Week 7 - External Files

This week is an extension of Week 6, in which we use the methods of Chapters 6, 7 and now Chapter 8. Inheritance is introduced here in that the data files inherit the iostream insertion (<<) and extraction (>>) operators, as well as all the other data formatting operations of iomanip. We also introduce examples of separating program files into header (.h) files and program (.cpp) files.

In Chapter 8 the following file related items are introduced:

#include <fstream> - basic file stream input/output.
#include <cassert> - if we wish to "assert" that a file opened successfully

ifstream infile("raw.data");
This declares a file stream variable named infile and associates this symbolic name with an actual disk file named "raw.data" which is opened for input.

ofstream outfile("process.data");
This declares a file stream variable named outfile and associates it with a new disk file named "process.data" which is opened for output.

After an attempt is made to open a file with the above statements, we should always check to see if the operation was successful. This can be done with the function fail(), typically as follows:

if(infile.fail())
   cout << "ERROR: Cannot open \"raw.data\"\n";
else {
   cout << "\"raw.data\" file opened\n";
   ...
}

Alternatively we can use the assert function to test if an input or output file opened successfully. We usually use this function for output data files:

assert(outfile);
Note that failing an assertion causes the program to abort with a "core" dump, nevertheless it is ok to use it, and you will get full grade in spite of Dr. Iz's comments and the fact that the assert function is not even mentioned in Friedman and Koffman.

As an example of file usage, if we have a sequence of integer numbers in infile which we would like to transfer to outfile:

int num;
while(infile >> num)
   outfile << num << endl;

Another very popular usage is that of reading a sequence of characters from a file and displaying them on the standard output as follows:

char ch;
while(infile.get(ch))
   cout << ch;

Note that after using a file it should always be closed:
infile.close();
outfile.close();

The various examples following have been loaded onto "condor" and are relevant to this chapter. After logging in, change directory to the week7 directory and list the various files as follows:

cd /home/condor/et181/week7
ls

• Section 6.1: Value and Reference Parameters (carryover from week 6)
  computeSumAve.cpp - computing the sum and average of two numbers, introducing Call-by-Value and Call-by-Reference parameters (p 294)
  void computeSumAve(float, float, float&, float&);
  
• Section 6.2: Functions with Output and Input Parameters
  testGetFrac.cpp - reads a fraction typed in by a user and returns the numerator and denominator (p 304)
  swap.cpp - swapping the values of 2 float variables (swap.pdf overheads)
  
• Section 8.1 - Standard Input / Output Streams
  countChars.cpp - Reading one character at a time (p393)
  
• Section 8.2: External Files
  copyFile.cpp - copying from one file to another (p398)
  gradefile/ - reading quiz grades from a datafile to determine the average grade
  • gradefile.cpp - previous method of reading datafile grade.data (trailer value)
  • grade_fail.cpp - new method of reading datafile grade.data using the new fail() function.
  • grade_ass.cpp - using the assert function
  • grade1file.cpp - to read datafile grade1.data (no trailer value)
  • grade2for.cpp - using a for loop to read grade2.data, in which the number of data elements is at the head of the data.
    
• Section 8.3: Using External Files for Communication between Programs
  payrollFile.cpp - (p408)
  
• Section 8.5: Input / Output Manipulators (Table 8.3)
  setw(int n)
  setprecision(int n)
  fixed
  scientific
  showpoint
  
• Lab 7 will concentrate on creating text data files of coordinate values, transferring them to your local computer using the FTP (File Transfer Protocol) application, and plotting them on your local computer using MATLAB.
  
  

Telnet: "condor"