CSC 213 Code Chapter 14, Fall 2007

File Processing 1 Entering data using the keyboard is not always convenient…Since engineers deal with big amount of data, keyboard entry become unsatisfactory… As an alternative, data can be entered once into a File and read as needed… And for many times… Therefore, the results of a program can be stored into a File instead of sending them to the screen… Thus, result data can be analyzed by another program… For permanent retention, the computer store files on secondary storage devices such as magnetic disks (Hard disk, Floppy), optical discs (CDROM, DVD) and Tapes… File Processing 2 A B d 9 ! 3 $ 1 L 6 y ; 990012 Joe Lee Computer Science 12/January/1973 The data Hierarchy ! The computer can construct a sophisticated way of representing data using only two states 0s and 1s… This two-states system is called Binary System ! Each 0 or 1 in a binary system is called a bit (binary digit) … ! But a single bit cannot store all the numbers, letters and special characters that the computer must process… The bits are put together in a group called a Byte… There are usually 8 bits in a byte, which represents one Character of data… ! A Character is the smallest element of data… letters digits special characters... ! A Field is a set of related characters… Student's ID, name, date of birth, major... File Processing 3 990012 John Lee Computer Science 12/January/1973 990012 Joe Lee Computer Science 12/January/1973 961234 Dany Eid Translation 23/Mars/1972 961234 Dany Eid Translation 23/Mars/1972 997765 Dory Haj Mathematics 30/Julia/1970 997765 Tom Haj Mathematics 30/Julia/1970 997765 Dory Haj Actuarial Science 30/Julia/1970 997932 Dory Hajjar Actuarial Science 30/Julia/1970 997765 GabbHoo Mechanical Eng. 30/Julia/1970 988295 GabbHoo Mechanical Eng. 30/Julia/1970 990012 John Lee Computer Science 12/January/1973 990012 Joe Lee Computer Science 12/January/1973 961234 Dany Eid Translation 23/Mars/1972 961234 Dany Eid Translation 23/Mars/1972 997765 Dory Haj Mathematics 30/Julia/1970 997765 Tom Haj Mathematics 30/Julia/1970 Student's Application File Student's Course load File ! A Record is a collection of related fields… Student's record... ! A File is a collection of related records… Student's application file... ! A Database is a collection of interrelated files stored together with a minimum of redundancy… Registrar’s database... File Processing 4 I/O Stream Main Memory Input / Output Stream (Recall) ! C++ I/O occurs in a stream …where a stream is simply a sequence of data (bytes)… ! In input operations, the data flow from the device (keyboard, disk, etc.) to the main memory… ! In output operations, data flow from the main memory to the output device (screen, printer, disk)… File Processing 5 I/O Stream Main Memory ! To enable a program to communicate with any I/O device, an object (identification entity) must be created and a stream is associated with this object… ! The association of the stream and the object provides a communication channels between the program and a particular input / output device… Programs File Processing 6 Any other stream object, must first be created by the program before being used … Standard Stream Objects (Recall) ! When a program is lunched by the operating system, four standard stream objects are automatically created and are made ready to be used by the program… Those objects are: # Standard Input… Usually connected to the keyboard. # Standard Output… Usually connected to the screen… # Standard Error… # Standard Log… ! In C++ those stream object can be referred to as follow: # Cin as the Standard Input… # Cout as Standard Output… # Cerr as Standard Error… # Clog as Standard Log… File Processing 7 ... for(;;) { cin >> val1; if (cin.eof()) break; cout << val1 * 2 << endl; } ... Stream Error States The state of a stream may be tested trough a series of flags (bits) … They are: # eofbit # failbit # badbit # goodbit ! The eofbit is set for an input stream after end-of-file is encountered… An end-of-file is signaled after an attempt to read data beyond the end of the stream… The eof() function can me used in the program to determine if end-of-file has been encountered on a stream… It return’s true if end-of-file has encountered and false otherwise… File Processing 8 ... int val1; for(;;) { cin >> val1; if (cin.fail()) break; cout << val1 * 2 << endl; } ... ! The failbit is set for a stream when a format error occurs on that stream … Such as entering non-digit characters into an integer variable… The fail() function reports if a stream operation has failed… It return’s true if an operation has failed and false otherwise… ! Note[JAN1] that rejected data is not lost, when the error occurs… It is up to the programmer to recover or to clear the buffer… File Processing 9 ! The badbit is set for a stream when a severe error occurs that results in the loss of data… Such failures are normally non-recoverable… The bad() function Reports if a stream operation has failed… It return’s true if an operation has failed and false otherwise… ! The goodbit is set for a stream when none of the bits (eofbit, failbit, badbit) is set for the stream… The good() function Reports if a stream operation can be used… It return’s true when all of the three functions eof(), fail() and bad() return’s false… To restore a stream’s state to “good”, the clear() function can be used so that I/O may proceed on that stream… File Processing 10 The status of an I/O stream object can also be tested under the control of a selection structure or a repetition structure… #include <iostream.h> void main() { int val1; while (cin[JAN2] >> val1) { if (val1 == -1) break ; cout << val1 * 2 << endl; } if (!cin[JAN3]) { if (cin.bad()) cout << "Bad Error...\n"; if (cin.fail()) cout << "Bad Input...\n"; if (cin.eof()) cout << "End of File...\n"; } } ! ios::operator void*[j4] is an operator that converts a stream to a pointer … It provides a conversion, that when used in a condition tests it returns a Boolean value… It returns 0 if either failbit or badbit is set in the stream’s error state… ! ios::operator ![j5] It returns a nonzero value if either failbit or badbit is set in the stream’s error state… File Processing 11 operator void* #include <iostream.h> class A { friend istream & operator >> (istream &, A&); public: A (int a) {X = a;} int getX () {return X;}; A& operator +=(int i) { X+=i; return *this; } operator void *() { cout <<"Ok\n"; return 0; } private: int X; }; istream & operator >> (istream &in,A &o) { in >> o.X ; return in; } void main() { A a1(10), a2(20); if (a1){} // Called. while(a1){} // Called. a1 = a2; // Not Called. a1; // Not Called. if (a2 = a1); // Called. if (a2= =a1); // Called for a2 then a1. if (a1 +=5); // Called. if (cin >>a1); // Not Called. } File Processing 12 100 Tony 123.99 231 Bob 341.87 112 Eddy 0 143 Jimmy 8012.9 99 Joe 200 100 Tony 123.99 231 Bob 341.87 112 Edward 143 Jimmy 8012.9 99 Joe 200 Updating Sequential Access Files It is difficult to modify the data in a sequential file without the risk of destroying other data in the file… ! If the name Eddy needs to be changed to Edward… Solutions: 1. The whole file is read into the memory, modified in memory and then written back to the file… 2. The file is read and modified as it is written to a temporarily file. Finally the temporarily file is written back to the original. Such solutions are awkward when updating single record at a time… They are acceptable when many records are update at a time… File Processing 13 // Updating data in a sequential file #include <iostream.h> #include <fstream.h> int main() { int account, accNb; char name[ 30 ], newName[30]; double balance; ifstream srcFile; ofstream dstFile; cout << "Enter the Account #"; cin >>accNb; cout << "Enter new name: "; cin >> newName; srcFile.open("clients.dat", ios::in); dstFile.open("temp.dat", ios::out); if ( !srcFile || !dstFile) { cout << "Error in file open\n"; return(1); } // Modify data in a temporary file… while (srcFile >> account >> name >> balance) { if (account == accNb) dstFile << account << " " << newName << " " << balance << endl; else dstFile << account << " " << name << " " << balance << endl; } srcFile.close(); dstFile.close(); File Processing 14 100 Tony 123.99 231 Bob 341.87 112 Eddy 0 143 Jimmy 8012.9 99 Joe 200 100 Tony 123.99 231 Bob 341.87 112 Edward 0 143 Jimmy 8012.9 99 Joe 200 100 Tony 123.99 231 Bob 341.87 112 Edward 0 143 Jimmy 8012.9 99 Joe 200 // Restore into original file… srcFile.open("temp.dat", ios::in); dstFile.open("clients.dat", ios::out); if ( !srcFile || !dstFile) { cout << "Error in file open\n"; return(1); } while (srcFile >> account >> name >> balance) { dstFile << account << " " << name << " " << balance << endl; } srcFile.close(); dstFile.close(); return 0; } clients.dat temp.dat clients.dat