Lab 12
CSci 127: Introduction to Computer Science
Hunter College, City University of New York
Spring 2021

Learning Objectives:

• Students will use the gcc (c++ compiler)
• Students will write Hello World! in C++
• Students will write programs that perform I/O and use variables in C++
• Students will write programs with Definite Loops (for Loops) in C++
• Students will use Unix commands to list file types and versions

Software tools needed: web browser and the C++ compiler, g++ and a graphical editor, such as gEdit.

Download the Skeletal Notes and Focus Questions to guide you while studying this lab.
These are a useful tool for note taking and you can keep these handy to study for and refer to during the final exam.

Quizzes

At the end of this lab, don't forget to take Lab Quiz 12! See the quiz page for details of the work due this week.

Using Python, Gradescope, and Blackboard

See Lab 1 for details on using Python, Gradescope, and Blackboard.

Getting gcc on Your Machine

We are using the gcc compiler for the C/C++ programming languages, which is freely available. We include directions below for downloading on your machine:

• Mac OSX: The compiler comes with the Mac's XCode tools package. You can download XCode from the Mac App Store.
• On-line: The on-line version used in lecture is OnlineGDB.
• Linux (Ubuntu or any other Debian based distribution): To get the most up-to-date version, first type:

  sudo apt update
  

  to update the package list, and then

  sudo apt install g++
  

  to install g++ and associated libraries and packages.
• Windows: If you installed the Windows Subsystem for Linux (see bottom of Lab 3), you can open Ubuntu terminal and follow the Linux (Ubuntu) instructions above. Accessing your C++ files in the Windows Subsystem for Linux can be a bit involved (see next section). These Windows installation instructions were written by Owen Kunhardt. You can find the full guide here.

Accessing Your Files in the Windows Subsystem for Linux

Accessing where your files are stored may be a little tricky. To access your files in Windows, you must first change into the directory /mnt, which you can do with the command:

cd /mnt

You can now use cd to access your files in Windows. If any file or folder contains a space in it, you must put it in quotation marks e.g. "CS Projects". For example, if your files are stored in your documents folder you can access them by typing the command:

 cd /mnt/c/Users/*enter your username here*/Documents 

You want to enter the username set for your account after Users. This is likely different than the username you created for Ubuntu. If you don't know your username, search cmd in the Windows search bar and open it. Then type the command:

echo %username% 

This should be the username you want to put for *enter your username here*. If your username contains a space, you must put it in quotation marks e.g. "Alan Turing".

These instructions were written by Owen Kunhardt. You can find the full guide here.

Using gcc

We will be using the 'Gnu Compiler Collection' (aka 'Gnu C Compiler'), or gcc, for our C++ programs. Originally designed for C programs, it was extended to include C++ programs and is one of the most common compilers for C/C++. The command, g++ calls gcc and automatically specifies the C++ library and treats all files as C++ (instead of the default as C files).

For C/C++ programs, we will do the following steps, at the command line:

• gedit hello.cpp

  which opens the gEdit editor with the file hello.cpp

• g++ hello.cpp -o hello

  which compiles hello.cpp and store the executable in the ouput file hello

• ./hello

  which runs the executable file, hello

Hello World

Let's write our first program that will say "Hello, World!". You can copy or type the program below into a text editor of your choice (gEdit in Linux, TextEdit in MacOs, notepad in Windows or any other editor that will allow you to save your file with .cpp extension). Text after the '//' is a comment (just like the '#' in Python). You should keep the first 3 lines of comments for gradescope:

//Name:  Your name here
//Email:  Your email here
//Date:  November 2019
//My first program in C++

#include <iostream>        //The built-in library for input/output
using namespace std;       //The names of standard definitions

int main()                 //C++ programs all have a main() function
{
  cout << "Hello, World!"; //Print "Hello, World!" to the terminal
  return 0;                //Standard way to end function
}

Let's go through line-by-line:

• //Name:  Your name here
  //Email:  Your email here
  //Date:  November 2019
  //My first program in C++
  

  These four lines are comments, containing information about the program.

• #include <iostream>        //The built-in library for input/output
  using namespace std;       //The names of standard definitions
  

  These two lines include commands that allow you to print to the screen and read input that the user has entered, using standard names and defintions. The #include statement is similar in functionality to the import statements in Python.

• int main()                 //C++ programs all have a main() function
  

  Code in C++ programs occurs in functions. Each program is expected to have a main() function that is called (invoked) when the program is run. The int before the function name is what type of data is returned when the function ends. In this case, the function will return an int or integer value.

• {
  

  C++ uses the curly brackets ('{' and '}') to indicate blocks of code, instead of indenting. Indenting code is considered good style, but you have more freedom to decide how much to indent versus Python. For function definitions and multiple lines of code in a block, the use of the brackets is required.

•   cout << "Hello, World!"; //Print "Hello, World!" to the terminal
  

  cout << is used for printing to the terminal. Note that, as a general rule, commands in C++ end in a semi-colon.

•   return 0;                //Standard way to end function
  

  When this function ends, it returns the integer 0, a common value for a program ending normally.

• }

  The closing bracket for the definition of the function, main(). All opening brackets must have a matching closing bracket.

Once you have it typed in and saved as hello.cpp, try compiling the program. In a terminal window, travel to the directory where you saved your file and type:

g++ hello.cpp -o hello

When it returns without an error, you can run it by typing:

./hello

When it compiles, and runs correctly (i.e. printing "Hello, World!" to the screen), see the Programming Problem List.

Simple I/O & Variables in C++

Unlike Python, you must declare variables before using them. For example, if you want a variable to hold the year, you would first declare it (i.e. request space for it):

int year;

Once we declare the variable, we can use it in the subsequent lines of the program:

//Another C++ program, demonstrating variables
#include <iostream>
using namespace std;

int main ()
{
  int year;
  cout << "Enter a number: ";
  cin >> year;
  cout << "Hello" << year << "!!\n\n";
  return 0;
}

For each variable, we need to specify what type of variable it is (whole number, real number, character, etc.) before we use it. For real numbers, we can use float, as in Python. We will introduce more data types next lab, but if you are curious, here is a chart of some common types. Our program above:

• Declares the variable year to be a whole number (integer).
• Asks the user for a number.
• Reads in the number the user entered and stores it in year.
• Prints out a messsage with that number.
• Ends the function (returning 0).

To summarize: cin is for input to the program, and cout is output from the program.

How could you modify this program to ask the user for a real (floating-point number)? Most arithmetic works the same in C++ as it does in Python. With this in mind, modify the above program to convert kilometers to miles. (Hint: see the Programming Problem List.)

Definite Loops in C++

C++ has for-loops that have are similar, but not identical to for-loops in Python. The basic format is:

int i;

for (i = 0; i < 10; i++)
{
   command1
   command2
   ...
}

For the loop above,

1. The variable i first takes on the value 0.
2. Next, check if the variable i < 10?
3. If no, leave the loop.
4. If yes, do the commands in the body of the loop.
5. Add one to the variable i (abbreviated: i++).
6. Go to #2.

Let's use a loop to write "Hello, World!" to the screen 10 times:

//Name:  Your name here
//Email:  Your email here
//Date:  November 2019
//Prints "Hello, World!" 10 times, using a loop

#include <iostream>
using namespace std;

int main ()
{
  int i;   //The index variable for the loop

  //Loop will repeat 10 times:
  for (i = 0; i < 10; i++)
  {
    cout << "Hello, World!\n";
  }
  return 0;
}

Note that cout doesn't automatically put output on a newline, so, we need to include the new line character ("\n") at the end of the line.

Follow the steps above to compile and run your program.

Next, modify the program so that it will print 10 times: the Hunter College motto ("Mihi cura futuri" which translates to: "The care of the future is mine"). When it compiles, and runs correctly, see the Programming Problem List.

More Useful Unix Commands

Now that we are creating executable programs, it's often useful to figure out what kind of file is in your directory. For example, if you were not sure what type of file, hello is, you could type at the command line:

file hello

file is the name of the command, and hello is the input parameter to the command. If you would like to find out the type of several files, you could type each separately:

file hello hello.cpp hello.py

(assuming all of those files are your working directory).

Or, you could use a "wildcard" that matches all files whose names match a pattern. For example:

file hello*

will tell you all type of every file that begins with the hello followed by 0 or more other characters.

Similarly,

file *

will tell you all type of every file in your current working directory.

It's often useful to figure out which version of a program you're using (since there could be multiple ones on your computer). To do that, there's a command, which that will tell which version of the program it's using by default. For example,

which g++

will show the location of the g++ program.

What's Next?

Now is a great time to take Lab Quiz 12! (15 minutes) It is directly based on Lecture and Lab 12.
After that, you can continue working on the programming assignments. The Programming Problem List has problem descriptions, suggested reading, and due dates next to each problem.
Keep in mind that the due dates are one week late for flexibility (if one week there is a setback and you can't submit your programs, you will have time catch up). Still, each week you should work on the programming assignments for that week, even if they are due a week later. If you are on a roll, you are welcome to work ahead!!!