Types of files Command line arguments File input and output functions Binary files Random access

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Claude Shelton
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2 Types of files Command line arguments File input and output functions Binary files Random access

4 Program data is not persistent across uses of an application To preserve data for use at another time it must be saved in a file To retrieve data from a previous session it must be read from a file Input and output streams can be created to read from and write to files

5 The fstream library includes definitions of classes that can be used to create streams The ifstream object for input from files The ofstream object for output to files To use these classes it is necessary to import the fstream library #include <fstream>

This function reads the first line from a file into a string and then returns the string string getfirstline(string fname){ string result; ifstream filein; //an ifstream variable

6 This function reads the first line from a file into a string and then returns the string string getfirstline(string fname){ string result; ifstream filein; //an ifstream variable filein.open(fname); //opens the file // Use the input operator to read the first line filein >> result; //just like using cin filein.close(); //always close files return result; }

7 Reading one line from a file isn't that useful It is more useful to read an entire file, which raises some questions If we want to read the entire file how do we return its contents? How big is the file? How do we know when we've got to the end?

It is natural to read the contents of a file into an array variable or some other sort of container And return the array from the function that reads the

8 It is natural to read the contents of a file into an array variable or some other sort of container And return the array from the function that reads the file Unless the size of the file will always be the same the array must be in dynamic memory So that an array of the appropriate size can be created and returned

9 An array must be created before values are inserted into it The size of a file must be known before reading its contents into an array There are different ways of obtaining this data Count the lines of the file before reading it into the array Include data in the file that records the size

10 If the size of a file is not known it is important to know when its end is reached An ifstream object (variable) has a number of useful functions Recall that functions that belong to an object are referred to as methods The.eof() method returns true when the end of a file has been reached

11 Here is pseudocode to read a file of integers into an array Declare an ifstream object Open the ifstream object Count the lines in the file While not end-of-file: increment count Declare a dynamic array of appropriate size Read the file into the array For 0 to count: store line in array

int* readfile(string fname, int & size){ // Reads a file into an array int* result = NULL; int temp = size= 0; //sets both variables to zero ifstream filein; //ifstream variable filein.

12 int* readfile(string fname, int & size){ // Reads a file into an array int* result = NULL; int temp = size= 0; //sets both variables to zero ifstream filein; //ifstream variable filein.open(fname); //opens the file // Count the number of lines without saving data while (!filein.eof()){ size++; // add one to the count filein >> temp; // don t do anything with the file data } filein.close(); // close the file filein.clear(); // make sure the stream is empty filein.open(fname); // reopen the file to read from start // Now read the file into the results array result = new int[size]; // make the array now the size is known for (int i=0; i < size; i++){ filein >> result[i]; //read line into array } filein.close(); return result; }

13 In the previous example the input file was processed twice Once to count the number of lines To find out how large to make the array Once to read the data into the array This can be avoided in a number of ways Use a different container to store the file data Identify the number of values in the file to be read Use the first value to record the number of values

14 If a file name does not exist, an attempt to read from that file will fail Resulting in the application terminating Or even resulting in an infinite loop ifstream objects have a.fail() method That returns true if a file could not be opened

15 int* readfile(string fname, int & size){ // Reads a file into an array int* result = NULL; int temp = size= 0; //sets both variables to zero ifstream filein; //ifstream variable filein.open(fname.c_str()); //opens the file // Check to see if the file was opened if (!filein.fail()){ // Repeat code from previous function // not shown here for space reasons }//end if } return result;

16 There are numerous errors that can be introduced when reading files Other than the file not existing These include The file being corrupt or unreadable The storage device being unavailable File format errors

17 A file is expected to be in a particular format If it is not it will probably be unreadable For example, Excel will not be able to read a file containing a saved game from Starcraft 2 The exception are text editors which are usually happy to read anything though it may appear as gibberish A file format specifies what data is being stored and how it is formatted A function to read file data must respect the file format

single rectangle With values for the x and y coordinates of the bottom left corner, and height and width

18 An example of a simple file format for a file containing rectangles might be The first number indicates the number of values So that a container of an appropriate size can be created Thereafter each line represents a single rectangle With values for the x and y coordinates of the bottom left corner, and height and width Separated by spaces Rectangles are separated by newlines

19 A robust program should deal with file format errors Where the file is not in the expected format Missing values Incorrect separators (e.g. commas instead of spaces) Values of an inappropriate type Once a file format error is detected the read process should be ended It is usually best to not try to have the program somehow fix the error

20 The same general techniques used for detecting input errors can be applied to files An ifstream object can be checked to see if input has failed The result of >> is zero when input has failed This can be used in if statements or loops to test for success We will write a read function To read a file of integers

21 int* readfile2(string fname, int & size) { int* result = NULL; ifstream filein; filein.open(fname); //opens the file // Assume first value contains size if (filein >> size) { result = new int[size]; // make array int i = 0; sets size and checks for errors } // Read the file into the results array while (i < size && filein >> result[i]) { ++i; } // If size numbers not read successfully file was bad if (i < size) { result = NULL; } } filein.close(); return result; reads each line one by one and checks for errors

23 Writing to files is very similar to reading from files Use an ofstream object to represent the file Open it Check that the file has successfully been opened Then write the desired data to the file Usually one line at a time In a loop Respecting the desired file format

24 This function writes an integer array to a file void writefile(int arr[], int size, string fname) { ofstream fileout; //an ofstream variable fileout.open(fname); //makes the file // Write each line of the array to the file for (int i = 0; i < size; i++){ fileout << arr[i] << endl; } fileout.close(); }

25 Writing to a file creates a new file Any file with the same name (in the same folder) will be replaced without warning To add to an existing file: Use an ofstream object as normal But set open to append fileout.open("myfile.txt", ios::app); Values written to the file will then be added to the end of the file

Reading from files File errors Writing to files

Reading from files File errors Writing to files Program data is not persistent across uses of an application To preserve data for use at another time it must be saved in a file To retrieve data from a