Reading from files File errors Writing to files

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Andrea Hawkins
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2 Reading from files File errors Writing to files

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? And 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 container like a vector 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 container like a vector And return the array from the function that reads the file Unless the size of the file will always be the same the array should 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 Vectors are very convenient to store data that is read from a file The size does not have to be know in advance As data can be inserted into the vector using push_back Or using more advanced STL methods The result of the input operator can be used to determine when the end of the file Assuming that the file contains no errors

16 There are two broad categories of errors that can occur when reading The file cannot be opened The file doesn t exist Or the path to the file is incorrect The drive containing the file cannot be read The file can be read but contains errors These errors could be file format errors Or simply that the wrong file is being read e.g. trying to open an image file in Excel

17 Failure to open a file can be detected ifstream objects have a.fail() method That returns true if a file could not be opened The programmer has to decide what should happen if a file cannot be opened This depends on the structure of the program but could include Asking the user to enter another file name Canceling the operation and continuing with the program Terminating the program

18 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;

19 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 An exception to this 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

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

20 An example of a simple file format for a file containing data about 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

21 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 The program should respond in some appropriate way to file format errors

22 Some possible responses to file format errors Ignore the data points with errors but process correct data Only process data up to the first error Fix the errors with some automated process Ask the user to fix the errors End the process And then terminate program, ask the user for another file, The choice depends on the program design Ending the read file process and informing the user is a good general option

23 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 It is common to embed the read in an if statement or while loop while (filein >> x)

int i = 0; sets size and checks for errors } // Read the file into the results array while (i < size && filein >> result[i]) {

24 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

26 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

27 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(); }

28 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

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

Types of files Command line arguments File input and output functions Binary files Random access Program data is not persistent across uses of an application To preserve data for use at another time it