2. Simple File Input

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Simple File Input

C++ programs can read and write files in many ways. For the sake of simplicity and uniformity, files are read and written using the same stream method already introduced for keyboard input.

For this page, we first need a test data file with known contents. Please compile and run the following program:


#include <iostream>
#include <fstream>

using namespace std;

int main()
{
	ofstream ofs("data.txt");
	for(int i =1;i <= 10;i++) {
		ofs << "This is line " << i << endl;
	}
	return 0;
}

If you want to see what this program's output is, simply examine the contents of the file "data.txt" that this program creates when run.

Not visible in the output file are some "control characters." A "control character" is a character that, instead of printing a symbol, causes an action to take place, like moving down to the next line on the display. A regular character is simply printed. A control character causes an action.

Here are some common control characters, their symbols, and what they do:

Linefeed	'\n'	Causes the printing position to move to a new line
Tab	'\t'	Causes the printing position to advance to a fixed column
Bell	'\a'	Causes a bell to ring (most platforms)

These special symbols can be used alone or in quoted strings to format the display:


	cout << "This is a test line\n\n";

This example will print a line followed by two linefeeds, which assures one blank line appears before the next line is printed.

Q: If I can add "\n" to the text of my printed lines, why use the special operator "endl" as in the example above?

The operator "endl" does two things. It (1) causes a newline to be printed, and it (2) causes the output to appear immediately.

In C++, input and output streams are "buffered." This means characters are read and written in groups, for the sake of efficiency. When keyboard input is being accepted, an entire line is read at once, which is why the user must press "Enter" to move along. When program data is being written, it is normally emitted in chunks.

To force output to be emitted at a particular time, either use "endl" as in the above example, or do this:


	cout << "This will appear right away." << flush;

The operator "flush" causes immediate output, which means these two lines are equivalent:


	cout << endl;
	cout << "\n" << flush;

Now let's read our data file in an obvious way — line by line. The following program contains the single most common student error in file reading. See if you can spot what the error is, what mistake it creates, and why. To reduce any chance for confusion, the error is in red :


#include <iostream>
#include <fstream>
#include <string>

using namespace std;

// this program contains an error

int main()
{
	ifstream ifs("data.txt");
	string line;
	// error in stream test
	while(!ifs.eof()) {
		getline(ifs,line);
		cout << "[ " << line << " ]" << endl;
	}
	return 0;
}

When you run this program, you will see a blank line is printed after the last valid data line. This is caused by the program error.

The error is attempting to test the stream for "end-of-file" without also trying to read it:


	while(!ifs.eof()) {

Remember: a C++ stream can have any origin — a file, a network connection, a keyboard, or any other source. Therefore the stream cannot detect that the data has ended until a read attempt fails.

Because of this, a program that tests for end-of-file without reading, then reads without testing, always fails.

A successful program always tests and reads at once. Here is a corrected version of the program:


#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
	ifstream ifs("data.txt");
	string line;
	while(getline(ifs,line)) {
		cout << "[ " << line << " ]" << endl;
	}
	return 0;
}

In the next example, we will use the stream extraction operator ">>" to read our file. Compile and run this program (it also has an error ):


#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
	ifstream ifs("data.txt");
	string word1, word2, word3;
	int num;
	while(ifs >> word1 >> word2 >> word3 >> num) {
		cout << "[ "
		<< word1
		<< word2
		<< word3
		<< num
		<< " ]"
		<< endl;
	}
	return 0;
}

Why does the error cause the output to look all squeezed together? To answer this question, we need to look at how the stream extraction operator ">>" works.

The extraction operator is actually a very sophisticated tool. It knows what kind of variable is receiving the data, and it conducts itself accordingly.

It works like this:

Phase 1 (search):
- Read characters.
- If a character is "whitespace" (control characters or spaces), discard it.
- If a non-whitespace character appears that is not appropriate to the target variable, stop, indicate an error, "break" the stream.
- If a character is appropriate to the target variable --
  - For integer variables, any of "+-0123456789".
  - For float/double variables, any of "+-.0123456789e" in a prescribed order.
  - For string variables, any non-whitespace characters.
  — begin phase 2.
Phase 2 (read):
- Read and accept characters that are appropriate to the target variable.
- If a character appears that is not appropriate to the target variable, either whitespace or some other character, don't discard it, stop reading, no error.

If you can commit this sequence of events to memory, it will greatly aid your stream programming.

Using the extraction operator is called "formatted reading." It is called "formatted" because the input is expected to have a particular format — groups of characters meant to be received by particular variable types, separated by whitespace. This kind of reading is ideal for text files that contain different kinds of data, data that is separated by whitespace.

Here is another common student error — mixing the extraction operator and getline() in the same program, without appropriate safeguards. Compile and run this program:


#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
	ifstream ifs("data.txt");
	string word1, word2, word3, line;
	int num;
	// read a line using the extraction operator
	if(ifs >> word1 >> word2 >> word3 >> num) {
		cout << "[ "
		<< word1 << ' '
		<< word2 << ' '
		<< word3 << ' '
		<< num << " ]"
		<< endl;
	}
	// read a line using getline
	if(getline(ifs,line)) {
		cout << "[" << line << "]" << endl;
	}
	return 0;
}

Why does this program fail — why can't it read the file's second line? Think:

In Phase 2, the extraction operator reads characters until it encounters whitespace, then it stops without discarding any of the whitespace.
getline() only reads until it encounters a linefeed.

Unfortunately for our test program, the whitespace character that is left behind by the extraction operator is a linefeed. getline() reads this single linefeed and stops without reading the line that follows it.

The solution is to remove the linefeed that was left behind by the extraction operator. Here is the corrected program:


#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
	ifstream ifs("data.txt");
	string word1, word2, word3, line;
	int num;
	// read a line using the extraction operator
	if(ifs >> word1 >> word2 >> word3 >> num) {
		cout << "[ "
		<< word1 << ' '
		<< word2 << ' '
		<< word3 << ' '
		<< num << " ]"
		<< endl;
	}
	// discard whitespace
	ifs.ignore(10000,'\n');
	// read a line using getline
	if(getline(ifs,line)) {
		cout << "[ " << line << " ]" << endl;
	}
	return 0;
}

As is true throughout all of programming, if you know how the various parts of your program work, you will also know why they don't.

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