Guided: Build a CLI File Reader in C

Welcome to this C++ guided lab!

Code lab organization

This lab is organized in steps, and each step has one or more tasks to be completed. Start with the FileReader.cpp source file. (If, for some reason, you need to undo your modifications, and resume the initial state of that source file, you can copy-and-paste the content of starter_files/FileReader1_initial.cpp).

For your convenience, if you are stuck in one task, partial solution files have been provided, so that you can use them to unlock and make progress. The solution files are provided under the solution directory. There are subdirectories for each step (e.g. step1, step2, etc.). Under each step subdirectory, you'll find solution files for each task of that step.

When you start a new step, you may want to copy the starter_files/FileReaderN_initial.cpp (where N = 1, 2, 3... is the step number) file content into FileReader.cpp, to be sure that you and the tool are on the "same page" for the current code status at the beginning of the step.

How to build C++ code

To compile the C++ source code, you can use the GNU C++ compiler from the terminal. For example, you can issue the command:

g++ -Wall -std=c++14 FileReader.cpp

This will compile the C++ source code FileReader.cpp, and produce the executable a.out. You can run the compiled code launching the executable file a.out previously generated:

./a.out

Of course, you can pass parameters via the command line. For example, you can specify the input text file to process:

./a.out test.txt

Note: The code of this code lab is written in modern C++, and can be compiled with C++ compilers supporting C++11/14/17 and next.

Enjoy the lab! Let's start with this simple C++ skeleton code for a console application. As you can see, a std::string variable has already been created to represent the input file name, and an instance of the std::ifstream object to read the file.

Now, you need to read the lines from that file. You can use the std::getline function for that. Now that you have successfully invoked std::getline to read a single line from the file, create a while loop to read all the lines from the input file, line by line.

The expected output are all the lines read from the input file.

You know how to read the content of the input text file line by line. Now, you need to create a memory representation for the file. You can use the std::vector combined with std::string. Basically, each line from the input file will be represented using a std::string, and you'll store all these lines into a std::vector<std::string>. In the code in FileReader2.cpp such a vector of strings has already been created to store the lines read from file. Now let's try to better organize your code. Instead of having the loading code directly inside main(), write a function LoadFileLines() that takes as input the file name, and returns a std::vector<std::string> containing the file lines. Invoke that function from main() to print out the file lines.

Now that you have the input text file loaded into memory using a std::vector<std::string>, you'll implement a search feature. In particular, you'll write C++ code to search for an input word in the memory representation of the input text file.

In FileReader3.cpp, you'll see the prototype of a function named SearchText that takes as input the string vector that stores the lines of the text file, and a search string. This function will return true if the input string was found in the text file, false otherwise.

So far both the file name and the search text have been hard-coded inside the C++ source code. In this step, you'll read those from user input, as parameters passed via the command line. The syntax for your program will be the following:

a.out <text file name> [<search text>]

The first parameter represents the input text file name.

The second parameter is optional, and represents the text to search for. If not present, no search is done.

Moreover, if nothing is specified from the command line, an help screen will be printed out. Continuing with the processing of the command line, if one parameter was passed, interpret it as the input text file. Add an if block to print out the file content if one parameter is passed to the command line (in this case, remember that argc is 2: the executable, and the first parameter). Now add another branch to the if statement, like an else if, to check if two parameters are passed on the command line. In this case, intepret the first parameter as the file name, and the second parameter as the text to search. Invoke the SearchText function already written and available in the source code, to print out the lines with the found text. If that function returns false, it means that no text was found. In this case, print out the message "Text not found in the input file."

In this step, you'll see how to handle exceptions to process error conditons. Now, to make things nicer, you can catch the exceptions inside main, and print an error message, instead of abruptly aborting the program with a core dump (as happens in the current state of the program). Congratulations on completing your text file reader app written in C++! :-)