File input and output  (Page 2/2)

It is considered good programming practice to determine if the file was opened properly. The device token should be a non zero value. It the operating system gives you the value of zero it was not able to open the file. The reason it usually can't open a file is because the filespec is wrong (misspelled or not typed case consistent in some operating systems) or the file is not stored in the location specified. We often test the device token by using an if then control structure with the action consisting of stopping the program if it is true that you got the zero. The first line of the if then control structure looks like this:

if (!inData)

Don’t be misled by the not operator. This reads "if it is true that the token stored in inData is zero". If inData is zero, noting zero is 1 or true.

Read – Moving data from a device that has been opened into a memory location defined in your program. When reading text files that have integer or floating-point constants, the operating systems converts the text symbols to a binary number. The operator used is the extraction or read operator. An example of reading is:

inData>>next_number

This expression is similar to reading from the standard input device (aka the keyboard):

cin>>next_number

The "cin" is a predefined device token associated with the Standard Input and Output devices. For our file reading example you might say, "Go to the device identified by the token stored in the inData variable and read in the next value storing it in the next_number variable within my program".

Write – Moving data from a memory location defined in your program to a device that has been opened. When writing integer or floating-point data types, the operating system converts the binary number into the proper text symbols. The operator used is the insertion or write operator. An example of writing is:

outData<<"Total is: "<<total<<endl;

This expression is similar to writing to the standard output device (aka the monitor):

cout<<"Total is: "<<total<<endl;

The "cout" is a predefined device token associated with the Standard Input and Output devices. For our file writing example you might say, "Go to the device identified by the token stored in the outData variable and write the items listed (the string constant then the value stored in my program variable named total then the endl or new line or the return character)".

Close – Your program requesting the operating system to release a file that was previously opened. There are two reasons to close a file. First, it releases the file and frees up the associated operation system resources. Second, if closing a file that was opened for output; it will clear the out the operating system’s buffer and insure that all of the data is physically stored in the output file. Some examples of closing files:

inData.close();

outData.close();

You need to study this module in conjunction with the demo file provided.

Demonstration program in c++

Creating a folder or sub-folder for source code files

Depending on your compiler/IDE, you should decide where to download and store source code files for processing. Prudence dictates that you create these folders as needed prior to downloading source code files. A suggested sub-folder for the Bloodshed Dev-C++ 5 compiler/IDE might be named:

• Demo_Programs

If you have not done so, please create the folder(s) and/or sub-folder(s) as appropriate.

Download the demo program

Download and store the following file(s) to your storage device in the appropriate folder(s). Following the methods of your compiler/IDE, compile and run the program(s). Study the source code file(s) in conjunction with other learning materials. You may need to right click on the link and select "Save Target As" in order to download the file.

Download from Connexions: Demo_File_IO.cpp

Download from Connexions: Demo_File_IO_Input.txt

After you run the program use a text editor to examine the Demo_File_IO_Output.txt file created by the program. You should see the output as: Total is: 33.3

Definitions

text file

A file consisting of characters from the ASCII character code set.

filename

The name and its extension.

filespec

The location of a file along with its filename.

open

Your program requesting the operating system to let it have access to an existing file or to open a new file.

device token

A key value provided by the operating system to associate a device to your program.

read

Moving data from a device that has been opened into a memory location defined in your program.

write

Moving data from a memory location defined in your program to a device that has been opened.

close

Your program requesting the operating system to release a file that was previously opened.