9: Files and Streams

What We Will Cover Announcements Please silence all cell phones and pagers Sit at a computer station, start your computer and login When class is over, please shut down your computer. Remember that you lose any files not saved in the student folder. Continuations 9.1: Streams and File I/O 9.1.1: About Program I/O 9.1.2: Files 9.1.3: Types of Files 9.1.4: Streams 9.1.5: Example of File I/O 9.1.6: Procedure For File I/O 9.1.7: Summary Exercise 9.1 9.2: Reading Data From Files 9.2.1: Reading Data Using an Input Stream 9.2.2: Reading Files Using getline() 9.2.3: Using Loops to Read Files 9.2.4: Reading Data into an Array 9.2.5: Summary Exercise 9.2 9.3: More I/O Topics 9.3.1: Alternative Syntax for Open 9.3.2: Appending to a File 9.3.3: File Names as Input 9.3.4: Stream Parameters 9.3.5: Other File Operations 9.3.6: Summary Exercise 9.3 9.4: Formatting Output 9.4.1: Formatting Functions 9.4.2: Manipulators 9.4.3: Example of Formatting Data 9.4.4: Summary Exercise 9.4 Wrap Up Due Next: A8: Tic-Tac-Toe (4/19/07) Exercise 8 and CodeLab Lesson 8 (4/19/07) A9: Calculate Your Grade (4/26/07) Exercise 9 and CodeLab Lesson 9 (4/26/07) Continuations Questions from last class? Homework Questions? A8: Tic-Tac-Toe (4/19/07) Exercise 8 and CodeLab Lesson 8 (4/19/07) Homework Discussion Questions (Thursday) What value do you store in the board array? What value does the user enter into the program? What is the syntax for declaring the board array? What does the displayBoard() function do? What does the move() function do? What does the checkWinner() function do? Did anyone use more functions than the required functions? Where did you place the main loop? Did anyone code the autoMove() function? ^ top 9.1: Streams and File I/O Objectives At the end of the lesson the student will be able to: List the types of files Describe the purpose of a stream Read and write text files ^ top 9.1.1: About Program I/O Program I/O = Program Input/Output Input to and output from programs Input can be from a keyboard, mouse or file Output can be to a display screen, printer or file Note that files can be both input and output devices for programs Advantages of file I/O Data still exists after the program ends Input can be automated (rather than entered manually) Output from one program can be input to another To store and retrieve data in a file, we need two items: A file A file stream object We will look at files first ^ top 9.1.2: Files File: a collection of data stored under a common name on a storage medium. Files provide long-term storage of large amounts of data Usually, you store files on non-volatile storage mediums Magnetic disks Optical disks Flash storage, like USB storage devices File are a single sequence of bytes Byte 0 Byte 1 Byte 2 ... Byte n−1 End-of-file marker The OS keeps track of the number of bytes in a file Using either an end-of-file marker or counting the number of bytes Files must have a name Naming requirements depend on the underlying operating system (OS) The OS organizes files into directories ^ top 9.1.3: Types of Files All data and programs are ultimately just zeros and ones Each binary digit can have one of two values: 0 or 1 A bit is one binary digit A byte is a group of eight bits Programs group files into two broad categories: text and binary Text Files In text files, the bits represent printable characters Files are usually stored as one byte per character (ASCII) Each record (like a line) are often delimited by end-of-line characters Macintosh (before OS-X): "\r" Unix: "\n" Windows: "\r\n" Individual fields within a record are often delimited by commas, tabs or other special characters Source code is stored as a text file You can read text files because each byte is translated by display devices into readable characters Binary Files Everything other than text is called binary Each bit represents some type of encoded information E.g.: program instructions or numeric data Binary files are easily read by the computer but not by humans For Example If we wanted to store an int value of 1234 in a file We know an int is four bytes long We can store our value as either 1234 or "1234" How we store it determines what is actually written to the file Description Byte 0 Byte 1 Byte 2 Byte 3 (int) 1234 as binary bits 00000000 00000000 00000100 11010010 (int) 1234 as bytes 0 0 4 210 "1234" as char's '1' '2' '3' '4' "1234" as ASCII codes 49 50 51 52 ^ top 9.1.4: Streams Stream: a one-way transmission path that either delivers data to a destination (screen, file, etc.) or that takes data from a source (keyboard, file, etc.) A stream connects a program to an I/O object Input stream: an object that provides a sequence of bytes to a program Output stream: an object that accepts a sequence of bytes from a program cin and cout are input and output streams File Streams File stream: a one-way transmission path used to connect a program to a file. File streams can be either input or output streams File input streams receive data from a file File output streams send data to a file File I/O also uses streams of type ifstream and ofstream Each file your program uses will need a separate file stream object Streams and objects Streams are objects and thus cin and cout are objects Objects are special variables that can have a function associated with them To call a function of an object, you use the dot operator That is why we use the following code to format output cout.setf(ios::fixed); // fixed notation, not scientific cout.setf(ios::showpoint); // show decimal point cout.precision(2); // show 2 decimal places ^ top 9.1.5: Example of File I/O As an example, let us write an application that reads and writes files We read a file named infile.txt, which contains the following 3 values: 10 20 30 We sum the numbers and write the sum to a second file named outfile.txt What do you notice that is different about the following code? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 /** * Reads three numbers from the file infile.txt, * sums the numbers, and writes the sum to the * file outfile.dat. */ #include <fstream> #include <iostream> using namespace std; int main() { ifstream fin; ofstream fout; fin.open("infile.txt"); if (fin.fail()) { cout << "Input file opening failed.\n"; exit(1); } fout.open("outfile.txt"); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } int first, second, third; fin >> first >> second >> third; fout << "The sum of the first 3\n" << "numbers in infile.txt\n" << "is " << (first + second + third) << endl; fin.close(); fout.close(); cout << "Processing completed\n"; return 0; } How does the program get its input? Where is the program output? ^ top 9.1.6: Procedure For File I/O Place the following include directives in your program file: #include <fstream> // for file I/O #include <iostream> // for cout using namespace std; Declare names for input and output streams ifstream fin; ofstream fout; Connect each stream to a file using open() and check for failure fin.open("infile.txt"); if (fin.fail()) { cout << "Input file failed to open.\n"; exit(1); } fout.open("outfile.txt"); if (fout.fail()) { cout << "Output file failed to open.\n"; exit(1); } Read or write the data. Read from a file with fin like using cin fin >> first >> second >> third; Write to a file with fout like using cout fout << "first = " << first << endl; Close the streams when finished fin.close(); fout.close(); ^ top 9.1.7: Summary Programs use streams for input and output cin and cout are types of streams File I/O also uses streams of type ifstream and ofstream Streams are objects and have functions associated with them Some of the functions we call for file I/O include: open("fileName"): establishes a connection from the stream to the file fail(): tests the stream for errors close(): closes the connection of the stream to the file You use input streams like you use cin fin >> first >> second >> third; You use output streams like you use cout fout << "first = " << first << endl; To halt execution of your program on error, use the exit function exit(1); Check Yourself What is used to connect a program to a file? What library is required for file I/O? How are streams declared? What stream function is used to open a file? What stream function is used to check that a file opened correctly? How do you read and write data using a stream? Why should you close a stream after you are done using it in your program? ^ top Exercise 9.1 In this exercise we write a program that copies two numbers from an input stream to an output stream. Specifications Save the following code as copytwo.cpp. Save the file infile.txt to the same directory as your program source code. Declare and open an input stream named fin that reads from infile.txt. Declare and open an output stream named fout that writes to a file named outfile.txt. Add code to read the first two numbers from the input stream and save it to the output stream. Add code to close both of the streams. Compile and run your program. View the outfile.txt file using TextPad or WordPad to verify that two numbers were copied. View the output with Notepad after you verify the program is working. Notice how Notepad cannot handle the end of line characters for Unix. Submit your program as the answer to this exercise. Starter Code for copytwo.cpp 1 2 3 4 5 6 7 8 #include <iostream> using namespace std; int main() { // Enter code here return 0; } ^ top 9.2: Reading Data From Files Objectives At the end of the lesson the student will be able to: Read data of various types from a file Check for end-of-file conditions Load data into an array using a loop ^ top 9.2.1: Reading Data Using an Input Stream You normally use the extraction operator >> to input data from a file ifstream fin; // ... more code here double num; fin >> num; cout << "Read data: " << num << endl; The variable fin is a stream operator and works just like cin: Skips whitespace Reads non-whitespace characters Stops reading when whitespace is found This works for all data types including characters and strings You can see this behavior in the following program If we change the data type, we can still read data from infile.txt The input stream changes the data into the type needed for the variable Reading From a File 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 #include <fstream> #include <iostream> using namespace std; int main() { ifstream fin; fin.open("infile.txt"); if (fin.fail()) { cout << "Input file opening failed.\n"; exit(1); } double num; fin >> num; cout << "Read data: " << num << endl; fin.close(); return 0; } Complications Reading Lines with Spaces However, just like with cin, there are complications when you want to read words with spaces between them >> skips whitespace and stops on encountering more whitespace Thus, you only get a single word for each input variable If you want to read a complete line of text like "Hello Mom!", you need to use getline() ^ top 9.2.2: Reading Files using getline() Sometimes you need to read and process an entire line as text For this, we can use the getline() function of the string class For example: ifstream fin; // ... more code here string line; getline(fin, line); cout << "Read data: " << line << endl; Recall that getline() stops reading when it encounters a '\n' By contrast, fin >> variable operates as follows: Skips whitespace Reads characters Stops reading when whitespace is found Thus if you mix fin >> variable followed by getline(fin, line), you get mysterious results Just like with cin, you get around this problem you by either: Only use getline() before using fin (and never after using fin) Use fin.ignore() after using fin and before using getline() fin.ignore(1000, '\n'); ^ top 9.2.3: Using Loops to Read Files Sometimes you do not know how many data items are in a file To solve this, the typical approach is to use a loop to process the file Then we use a sentinel value from the file input to end the loop Whenever a program does a read operation it returns a value that can be interpreted as a boolean A 0 if it cannot read, otherwise a non-zero value Thus we can use this returned value as our sentinel For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 #include <fstream> // for file I/O #include <iostream> using namespace std; int main() { ifstream fin; fin.open("infile.txt"); if (fin.fail()) { cout << "Input file failed to open.\n"; exit(1); } double nextNum, sum = 0; int count = 0; while(fin >> nextNum) { cout << "Read: " << nextNum << endl; sum = sum + nextNum; count++; } cout << "average = " << (sum / count) << endl; fin.close(); return 0; } The expression (fin >> next) returns true if successful Also, it returns false if the program attempts to read beyond the end of a file Reading Complete Lines Sometimes you need to read and process an unknown number of lines in a file For this you can use the getline() function in a loop as well: while(getline(fin, line)) { cout << line << endl; } ^ top 9.2.4: Reading Data into an Array Sometimes you want to process the data in a file several times One way to do this is to load the data into an array Then you can process the data as an array For example: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 #include <fstream> // for file I/O #include <iostream> using namespace std; const int MAX_SIZE = 100; int main() { int data[MAX_SIZE]; ifstream fin; fin.open("infile.txt"); if (fin.fail()) { cout << "Input file failed to open.\n"; exit(1); } // Load data into array int count = 0; while(fin >> data[count] && count < MAX_SIZE) { cout << "Read: " << data[count] << endl; count++; } fin.close(); // Process array data double sum = 0; for (int i = 0; i < count; i++) { sum = sum + data[i]; } cout << "average = " << (sum / count) << endl; return 0; } ^ top 9.2.5: Summary You can use the extraction operator to read data from a file ifstream fin; // ... more code here double num; fin >> num; cout << "Read data: " << num << endl; The variable fin is a stream operator and works just like cin Similarly, you use the getline() function to read complete lines of text with spaces between words ifstream fin; // ... more code here string line; getline(fin, line); cout << "Read data: " << line << endl; Sometimes you do not know how many lines are in a file To solve this, the typical approach is to use a loop to process the file while(fin >> line) { cout << line << endl; } If you want to process the data from a file several times, you can use an array to store the data Check Yourself How do you code a statement to read a string from a file that includes spaces? What function do you use after using fin and before using getline()? What algorithm do you use to read every number in a file when you do not know how many numbers are in the file? What loop condition do you use to read every number in a file? What loop condition do you use to read every line in a file? If you want to process the data from a file several times, what can you do? ^ top Exercise 9.2 In this exercise we explore reading all the lines of a file using a loop. Specifications Save the following code as loadfile.cpp. Save the file infile.txt to the same directory as your program source code. Declare and open an input stream named fin that reads from infile.txt. Declare and open an output stream named fout that writes to a file named outfile.txt. Add code to read all the numbers from the input stream and save it to the array named data Add code to find the average, minimum and maximum from the array of data. Add code to display the average, minimum and maximum to the screen using cout. Add code to write the average, minimum and maximum to outfile.txt. Add code to close both of the streams. Compile and run your program. Submit your program as the solution for this exercise. 1 2 3 4 5 6 7 8 9 10 11 12 13 #include <fstream> // for file I/O #include <iostream> using namespace std; const int MAX_SIZE = 100; int main() { int data[MAX_SIZE]; // Enter code here return 0; } ^ top 9.3: More I/O Topics Objectives At the end of the lesson the student will be able to: Append data to files Code user input as file names Code stream parameters in functions ^ top 9.3.1: Alternative Syntax for Open It is possible to create a stream and open a file in one step Rather than: ofstream fout; fout.open("messages.txt"); You can use: ofstream fout("important.txt"); ^ top 9.3.2: Appending to a File The standard open operation for writing begins with an empty file Even if the file exists you loose all the contents To prevent loosing the information, you must open for appending to the file ofstream fout; fout.open("important.txt", ios::app); If the file doesn't exist then ofstream creates it If the file exists then ofstream positions itself to append to the end The second argument is a constant defined in class ios For Example The following program will add a message to the file every time we run it 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 #include <fstream> #include <iostream> using namespace std; int main() { string message; cout << "Enter a message to add to the file: "; getline(cin, message); ofstream fout; fout.open("messages.txt", ios::app); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } fout << message << endl; fout.close(); cout << "See you later!\n"; return 0; } ^ top 9.3.3: File Names as Input Note that the argument to open() is a string type fin.open("infile.txt"); Instead of a literal string, we can use a string variable However, we must use the c_str() function to convert to a C-string For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 #include <fstream> // for file I/O #include <iostream> using namespace std; int main() { string filename, line; cout << "Enter a file name: "; cin >> filename; ifstream fin; fin.open(filename.c_str()); if (fin.fail()) { cout << "Input file failed to open.\n"; exit(1); } while(getline(fin, line)) { cout << line << endl; } fin.close(); return 0; } About C-strings C++ offers a special kind of char array known as a C-string A C-string is a hold over from the C programming language However, many library functions still use C-strings and not string variables Thus we need to convert string variables to C-strings in these cases We will study C-strings later in the course ^ top 9.3.4: Stream Parameters Stream types can be formal parameters in functions However, they must be call-by-reference parameters void sayHello(ofstream& aStream) { aStream << "hello\n"; } Note that it is possible to leave out the 'f' in the stream parameter This would give us a function like: void sayHello(ostream& aStream) { aStream << "hello\n"; } An ostream (no 'f') parameters accepts either cout or ofstream objects as arguments Similarly, istream (no 'f') parameters accept either cin or ifstream objects as arguments Using this feature, we can make functions work for both files and user I/O For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 #include <fstream> // for file I/O #include <iostream> using namespace std; /** * Outputs a line to the ostream. * * @param aStream the output stream * @param line The string to output */ void output(ostream& aStream, string line); int main() { ifstream fin; fin.open("infile.txt"); if (fin.fail()) { cout << "Input file opening failed.\n"; exit(1); } ofstream fout; fout.open("outfile.txt"); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } string line; while(getline(fin, line)) { output(cout, line); output(fout, line); } fin.close(); fout.close(); return 0; } void output(ostream& aStream, string line) { aStream << line << endl; } ^ top 9.3.5: Other File Operations Some file operations are not supported by C++ streams In these cases, you use the C-style functions Note that these functions take C-string arguments Thus you must use the c_str() function when using string variables Commonly Used C-Functions for File Manipulation Function Description remove(fileName) Deletes the file specified by the C-string fileName. rename(oldName, newName) Changes the file or directory name specified by the C-string oldName to the newName. perror(message) Print the error message specified by the C-string along with the system error message. system(command) Executes a command specified by the C-string like you were using the command line. Example to Remove a File string fileName; cout << "File to remove: "; cin >> fileName; int result = remove(fileName.c_str()); if (result == 0) { cout << "File successfully removed\n"; } else { perror("Error removing file\n"); } Example to Rename a File int result; string oldName, newName; cout << "Old file name: "; cin >> oldName; cout << "New file name: "; cin >> newName; result = rename(oldName.c_str(), newName.c_str()); if (result != 0 ) { perror( "Error renaming file" ); } Examples Using system() Function system("ls"); // list files system("clear"); // clear the screen system("cmd.exe /c color 1E"); // change colors Changing Console Colors Since we are running Cygwin on Windows, we can use Windows to change the console colors The Windows command function is a program named cmd You can set the console colors using the COLOR command using: system("cmd.exe /c color attr"); Where attr is TWO hex digits The first specifies the background The second specifies the foreground Each digit can be any of the following values: 0 = Black 8 = Gray 1 = Blue 9 = Light Blue 2 = Green A = Light Green 3 = Aqua B = Light Aqua 4 = Red C = Light Red 5 = Purple D = Light Purple 6 = Yellow E = Light Yellow 7 = White F = Bright White ^ top 9.3.6: Summary The standard open operation will create an empty file You can open a file for appending data by using an extra argument ofstream fout("important.txt", ios::app); You can use string variables to open files However, we must use the c_str() function to convert to a C-string ifstream fin; fin.open(filename.c_str()); Streams can be arguments to a function, but you must use call-by-reference Type istream for function parameters works for both cin and ifstream Type ostream for function parameters works for both cout and ofstream Check Yourself How do you keep a file opened for writing from destroying existing information? What string function do you use to convert a string to a C-string? How do you code streams as parameters for a function? ^ top Exercise 9.3 In this exercise we explore reading all the lines of a file using a loop. Specifications Copy the following code into a text editor and save the file as showfile.cpp. Stub out the function toScreen() and call the function toScreen() from main(). Compile your program to make certain it is correct so far. Define the function toScreen() so that it reads one line at a time from the input stream parameter using function getline() and displays each line it reads to the screen. Compile, run and test the program to make sure it is correct. Submit your program as the solution for this exercise. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 #include <fstream> // for file I/O #include <iostream> using namespace std; void toScreen(ifstream& fin); int main(void) { string filename, line; cout << "Enter a file name: "; cin >> filename; ifstream fin; fin.open(filename.c_str()); if (fin.fail()) { cout << "Input file failed to open.\n"; exit(1); } // Call function toScreen() here return 0; } // Add new function toScreen() here ^ top 9.4: Formatting Output Objectives At the end of the lesson the student will be able to: Write code using output formatting functions Write code using manipulators ^ top 9.4.1: Formatting Functions Formatting functions are used to set the layout of the output These same functions apply to cout precision: sets the number of digits to display for floating-point numbers fout.precision(2); setf: sets a format flag for the stream fout.setf(ios::fixed); // Fixed notation fout.setf(ios::scientific); // Scientific notation fout.setf(ios::showpoint); // show decimal point fout.setf(ios::showpos); // Show + signs fout.setf(ios::right); // Right justify fout.setf(ios::left); // Left justify unsetf: clears a format flag fout.unsetf(ios::showpos); // No more + signs width: set or return the field width Only applies to next item that is output Width expands automatically so that entire item is always output fout.width(3); fout << 3 << endl; // ouputs <space><space>3 fout << 123.45 << endl; // ouputs 123.45 For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 #include <fstream> // for file I/O #include <iostream> using namespace std; /** * Outputs a line to the ostream. * * @param aStream the output stream * @param line The string to output */ void formatOutput(ostream& aStream, double number); int main() { double number; cout << "Enter a number and I will format it: "; cin >> number; ofstream fout; fout.open("formatted.txt", ios::app); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } formatOutput(cout, number); formatOutput(fout, number); return 0; } void formatOutput(ostream& aStream, double number) { aStream << "The formatted number is ("; aStream.setf(ios::fixed); aStream.setf(ios::showpoint); aStream.setf(ios::showpos); aStream.setf(ios::right); aStream.precision(3); aStream.width(10); // only applies to next output aStream << number << ")" << endl; } ^ top 9.4.2: Manipulators Manipulator: a function called in a nontraditional way Manipulators in turn call member functions Manipulators may or may not have arguments To use manipulators you may need to include: #include <iomanip> using namespace std; endl: outputs a newline character setw: sets the field width from that point onward cout << "Start" << setw(4) << 10 << setw(6) << 20; setprecision: sets the number of digits of precision cout << "$" << setprecision(2) << 9.4 << endl; Further information: C++ I/O Flags (includes manipulators) For Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 #include <fstream> // for file I/O #include <iostream> #include <iomanip> using namespace std; /** * Outputs a line to the ostream. * * @param aStream the output stream * @param line The string to output */ void formatOutput(ostream& aStream, double number); int main() { double number; cout << "Enter a number and I will format it: "; cin >> number; ofstream fout; fout.open("formatted.txt", ios::app); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } formatOutput(cout, number); formatOutput(fout, number); return 0; } void formatOutput(ostream& aStream, double number) { aStream << "The formatted number is (" << fixed << showpoint << showpos << right << setprecision(3) << setw(10) << number << ")" << endl; } ^ top 9.4.3: Example of Formatting Data Following example is adapted from textbook pages 526-527 As an example of formatting data, consider the following data file: 1 2 3 4 5 12.34 -9.87654 2.3131 -89.506 12.33333333 92.8765 -1.234567e2 We want to write a program to take the raw data from the file and format it neatly Also, we will write first to the screen and a then to a file 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 /** * Reads all the numbers in the file rawdata.txt * and writes the numbers to the screen and to * the file neat.txt in a neatly formatted way. */ #include <iostream> #include <fstream> #include <iomanip> using namespace std; void formatData(istream& messyFile, ostream& neatFile, int precision, int fieldWidth); int main() { ifstream fin; fin.open("rawdata.txt"); if (fin.fail()) { cout << "Input file opening failed.\n"; exit(1); } ofstream fout; fout.open("neat.txt"); if (fout.fail()) { cout << "Output file opening failed.\n"; exit(1); } formatData(fin, cout, 5, 12); // Close and reopen input file fin.close(); fin.clear(); // allows reuse of stream fin.open("rawdata.txt"); if (fin.fail()) { cout << "Input file opening failed.\n"; exit(1); } formatData(fin, fout, 5, 12); fin.close(); fout.close(); cout << "End of program.\n"; return 0; } void formatData(istream& messyFile, ostream& neatFile, int precision, int fieldWidth) { neatFile.setf(ios::fixed); neatFile.setf(ios::showpoint); neatFile.setf(ios::showpos); neatFile.precision(precision); double next; while (messyFile >> next) { neatFile << setw(fieldWidth) << next << endl; } } ^ top 9.4.4: Summary C++ has many formatting functions to set the layout of the output You can use them with cout and any ofstream type You can use manipulators to change formatting within the stream Affects output after the manipulator is used and onwards You can use manipulators to format one value at a time Check Yourself How do you set the width of a field when outputting numbers? After you set the width of a numeric field, how long does the setting last? If the number you display needs more digits than the specified width, does the entire number get displayed? By default, is numerical output left or right justified? What is meant by the term manipulator? ^ top Exercise 9.4 In this exercise we explore how to format output. Specifications Create a file named formatting.txt Complete the following and record the answers to the questions in the file. Run the following code: #include <iostream> #include <iomanip> using namespace std; int main(void) { cout << "*"; cout.width(5); cout << 123 << "*" << 123 << "*" << endl; cout << "*" << setw(5) << 123 << "*" << 123 << "*" << endl; return 0; } Q1: To how many number fields does width() or setw() apply? Run the following code: #include <iostream> #include <iomanip> using namespace std; int main(void) { cout << "*" << setw(5) << 123; cout.setf(ios::left); cout << "*" << setw(5) << 123; cout.setf(ios::right); cout << "*" << setw(5) << 123 << endl; return 0; } Q2: By default, are numbers output ios::left or ios::right? Run the following code: #include <iostream> #include <iomanip> using namespace std; int main(void) { cout << "*" << setw(3) << 12345 << "*\n"; return 0; } Q3: How many digits are displayed? Why? ^ top Wrap Up Reminders Due Next: A8: Tic-Tac-Toe (4/19/07) Exercise 8 and CodeLab Lesson 8 (4/19/07) A9: Calculate Your Grade (4/26/07) Exercise 9 and CodeLab Lesson 9 (4/26/07) When class is over, please shut down your computer You may complete unfinished exercises at the end of the class or at any time before the next class. ^ top Home | Blackboard | Announcements | Day Schedule | Eve Schedule Course info | Help | FAQ's | HowTo's | Links Last Updated: May 19 2007 @20:49:09