The document discusses using files for input/output in C++ programs. It outlines the 5-step process: 1) include fstream header, 2) declare file stream variables, 3) associate variables with files, 4) use stream variables for input/output, 5) close files. It provides examples of opening files for input/output, reading/writing data, seeking to different positions in a file, and challenges the reader to process Dr. King's speech stored in a file.

1. An Introduction To Software
Development Using C++
Class #21:
Files

2. Say Hello To Homework #4!
• Last design did not work out – overloaded the traffic light
processors. Going back to HW#2 solution.
• Mayor’s son collected traffic data.
• The city would like you to use the mayor's son's file to
simulate the traffic pattern that happened on that day. They
are interested in having you calculate several values:
– How many cars came from each direction.
– The maximum size of the line of cars that had to wait to get through
the light.
– How long each car had to wait to make it through the stoplight.
– The average amount of time that cars wait based on which direction
they are coming from (four values)

3. Data, Data, Data
• In the real world, your program will need to process data. A
lot of data.
• In this class we’ve already used two different ways to get data
into your programs:
– You typed it in (“input”)
– It was given to you in the form of constants (6 seconds, 9 seconds,
12 seconds)
• Now it’s time to deal with A LOT OF DATA.
• Say hello to files…
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4. What Is A File?
• C++ considers a file just a sequence of bytes
• Each file ends either with an end-of-file marker or at a
specific byte number recorded in an operating-system-
maintained, administrative data structure.
• When a file is opened, an object is created, and a
stream is associated with the object

5. How Do We Teach C++ How To Get
Data From A File
• The standard I/O header file, iostream, contains the
data types and variables that are used only for input
from a standard input device and output to a
standard output device.
• C++ provides a header file called fstream which is
used for file I/O
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6. The 5 Step File I/O Process
1. Include the header file fstream in the
program.
2. Declare file stream variables.
3. Associate the file stream variables with the
input/output sources.
4. Use the file stream variables with >>, << or
other input / output functions.
5. Close the files.
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7. Step #1: the header file fstream in the
program
• The following statement accomplishes this
task:
#include <fstream>
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8. Step #2: Declare file stream variables
• Consider the following statements:
ifstream indata;
ofstream outdata;
• The first statement declares indata to be an
input file stream variable.
• The second statement declares outdata to be
an output file stream variable.
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9. Step #3: Associate the file stream variables
with the input/output sources
• This step is called “opening the files”
• The stream member function
open is used to open files.
• The syntax for opening a file is:
fileStreamVariable.open(sourceName);
• Here fileStreamVariable is a file stream variable
and sourceName is a file name.
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10. Example Of Opening Files
• Assume input data is stored in a file called
prog.dat and prog.out is to be used for output.
• The following statements open prog.dat for
input and prog.out for output.
inData.open(“prog.dat”);
outData.open(“prog.out”);
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11. Dealing With
• If the input / output files are located in the same subdirectory as
the C++ source code file, then you do not have to specify the path –
just a file name.
• If the input / output file is located in a different subdirectory, then
you must specify both a path and file name.
• If our data file was located on a flash memory in drive H, then we
would have to use the statement:
inData.open(“h:prog.dat”);
• Note the use of two “” – “” is an
escape character and so to produce
• a “” you need to use “”.
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12. Step 4: Use the file stream variables with
>>, << or other input / output functions
• You use the file stream variables with >>, << or other input /
output functions.
• The syntax for using >> or << with file stream variables is
exactly the same as the syntax for using them with cin and
cout.
• However, instead of using cin or cout,
you use the file stream variables that
were declared.
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13. Examples Of File Input / Output
• Example: indata >> payrate;
reads data from the file prog.dat and stores it in the variable payrate
• outdata << “The paycheck is: $” << pay << endl;
stores the output The paycheck is $565.78 – in the file prog.out.
• The following code reads one character from a file:
char c;
while (indata.get(c)) // loop getting single characters
cout << c;
• The following code reads one line from a file:
string s;
while (getline(indata,s)) // loop getting lines
cout << s;
Image Credit: groups.etown.eduHW #4 checker

14. Step 5: Close the files
• Closing a file means that the file stream variables are
disassociated from the storage area and are freed.
• Once freed, they can be used for other file I/O.
• Closing an output file ensures that the entire output is sent
to the file – the buffer is emptied.
• You close files by using the stream function close
• Example:
indata.close()
outdata.close()
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15. The Existence Of Files
• In the case of an input file, the file must exist before the open statement
executes.
• If the file does not exist, then the open statement fails and the input
stream enters the fail state.
– Once a failure has occurred, you must use the function clear to restore the
input stream to a working state.
• An output file does not have to exist before it is opened.
• If the output file does not exist, then the system prepares an empty file for
output.
• If the output file does exist, then
by default its old contents are erased
upon being opened.
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16. How To NOT Destroy An Output File
• To add output to the end of an existing file, you can use the
option ios::app as follows.
• Suppose that outData is declared as before and you want
to add the output to the end of the existing file, say
firstProg.out.
• The statement to open this file is:
outData.open(“firstProg.out”, ios::app);
• If the file firstProg.out does not
exist, the system simply creates
an empty file.
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17. How To Move Around In Both
An Input And An Output File
• Both input and output files provide member functions for
repositioning the file-position pointer (the byte number of
the next byte in the file to be read or written).
• These member functions are seekg (“seek get”) for input
and seekp (“seek put”) for output.
• Each input object has a get pointer, which indicates the
byte number in the file from which the next input is to
occur, and each output object has a put pointer, which
indicates the byte number in the file at which the next
output should be placed.
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18. How To Move Around In
An Input File
• The statement: inClientFile.seekg( 0 );
repositions the file-position pointer to the beginning of the file
(location 0) attached to inClientFile.
• A second argument can be specified to indicate the seek direction,
which can be ios::beg (the default) for positioning relative to the
beginning of a stream, ios::cur for positioning relative to the current
position in a tream or ios::end for positioning relative to the end of
a stream.
• The file-position pointer is an integer value that specifies the
location in the file as a number of bytes from the file’s starting
location (this is also referred to as the offset from the beginning of
the file).
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19. How To Move Around In An Input File
• Some examples of positioning the get file-
position pointer are:
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20. How To Move Around In
An Output File
• The same operations can be performed using
output member function seekp.
• Member functions tellg and tellp are provided to
return the current locations of the get
and put pointers, respectively.
• The following statement assigns the get file-
position pointer value to a variable:
location = fileObject.tellg();
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21. In Class Programming Challenge:
MLK’s “I Have A Dream Speech”
• Open the file that contains Dr. King’s “I have a dream” speech.
• Read in each line and print the number
of times that you find the words “I”, “we”,
“dream”, and “freedom”.
• Print the total number of words in the
speech.
• Write a version of this speech out to a file
with HTML paragraph formatting: “<p>”
at the start of the paragraph and “</p>”
at the end of each paragraph.
Image Credit: http://www.americanrhetoric.com/speeches/mlkihaveadream.htm

22. What’s In Your C++ Toolbox?
cout / cin #include if/else/
Switch
Math Class String getline While
For do…While Break /
Continue
Arrays Functions