University:
California State University, Northridge
Course:
COMP 380/L | Introduction to Software Engineering and Lab
Academic year:

2006
Views:

418

Pages:

4
Author:

litauiskzzn

File input and output Outline • Last Tuesday – Formatting output – Controlling appearance compared with default output of six significant figures – Manipulators for width and precision Format Review and File Input and Output • Today – File input and output – Uses same operators as screen output and keyboard input – Associate program variable name for file with operating system name 2 How do We Format Output? Persistence of Manipulators • Use manipulators in output statements • Requires use of #include • Manipulators we will use • setw(w) is in effect for one output item only even in same output statement – cout << setw(10) << x << setw(10) << y; – fixed forces fixed format output – scientific forces scientific format output – setw(w) assigns w spaces for output (right justified by default; see left and right) – setprecision(p) uses the value of p to set the number of significant figures • All other manipulators are in effect until changed (even over multiple output statements) – cout << fixed << x << “ “ << y; – cout << z; // fixed still in effect! 3 Use of setprecision(p) 4 setw and setprecision • Effect depends on other options • setprecision controls the number of digits to be printed • setw controls how many spaces are used for the output • What happens if the setw manipulator does not give enough spaces – For default output (not using fixed or scientific manipulators) setprecision(p) gives p significant figures – If fixed or scientific manipulators are used, setprecision(p) gives p decimal places • For scientific output p decimal places is p+1 significant figures – The entire output item is printed – There is no spaced between the output item and the previous output (if any) • Once used, setprecision(p) remains in effect until changed • Setprecision(6) restores default 5 6 1 File input and output setw and setprecision Question setw and setprecision Question • double w = -3.56878e5, x = 1.234; y = 784.525, z = 23456.789; • Write the output statements to print w and x on the first line and y and z on the second line • Print each number with 3 decimal places and allow at least two spaces between the numbers • What is output without formats? • • • • – cout << w << x << endl << y << z; 7 How do you print 3 decimal places? cout << fixed << setprecision(3); What is width in setw(width)? Have to consider number of spaces required for largest number • How many spaces are required? • First, we need 3 spaces numbers after the decimal and 1 for the decimal point for a total of 4 spaces 8 setw and setprecision Question setw and setprecision Question • How many spaces are required in front of the decimal place? • Which number to be printed has the maximum size: w = -3.56878e5, x = 1.234; y = 784.525, z = 23456.789 • Maximum characters before decimal is seven for -3.56878e5 = -356878 • Don’t forget that we want at least two spaces before this number • Now, let’s recap the number of spaces we need for the setw manipulator • Add spaces required after decimal, before decimal, spacing between numbers and decimal point 3 + 7 + 2 + 1 = 13 => setw(13) • What is cout statement? • cout << fixed << setprecision(3) << setw(13) << w << setw(13) << x << endl << setw(13) << y << setw(13) << z ; 9 Input and Output Files 10 Input and Output Files II • Want to be able to read from and write to files on disk storage • Such files can be accessed by computer operating system • Have operating system file name that has short and long form – Short: program.dat – Long: C:\temp\program.dat • File name structure is different for other operating systems 11 • Other programs can use output files prepared by C++ programs • Can use other programs to prepare input files for C++ programs • File input and output on C++ requires #include for file library • C++ commands use a program variable file name to refer to files • Must associate program variable file name with operating system file name 12 2 File input and output C++ File Input and Output C++ File Input • Three steps • Use #include directive to include file library in program • Associate program variable file name with operating system file name • Use same syntax as cout and cin • Replace cout by and cin by program variable names for output and input files • Use #include • Use ifstream command to create program variable name for file and link to operating system file name 13 14 12 20 32 55 43 19 27 88 – ifstream myInput(“input.dat“); – ifstream in(“januaryData.txt”); – ifstream inFile(“exercise4.001”); You choose both file names • Use name to read from file – inFile >> x >> y >> z; (like cin >> x >> y >> z;) – myInput >> date >> time; Input File Examples Output Files 12 20 32 55 43 19 27 88 • Like input, but use ofstream not ifstream • ofstream command associates program and operating system names x w 12 32 43 27 20 55 19 88 u v a b c • All three files are the same to C++ – Same sequence of numbers • ifstream inDat(“today.dat”) • inDat >> x; • inDat >> w >> u >> v; • inDat >> a >> b >> c; – ofstream outFile(“results.out”); – ofstream out(“case.012”); • Use program name in place of cout to write to file – outFile << setw(10) << x; – out << “Results for Case “ << case; • What values do u and c have? 15 General File Syntax 16 Program File Example • ifstream (“”); • ofstream (“”); • << // like cout • >> // like cin • You choose both names 17 // assumes #include used ifstream iFile(“myInput.txt”); ofstream out(“prog.1”); Operating system names double x, y, z; Program variable names iFile >> x >> y; z = sqrt( x * x + y * y ); out << “ x y z\n”; out << setw(12) << x << setw(12) << y << setw(12) << z; 18 3 File input and output Accessing Files File Questions • Access file from any windows program using operating system file name • Can use Visual C++ (or other text editors) to create input files and view output files • Input files must be in project folder • To display available output files, select “All Files” from the pulldown menu next to files of type in the Open menu • Write statement to associate program name myInput with the operating system name, data.txt, as an input file ifstream myInput( “data.txt” ); • Read the first three data items on the file data.txt into the variables x1, x2, and x3 in that order myInput >> x1 >> x2 >> x3; 19 More File Questions 20 Summary • File input and output uses same syntax and commands as keyboard input and screen output • Replace cin by program name for input file and cout by program name for output file • Use ifstream and ofstream to associate operating system file names with program names for input and output • Write statement to associate the operating system name, results.out, with a program variable name to be used as an output file • Write the variables x and y to results.out, preceeded by the strings “x = “ and “y = “ ofstream o( “results.out” ); o << “x = “ << x << “, y = “ << y; 21 22 4

Format Review and File Input and Output

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