dr. Mark L. Hornick

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