06 file processing


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06 file processing

  1. 1. File Processing
  2. 2. Hardware Model CPU Input Devices Memory (RAM) Output DevicesTo this point we have been using the Storage Deviceskeyboard for input and the screen for output. Both of these are temporary. 2
  3. 3. Information Processing Cycle Input Process Output Raw Data (Application) InformationOutput from one process can Storageserve as input to another process. Storage is referred to as secondary storage, and it is permanent storage. Data is permanently stored in files. 3
  4. 4. Input and Output (I/O)• In the previous programs, we would enter the test data and check the results. If it was incorrect, we would change the program, run it again, and re-enter the data.• For the sample output, if we didn’t copy it when it was displayed, we had to run it again.• Depending on the application, it may be more efficient to capture the raw data the first time it is entered and store in a file.• A program or many different programs can then read the file and process the data in different ways.• We should capture and validate the data at its points of origination (ie cash register, payroll clerk). 4
  5. 5. File I/O handled by Classes• Primitive data types include int, double, char, etc.• A class is a data type that has data (variables and values) as well as functions associated with it.• The data and functions associated with a class are referred to as members of the class.• Variables declared using a class as the data type are called objects.• We’ll come back to these definitions as we go through an example.• Sequential file processing – processing the records in the order they are stored in the file. 5
  6. 6. Example Scenario• We have a file named infile.txt• The file has 2 records with hours and rate.• We want to process the file by – reading the hours and rate – calculating gross pay – and then saving the results to outfile.txt 6
  7. 7. infile.txt40 10.0050 15.00• This would be a simple text file.• The file can be created with Notepad or some other text editor.• Just enter two values separated by whitespace and press return at the end of each record. 7
  8. 8. fstream• We have been using #include <iostream> because this is where cin and cout are defined.• We now need to #include <fstream> because this is where the commands to do file processing are defined.• fstream stands for File Stream.• We can have input and output file streams, which are handled by the classes ifstream and ofstream.• Recall that a stream is a flow characters. – cin is an input stream. – cout is output stream. 8
  9. 9. Sample Program#include <iostream> // cin and cout#include <fstream> // file processingusing namespace std;void main ( ){ int hours; double rate, gross; ifstream inFile; //declares and assigns a ofstream outFile; //variable name to a stream 9
  10. 10. Open Functionifstream inFile;//ifstream is the class (data type)//inFile is the object (variable)inFile.open(“infile.txt”);//open is a function associated with the object inFile//open has parenthesis like all functions ( )//The function call to open is preceded by the objectname and dot operator.//We pass open( ) an argument (external file name). 10
  11. 11. Internal and External File Names ifstream inFile; inFile.open(“infile.txt”);• inFile is the internal file name (object).• infile.txt is the external file name.• open connects the internal name to the external file.• Naming conventions for the external file names vary by Operating Systems (OS).• Internal names must be valid C++ variable names.• In the open is the only place we see the external name. 11
  12. 12. Check if Open was Successful• after calling open ( ), we must check if it was successful by using fail(). if ( inFile.fail() ) //Boolean { cout << “Error opening input file…”; exit (1); //<cstdlib> } 12
  13. 13. Open outfile.txtoutFile.open(“outfile.txt”);if ( outFile.fail() ){ cout << “Error opening output file…”; exit (1);} 13
  14. 14. Quick Reviewvoid main ( ){ int hours; double rate, gross; ifstream inFile; outFile.open(“outfile.txt”); ofstream outFile; if ( outFile.fail() ) inFile.open(“infile.txt”); { cout << “Error…”; if ( inFile.fail() ) exit (1); { cout << “Error …”; } exit (1); } 14
  15. 15. Recall infile.txt?40 10.0050 15.00• Recall that we are processing infile.txt.• The next slide shows the while loop.• The two values have been entered separated by whitespace and there is carriage return at the of each record (also whitespace). 15
  16. 16. Process the fileinFile >> hours >> rate; // read first record// cin >> hours >> rate; //same extractors infile.txt 40 10.00while (! inFile.eof() ) 50 15.00{ eof gross = hours * rate; outFile << hours << rate << gross << endl; //write// cout << hours << rate << gross << endl; inFile >> hours >> rate; //read next record} 16
  17. 17. outfile.txt40 10.00 400.0050 15.00 750.00• Assumes decimal formatting was applied. 17
  18. 18. Close Files inFile.close( ); outFile.close( );• Release files to Operating System (OS).• Other users may get file locked error.• Good housekeeping. 18
  19. 19. Outfile.txt while vs do-while • 10.00 400.00 50 15.00 750.00inFile >> hours >> rate; 50 15.00 750.00while (! inFile.eof() ) do{ { inFile >> hours >> rate; gross = hours * rate; gross = hours * rate; outFile << hours << rate outFile << hours << rate << gross << endl; << gross << endl; inFile >> hours >> rate; } while (! inFile.eof() )} //writes last record twice. 19
  20. 20. Outfile.txt Empty infile.txt ?? ??.?? ??.??inFile >> hours >> rate;while (! inFile.eof() ) do{ { gross = hours * rate; inFile >> hours >> rate; outFile << hours << rate gross = hours * rate; << gross << endl; outFile << hours << rate << gross << endl; inFile >> hours >> rate;} } while (! inFile.eof() )//while will leave outfile //do-while writes garbage empty. 20
  21. 21. Do-while and File Processing• Do-while processes last record twice.• Do-while does not handle empty files.• Use a while loop for file processing. 21
  22. 22. Constructors and Classes• Classes are defined for other programmers to use.• When an object is declared using a class, a special function called a constructor is automatically called.• Programmers use constructors to initialize variables that are members of the class.• Constructors can also be overloaded to give programmers options when declaring objects. 22
  23. 23. ifstream Constructor Overloaded• Default constructor takes no parameters. ifstream inFile; inFile.open(“infile.txt”);• Overloaded constructor takes parameters. ifstream inFile (“infile.txt”); Declare object and open file on same line. Gives programmer options. 23
  24. 24. Magic Formula• Magic formula for formatting numbers to two digits after the decimal point works with all output streams (cout and ofstreams).cout.setf(ios::fixed) outFile.setf(ios::fixed)cout.setf(ios::showpoint) outFile.setf(ios::showpoint)cout.precision(2) outFile.precision(2)• Note that cout and outfile are objects and setf and precision are member functions. 24
  25. 25. Set Flag• setf means Set Flag.• A flag is a term used with variables that can have two possible values. – On or off, 0 or 1 – Y or N, True or false• showpoint or don’t showpoint.• Use unsetf to turn off a flag. cout.unsetf(ios::showpoint) 25
  26. 26. Right Justification• To right justify numbers, use the flag named right and the function named width together.• Numbers are right justified by default.• Strings are left justified by default. cout.setf(ios::right); cout.width(6); 26
  27. 27. Using right and width( )outFile.setf(ios::right);outFile.width(4); //use four spacesoutFile << hours; //_ _40outFile.width(6);outFile << rate; //_ _40 _10.00Have to enter four different commands; it’s a lot of typing… 27
  28. 28. Manipulator• Short cut to call a formatting function.outFile.setf(ios::right);outFile << setw(4) << hours << setw(6) << rate;//need #include <iomanip> 28
  29. 29. Formatted I/O• cin uses formatted input.• If you cin an integer or double, it knows how to get those values.• If you enter an alpha character for a cin that is expecting a number, the program will bomb (try it).• To control for this, you need to get one character at a time and then concatenate all of the values that were entered.• For example, for 10.00, you would get the 1, 0, a period, 0, and then another 0.• If the values were all valid, you would convert these characters into a double using some built in predefined functions (atof, atoi). 29
  30. 30. Character I/O Functions• We actually use some of character I/O functions in our displayContinue().• To get one character including white space use get. cin.get(char);• To get entire line up to the newline use getline. cin.getline(char, intSize);• To put out one character use put. cout.put(char); 30
  31. 31. Functions to Check Value• After you get one character, you will need to check what it is.isalpha(char) - (a-z)isdigit(char) - (0-9)isspace(char) - (whitespace)isupper(char) – (checks for uppercase)islower(char) – (checks for lowercase) 31
  32. 32. Functions to Change Value• These functions can change the case if the value in the char variable is an alpha. tolower(char) toupper (char)Is the following statement true or false? if (islower(tolower(‘A’))) //true or false 32
  33. 33. ASC II Table• Review the ASCII table available on the website.• ASCII table shows the binary value of all characters.• The hexadecimal values are also presented because when a program aborts and memory contents are displayed, the binary values are converted to hexadecimal (Hex).• The Dec column shows the decimal value of the binary value.• Look up CR, HT, Space, A, a• It shows that the values for a capital A is the same as a lowercase a.• The values also inform us how data will be sorted. 33
  34. 34. Control Structures Reviewed• 1st Sequence Control• 2nd Selection Control (if, if-else)• 3rd Repetition Control (while, do-while)• 4th Case Control – Also known as Multiway Branching (if-else) – Related to Selection Control• The switch statement is an alternative to if-else statements for handling cases. 34
  35. 35. Switch Example – main()void main(){ char choice; do // while (choice != D) { cout << "Enter the letter of the desired menu option. n" << "Press the Enter key after entering the letter. n n" << " A: Add Customer n" << " B: Up Customer n" << " C: Delete Customer n" << " D: Exit Customer Module n n" << "Choice: "; cin >> choice; 35
  36. 36. switch (choice) //controlling expression{ case ‘a’: case A: addCustomer(); break; case B: upCustomer(); break; case C: deleteCustomer(); break; case D: cout << "Exiting Customer Module...please wait.nn"; 36
  37. 37. Controlling Expression• The valid types for the controlling expression are – bool – int – char – enum• Enum is list of constants of the type int – Enumeration – Enum MONTH_LENGTH { JAN = 31, FEB = 28 …}; – Considered a data type that is programmer defined. 37
  38. 38. Switch Statement Summary• Use the keyword case and the colon to create a label. – case ‘A’: – A Label is a place to branch to.• Handle each possible case in the body.• Use the default: label to catch errors.• Don’t forget the break statement, or else the flow will just fall through to next command.• Compares to an if-else statement. 38
  39. 39. Break Statement• The break statement is used to end a sequence of statements in a switch statement.• It can also be used to exit a loop (while, do-while, for-loop).• When the break statement is executed in a loop, the loop statement ends immediately and execution continues with the statement following the loop. 39
  40. 40. Improper Use of Break• If you have to use the break statement to get out of a loop, the loop was probably not designed correctly.• There should be one way in and one way out of the loop.• Avoid using the break statement. 40
  41. 41. Break Exampleint loopCnt = 1;while (true) //always true{ if (loopCnt <= empCnt) //Boolean is inside the loop break; getRate() getHours() calcGross() displayDetails() loopCnt++;} 41
  42. 42. Continue Statement• The continue statement ends the current loop iteration.• Transfers control to the Boolean expression.• In a for loop, the statement transfers control to the up expression.• Avoid using the continue statement. 42
  43. 43. Continue Exampleint loopCnt = 1;while (loopCnt <= empCnt){ getRate() getHours() calcGross() if (gross < 0 ) //skip zeros { loopCnt++ continue; //go to top of loop } displayDetails() loopCnt++;} 43
  44. 44. Summary• File Processing• Introduced classes and objects• Formatting flags and functions• Character I/O and functions• ASCII Table• Switch Statement 44
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