Lab - File I/O

Objectives

Discussion

First go over doing File I/O in C++. We will use the bigint data files as examples.

You will need to include the following to do file I/O. fstream is for file streams. 


#include <iostream>
#include <fstream>

int main () {
    //Creates a stream inFile that points to a file.
    //Opens the file data1-1.txt for input (reading).
    std::ifstream inFile("data1.txt");

    // Process file
    //

    infile.close(); //Need to close file
}

After opening the file, we should check to see if the file is opened properly. If not, we deal with the error. The file may not exist and hence, can not be open properly. 


if (!inFile.is_open()) {
    std::cout << "Error: Unable to open file";
    // exit or return
}

Now that the we know the file is properly open, it can be read from. Let us go over how get() works. It is the most simple form of input for C++. It reads one character at a time.


    char ch;
    // ch is passed by reference and assigned a SINGLE character
    inFile.get(ch);

The stream inFile is basically a pointer to the location in the file to read next. get(ch) reads the next single character from the stream inFile and assigns it to ch. Also, the stream now points to the next character to read in the file.

However, you maybe at the end of the file (EOF), and there is nothing to read. This is a problem and can cause errors and infinite loops. The eof method on the stream checks the state of the stream. Returns if it is at the end of file or not.

ALWAYS check for end of file before EVERY read.

Normally we loop on not end of file and use get to read the contents of the file.

Below is an example of using eof() with get. It will read each character from a file and print it out. Thus making an exact copy of the file. 


#include <iostream>
#include <fstream>

int main() {
    std::ifstream inFile("data1.txt");
    if (!inFile.is_open()) {
        std::cout  << "Unable to open file\n";
        return 1;
    }

    char ch;
    if (!inFile.eof()) inFile.get(ch);  //Read the first character in the file
    while (!inFile.eof()) {             //It may now be end of file
        std::cout << ch;                //Print out the character
        inFile.get(ch);                 //Read the next character
    }
    std::cout << std::endl;
    inFile.close();
    return 0;
}

Try the above code and get it working.

Why are we doing a read before the loop?

If a read fails (e.g., at the end of the file) it does not modify ch and end of file is true.

What will happen if we did not do a read before the loop and in the loop did a read than print?

Let us now examine the behavior of the >> operator for reading characters from a file.

  1. It skips all leading whitespace
  2. Reads one character
  3. Unlike get() in that whitespace is always skipped

Whitespace is skipped when using >>. Whitespace characters are space (' '), tab ('\t'), new line ('\n'), carriage return ('\r') and end of file.


   char ch;
   while (!inFile.eof()) {
     std::cout << ch;
   }

Try it! Replace the input loop above with the >> operator version. IMPORTANT: Whitespace is skipped with >> operator, Whitespace is NOT skipped when using get().

Notes:



Setup

Need to:

  1. Create a directory called fileio in your cs23001 svn folder.
  2. Copy the provided data and code files. These will be in the shared/labs/ folder. The instructors will give the location/name of the file in lab.
  3. There is a file called fileio.cpp, which will contain the code for this lab assignment.


Assignment

Write code to solve a set of different reading tasks. Use the provided fileio.cpp as a starting point. Compile the code as follows:

clang++ -Wall fileio.cpp -o fileio

This will result in an executable called fileio. The program takes a file name as input. Use the following to run the program:

./fileio data1.txt

Part 1

Count all the characters in a file, including the end of file. Use get().

Example input file contents:

Hello Jan

Example output:

1: H
2: e
3: l
4: l
5: o
6:  
7: J
8: a
9: n
Total: 9

Part 2

Count all the non-whitespace characters in a file. Use >>.

Example input file contents:

Hello Jan

Example output:

1: H
2: e
3: l
4: l
5: o
6: J
7: a
8: n
Total: 8

Part 3

Count the number of words a file. Use >>. A word is any sequence of characters separated by whitespace.

Example input file contents:

Hello Jan,
how are you today?

Example output:

1: Hello
2: Jan,
3: how
4: are
5: you
6: today?
Total: 6

Part 4

Count the number of lines a file. Use getline().

Example input file contents:

Hello Jan,
how are you today?

Example output:

1: Hello Jan,
2: how are you today?
Total: 2

Part 5

Read a sentinel value (integer) from a file (on first line), than read that many lines of data (one word per line).

Requirements

The following should be true when you are done, NAMES MUST MATCH EXACTLY:

URL: https://data-structures.cs.kent.edu/labs/Lab4
Last update: EST