7/24/2018 C/C++ Programming for Engineers: Streams and Arguments John T. Bell Department of Computer Science University of Illinois, Chicago Review Given the following input: A. blank ( empty string ) Every one is great. B. is great. That's right. What does the following get into str2? C. great. cin >> str0; cin >> str1; D.That’s right getline(cin, tmpStr); getline(cin, str2); E. Error. str2 is unaffected. 2 1 7/24/2018 The Stream Concept • In C++, a “stream” is a class, encapsulating support for a flow of information. – Input streams, “istream”s, flow information into a variable, from some other source. – Output streams, “ostream”s, flow information out of a program, to some other destination. – Sources and destinations may include the keyboard, screen, data files, or memory buffers. 3 Standard Streams The following streams are automatically available, when you #include <iostream> and state “using namespace std;” : • cin - Reads from standard input ( stdin, keyboard ), using the >> operator. • cout - Writes to standard output ( stdout, screen), using the << operator. • cerr - Writes to standard error ( stderr, screen), using the << operator. 4 2 7/24/2018 cout Versus cerr • Normally data sent to either cout or cerr both go to the screen. • However either stream may be redirected independently of the other. • Technically only “results” should be sent to cout; All other output ( e.g. diagnostics, questions, etc. ) should be sent to cerr. • cout is also buffered, whereas cerr is direct. 5 Buffered Output • Because of the overhead involved in sending individual characters out to hardware, cout normally buffers data until there is enough to be worth sending to the device. • The flush( ) method ( e.g. cout.flush( ) ) and endl force buffers to be flushed. ( not “\n” ) • Unflushed buffers may be lost in a crash. • cerr is unbuffered - Data goes directly to HW. 6 3 7/24/2018 The << Operator • Technically the << operator is a method, defined in the ostream class for standard types. • The method returns the ostream by reference, allowing daisy chaining: – cout << prompt << x; ( cout << prompt ) << x; • Advanced programmers can define the << operator for specialty data types that they create. E.g. cout << myAirplane << endl; 7 istreams (cin) and the >> Operator • istreams define the >> operator for all standard types. (Programmers can add more.) • The >> operator reads characters from stdin and converts to the proper data type for storage. E.g. ‘3’ ‘.’ ‘1’ ‘4’ ‘1’ ‘5’ ‘9’ stores PI • >> can also daisy chain: cin >> x >> y >> z; • When reading strings, >> reads up to the next “whitespace” character. ( space, tab, newline ) • The getline( ) method will read in entire lines. 8 4 7/24/2018 Formatting of ostreams • There are a number of manipulators that can be added into output streams, in order to format the following data. • #include <iomanip> to use these manipulators. • Most remain in effect until specifically changed. – setw( ) is the most notable exception. setw( ) only affects the next data item that is output. 9 Floating Point Manipulators Manipulator Description Example Use fixed-point notation. fixed 12.34 From <ios> Use scientific notation. scientific 1.234e+01 From <ios> If stream has not been manipulated to fixed or scientific: p=3 yields 12.3 setprecision(p) Sets max number of digits in number p=5 yields 12.34 If stream has been manipulated to fixed or scientific: fixed: p=1 yields 12.3 Sets max number of digits in fraction only (after the decimal point). scientific: p=1 yields 1.2e+01 From <iomanip> Even if fraction is 0, show decimal point and trailing 0s. For 99.0 with precision=2 and fixed: showpoint Opposite is noshowpoint. 99 (default or noshowpoint) From <ios> 99.00 (showpoint) 10 5 7/24/2018 Alignment Manipulators Manipulator Description Example (for item "Amy") Sets the number of characters for the next output item only (does not persist, in contrast to other manipulators). For n=7: setw(n) By default, the item will be right-aligned, and filled with spaces. “ Amy" From <iomanip> Sets the fill to character c. For c='*': setfill(c) From <iomanip> "****Amy" Changes to left alignment. left "Amy " From <ios> Changes back to right alignment. right “ Amy" From <ios> 11 Additional Manipulators Manipulator Description Inserts a newline character '\n' into the output buffer, endl and informs the system to flush the buffer. From <iostream> Informs the system to flush the buffer. flush From <iostream> Others Exist See http://www.cplusplus.com/reference/iostream/manipulators 12 6 7/24/2018 printf( ) • printf( ) is the “print formatted” function from C, inherited by C++. • It is both more powerful and more complicated to use than stream manipulators. • int printf( const char * format, . ); – First argument is a C-style character string, containing format specifiers denoted by %. – Each time a format specifier is encountered, printf( ) processes another input data argument. – http://www.cplusplus.com/reference/cstdio/printf/ 13 The Form of a Format Specifier • %[flags][width][.precision][length]specifier – flags are modifiers, e.g. left or right justified – width is the total width of the field, including all characters, signs, decimal points, and spaces. – precision is the number of digits right of the decimal point – specifier indicates the type of data: • %d = decimal integer • %f = floating point ( floats, doubles ) • %c = character • %e %E - Exponential notation • %s = string • %g %G - Uses %f/F or %e/E as fits. 14 7 7/24/2018 Review Given that “temp” is a double with value 98.63, which would cause the output to appear as "9.86e+01"? A. cout << fixed; B. cout << setprecision( 2 ); C. cout << setprecision( 3 ); D. cout << scientific << setprecision( 2 ); E. cout << scientific << setprecision( 3 ); 15 Preview Suppose you have a C++ ifstream variable named “fin” that has been successfully connected to a data file, and that there is an integer in the file ready to be read. How can you read in the integer into the variable “ndata”? A. fin << ndata; B. fin >> ndata; C. ndata << fin; D. fin.read( ndata ); E. fscanf( fin, “%d”, &ndata ); 16 8 7/24/2018 Other Types of Streams • File streams and string streams are special types of I/O streams, transferring data to/from data files and memory buffers respectively. • These special streams provide all the functionality of regular streams ( see next slide ), plus some additional features. 17 The Stream Class Hierarchy Classes lower in the diagram are more specialized versions of those above them. They “inherit” properties from their “parents”, and then istringstream ostringstream add additional features and An ifstream IS an istream. Anything you can do with an functionalities. istream you can also do with an ifstream. 18 9 7/24/2018 File Streams • #include <fstream> to use file streams. • Declare variables of type ifstream or ofstream • Attach file stream to a specific file either when declaring or later using open( ) : – ifstream infile( “myInputData.txt” ); – ofstream outfile; – outfile.open( “myOutputData.txt” ); • Verify the file opened before using stream. 19 Reading in File Names • The “string” data type must be converted to C-style character strings for pass to open( ): string filename; ifstream infile; cerr << “Please enter file name: “; cin >> filename; infile.open( filename.c_str( ) ); 20 10 7/24/2018 Using File Streams • Once file streams are created and connected to open files, they are used the same as any other iostreams, e.g. just like cin and cout: ifstream infile( “myInFile.txt” ); // check ! ofstream outfile( “myOutFile.txt” ); // “ double x; infile >> x; outfile << “x = “ << x << “, x^2 = “ << x * x << endl; 21 Checking Stream Conditions, etc. The following boolean stream methods check error state. E.g. for stream “infile”, check infile.good( ) Function Meaning good( ) True when there are no errors. True when the end-of-file is reached on extraction. eof( ) Occurs only when attempting to read past end of file. fail( ) True when a logical error occurs on extraction or insertion. bad() True when a read (or write) error occurs. clear( ) Clears all bad bits and restores good( ) condition. close( ) Flush all buffers and close file stream. 22 11 7/24/2018 When checking to see if a file opened, clear bad bits between tries while( true ) { cerr << “Please enter file name : “; cin >> filename; infile.open( filename.c_str( ) ); if( infile.good( ) ) break; infile.clear( ); } 23 Likewise When Checking Keyboard Input while( true ) { cerr << “Please enter your age: “; cin >> age; if( cin.good( ) && age > 0 && age < 120 ) break; cin.clear( ); cerr << “Invalid input, please try again.\n”; } 24 12 7/24/2018 Review Given the following code, which correctly writes "apples" to file outfile.txt? ofstream outFS; outFS.open("outfile.txt"); A. outfile.txt << “apples”; B. outFS << “apples”; C. outfile.txt >> “apples”; D. outFS >> “apples”; E. ofstream << “apples”; 25 Three ways to know how much data to read from a file: 1. Using a counter, usually read from the file, but it could come from other sources. 2. Using a sentinel value, indicating the end of valid data. Requires that there exist some ( combination of ) value(s) that are not valid. 3. Detecting the end of the file. • So suppose you wanted to read triplets of ( x, y, z ) doubles from a file . 26 13 7/24/2018 Reading data from a file using a data counter infile >> nData; // Two checks for( int i = 0; i < nData; i++ ) { infile >> x >> y >> z; // Check, then use 4 } 3.14 27.5 42.0 109.9 -15 6.02e23 1.0 2.0 3.4 0.0 15.8 17.323 27 Reading data from a file using a sentinel value int nData = 0; while( true ) { infile >> x >> y >> z; // Check this worked if( x == 0.0 && y == 0.0 && z == 0.0 ) break; 3.14 27.5 42.0 nData++ 109.9 -15 6.02e23 1.0 2.0 3.4 // use data 0.0 15.8 17.323 } 0.0 0.0 0.0 28 14 7/24/2018 Detecting the end of file • The eof( ) method of the stream class will return true when an error has occurred trying to read past the end of the file.