Intro to File Input/Output in C++

Background material

Lab assignment.

Part 1. Formatted (structured) input/output

For this part you need to download files grade.cpp, data and bad_data.

Brief overview.

Input and output files must be opened before reading and writing may begin. Each file used by a program is refered to by a variable. Variables corresponding to an input file are of the class ifstream, and those corresponding to an output file are of class ofstream:
   ifstream inFile;
   ofstream outFile;
A file is opened using a method open(filename,mode), where filename is the name of the file (in quotation marks), and mode is the mode in which a file is opened, s.a. reading, writing, appending, etc. For instance, the code"myfile",ios::in);"myfile2", ios::out);
opens a file myfile for reading, and myfile2 for writing. After the files have been opened, their variables (in this case inFile and outFile) can be used in the rest of the program the same way as cin and cout. These variables are sometimes called file handles.

In formatted input and output operator >> automatically figures out the type of variables that the data is being stored in. Example:

        int a, b;

        inFile >> a >> b;
Here the input is expected to be two integers. If the input is anything other than two integers (say, an integer and a float), the input operator returns value 'false'. This allows to check if the data entered by the user is of the correct type, as follows:
	if (inFile >> a >> b)
	    outFile << a << b << endl;
	    cerr << "Invalid data" << endl;
Similarly we can place input operator into the condition of a loop. Since operator >> returns 'false' at the end of file as well, in the case when all the data is valid the loop will stop at the end of file.

All files opened in the program must be closed in the end of the program. To close a file, use method close:

Closing an output file causes all the data from the output buffer to be actually written to the file.

Problem 1

File data contains a list of names and scores in the following format:
john 85
mary 88
jake 99
The assignment for this part of the lab is to write a program that reads such data from a file data, increase every score by 10, and write the output to a new file result. The output should look as follows:
john 95
mary 98
jake 109
Exercise 1. Fill in the missing code in the file grade.cpp. Compile and run your program. Check the results in the file result to make sure that the program works correctly.

Question 1. Change your program so that the input is taken from the file bad_data rather than data. What gets written to the file result? Why?

Question 2. Give another example of a file with invalid input. What is the output for this file?

Part 2. Unformatted (unstructured) input/output

For this part of the assignment you need to download file modify.cpp. Also download files data and bad_data if you have not done so already.

Brief overview.

The difference between formatted and unformatted input (or output) is that unformatted input treats a file as just a sequence of bytes with no structure. Consequently, rather than expecting integers, floats, or strings as input, we specify a number of bytes for reading (or writing in the case of output). Methods used for unformatted input are: get() (several versions), getline(), and read(). Methods for unformatted output are put() and write().

The simplest case of unformatted input/output is reading or writing a file character-by-character (recall that the size of a character is exactly one byte). This is a convenient way of processing text files. Example:

	char c;

The first command reads a character from an input file associated with inFile and stores this character in c. The second writes character c to an output file associated with outFile.

In the case of unformatted input there is no such thing as invalid data, since any sequential file can be read byte-by-byte. However, the method call to get() returns 'false' when the end of the file is reached. Placing the input statement in the condition of the while loop (as we did for the formatted input) will cause the loop to stop at the end of the file.

Opening and closing of input/output files for unformatted data is the same as for formatted input/output.

Problem 2

Given a text file, create a copy of this file in which all lower-case letters are changed to upper case. To check if a character is a lower-case letter, we use a function islower(), to change it to upper case use a function toupper().

Exercise 2. Fill in the missing code in the file modify.cpp. Compile and run the program. Check the results in the file result2 to make sure that the program works correctly.

Question 3. Which line of code makes the program stop at the end of the file?

Question 4. Change the program so that the input is taken from the file bad_data. What is the output in this case?

BU CAS CS - Intro to File Input/Output in C++
Created by Elena Machkasova <>
Modified from material by Saratendu Sethi <>.
Material adapted for C++ from Intro to File Input/Output in C.