File input and output February 16, 20061Format Review and File Input Format Review and File Input and Outputand OutputLarry CarettoComputer Science 106Computing in Engineering and ScienceFebruary 16, 20062Outline• Last Tuesday – Formatting output– Controlling appearance compared with default output of six significant figures– Manipulators for width and precision• Today – File input and output– Uses same operators as screen output and keyboard input– Associate program variable name for file with operating system name3How do We Format Output?• Use manipulators in output statements• Requires use of #include<iomanip>• Manipulators we will use– 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 figures4Persistence of Manipulators• setw(w) is in effect for one output item only even in same output statement– cout << setw(10) << x << setw(10) << y;• All other manipulators are in effect until changed (even over multiple output statements)– cout << fixed << x << “ “ << y;– cout << z; // fixed still in effect!5Use of setprecision(p)• Effect depends on other options– 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• Once used, setprecision(p) remains in effect until changed• Setprecision(6) restores default6setw and setprecision• 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– The entire output item is printed– There is no spaced between the output item and the previous output (if any)File input and output February 16, 200627setw 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;8setw and setprecision Question• 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 spaces9setw 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 number10setw and setprecision Question• 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 point3 + 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 ;11Input and Output Files• 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 12Input and Output Files II• 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 <fstream> 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 nameFile input and output February 16, 2006313C++ File Input and Output• Three steps• Use #include<fstream> 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 files14C++ File Input• Use #include<fstream>• Use ifstream command to create program variable name for file and link to operating system file name– ifstream myInput(“input.dat“);– ifstream in(“januaryData.txt”);– ifstream inFile(“exercise4.001”); • Use name to read from file– inFile >> x >> y >> z; (like cin >> x >> y >> z;)– myInput >> date >> time;You choose both file names15Input File Examples122032554319278812 20 32 55 43 19 27 8812 2032 5543 1927 88• 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;• What values do u and c have?x w u v a b c16Output Files• Like input, but use ofstream not ifstream• ofstream command associates program and operating system names– 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;17General File Syntax• ifstream <InputFileVariableName> (“<OperatingSystemInputFileName>”);• ofstream <OutputFileVariableName>(“<OperatingSystemOutputFileName>”);• <OutputFileVariableName> << <OutputList> // like cout• <InputFileVariableName> >> <InputList> // like cin• You choose both names18Program File Example// assumes #include<fstream> usedifstream iFile(“myInput.txt”);ofstream out(“prog.1”);double x, y, z;iFile >> x >> y;z = sqrt( x * x + y * y );out << “ x y z\n”;out << setw(12) << x << setw(12) << y << setw(12) << z;Program variable namesOperating system namesFile input and output February 16, 2006419Accessing Files• 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