Lab 1: Your First Program

Part 0: Lab Ground Rules

• Our labs are available for your use many hours during the day, as well as after hours.
• No eating is allowed in the lab. Drinks are allowed only in closed containers (so soda cans are not permitted).
• Be courteous and log off the computer when you leave the lab.
• Do not attempt to physically alter any equipment in the lab.
• Do NOT reboot the machines.

Part 1: The Computing Environment

Login: Ubuntu

• Your username and password are the same as those of your MIDN account. Enter your username and password to log on. Note: If you're reading this in lab, you've probably already done this step!

Your Unix Account

• You already have an account on the CS Department's Unix machines, which are actually running GNU/Linux.
• The Unix environment is quite different than Windows and will take some getting used to. I strongly recommend doing all of your programming work on the Unix machines so that you comfortable with it by the 6-week exam. A quick reference guide to common Unix commands is provided on the Resources page.
• You are always welcome to work in the Unix labs throughout the semester; however you must also get comfortable with some other option that will work from your room. Your two primary options are:
  • Use VMware to emulate Ubuntu Linux on your Windows machine (highly recommended).
  • Use the Unix lab machines remotely by connecting to them from your Bancroft machine. To do this, you will use the program "PuTTy". This is not recommended, but if you really must, ask your instructor about how to use something like 'Xming' to make it work better.
  See the Resources section of the SI204 home page for info on how to set these up.

Part 2: Editor and project

Choose your editor

• emacs - feature-rich and powerful text editor. ←Your instructors' choice!
• vi/vim/gvim - lightweight and powerful text editor. ← Many like it!
• gedit - similar to Windows Notepad. ← Feels nice and comfortable, like an old sweatshirt. Do you want to program with a sweatshirt?
• atom - a new pretty-to-look-at editor. However, for SI204 we do not want you to use this. It "helps" by auto-completing code as you type, but this is harmful to the learning process. Once you get the hang of coding yourself, you can try atom out later.

Header files and project

Our C++ program (source code) will be entered as a file. This is the actual file that you will type using the editor. Other files will be used along with your source code to enable the execution of your program. For example, another type of file that may make up your project is a "header" file, or "include" file. An "include" file allows you to use additional source code that you or others have written. They comprise a library of helpful tools -- for example, they can provide input/output capabilities, standard math functions (like cosine, or finding the square-root), among others. In fact, they are often called standard library files. You reference a "header" file by "including" it into your source file. You will include a library file today that provides basic input and output services for your program (details to follow).

The complete collection of files needed to run our program is termed a project. So, a project is the collection of files that make up the program (or application) that you are developing.

Part 3: Step-by-step instructions to creating the "HELLO WORLD" program

1. Directory setup.
   • Open a terminal by selecting the Terminal application (to do so, look at the "Launch Bar" on the left side of your screen). Click on the top bottom (text saying "Dash Home" appears when you hover over this), wait for a box to appear, and type "Terminal". Or, if the Terminal icon already appears lower down in the Launch Bar, just click on that).
     
     This will open a window where you can type in Linux commands.
     USEFUL SHORTCUT: Pressing Ctrl+Alt+T will also open a terminal.
   • Make a new directory (what Windows calls a "folder") to hold your work. You use the "mkdir" command for this. In the terminal that you just opened, type exactly this:

     mkdir si204

   • "Permissions" control who is allowed to view, modify, and use your files. Later you'll learn the details of this, but for now we just want to ensure that the permissions of your directory allow only you to use your files. The "chmod" (change file mode) command is used for this. First, you will change the permissions mode as follows: for users that belong to the same group(g) as this file or directory, you want to subtract permissions (a minus sign) for read(r), write(w), and execute(x). So enter this:

     chmod g-rwx si204 

     Next, you will do the same thing, but also for all "other"(o) users (all users that don't either own the file or belong to the same group). Enter this:

     chmod o-rwx si204 

     If during this or any other step you get unexpected error messages, talk to your instructor.
   • Now we want to change the current directory to the new si204 directory. The "cd" command changes the directory. Then, make a new directory to hold your files for lab01 using "mkdir" command, and finally change to this new directory:

     cd si204 
     mkdir lab01 
     cd lab01
     

   • Let's verify that we are where we expect to be. Use the "pwd" command ("print what directory"). Enter this:

     pwd  

     You should see a full listing of the "path" to your current working directory, which should look approximately like /home/mXXXXXX/si204/lab01
   • NOTE: For all labs and other work with your Unix account, keep all of your files inside your si204 folder. Inside the si204 directory, make a new directory for each lab, homework, project, etc. Do this regardless of whether you are using a lab machine or your own Bancroft machine.

2. Edit the source code

   Open your preferred text editor. For example, if you use vi, in your terminal window type

   emacs main.cpp &

   or if you use vim,

   gvim main.cpp & 

   Explore emacs	Explore vim
   Try and learn a bit about emacs using the below references: Emacs Essentials a bigger list of commands a much bigger list of commands	Try and learn a bit about Vim using the below references: Vim cheatsheet the interactive Vim tutorial Vim Introduction and Tutorial

   Now use your text editor to copy in the following source code into the main.cpp file:

   #include <iostream>
   using namespace std;
   
   int main() {
     cout << "Hello World!" << endl;
     return 0;
   }
   

   When finished, save your file.

3. Compile (and link).

   If you want the programs you write to actually do something then you must compile them (turn source code into machine language). Converting high-level C++ source code into machine language is very complex, but compiler vendors solve this problem for you. To compile your program, first make sure that you have a terminal open from before, whose current directory is where you saved main.cpp. Once you are there, compile by typing into the terminal

    g++ main.cpp -o m012345

   If your program compiled successfully, you should not see any output from g++ (you will just see the prompt, waiting for the next command) and there should be a new file in your working directory called m012345. (If you omit the "-o m012345", which we will often do, then a file with default name of "a.out" is created.) The new file is the "executable file" for your program.

   If you do see any text output from g++, then get assistance from the instructor. If there are errors, they will be listed in the output. Tips for deciphering error messages are given at the end of this tutorial.

   Note: technically, the single command you issued actually "compiled" and "linked" your program. We'll talk much later about this, but in short "linking" (amongst other things) takes your code and combines it with other code provided elsewhere to do standard things like input and output (this is referenced by the #include <iostream> line in your code). Often, we'll informally call this whole process "compiling."

4. Run!

   Once you successfully compiled and linked your program (no errors), then you are ready to execute your program. In your current working directory, you will see a new file called m012345 (or whatever you have named it). That is your program. To execute a file, type the file's name preceded by a "dot slash" (literally, "./") and hit the enter key.

   
   Note: the "dot slash" is telling the computer to look for the program (m012345) in the current directory (indicated by the "dot"). In some cases you can get away with not including this, because your computer is set up to automatically look for commands in the current directory (amongst other places), but including the "dot slash" is safer because it will always work.
5. Program Output:

   Your program will output to the same terminal from where it was executed, as seen above.

   Now, let's view the executable file you just executed. Type

   ls -l

   to list details of the contents of your current working directory. There is a file named main.cpp – it is your source code file. Note the size of the file (95 byes in our example). The file called m012345 is your program's executable file, which is combined with all of the other necessary machine code to run your program. Note the size (7782 bytes in our example)!! The increase in size is due to the code that was “included” or linked into your program.
   

Part 4: Let's do it all over again!

Repeat Part 3 of this lab all over again, with these changes:

• First, close all open windows (including the editor and terminal).
• In step 1, you've already created directories (mkdir) and change permissions (with chmod). You still need to open a terminal and "cd" to the correct directory for lab01.
• Name your source file add.cpp instead of main.cpp. Now, what do you need to change when you compile your program?
• Instead of entering the "Hello World" program, enter in this simple addition program:

// (Your name and alpha)
// This program adds two numbers

#include <iostream>
using namespace std;

int main() {
  int number1, number2, sum;
  number1 = 12;
  number2 = 13;
  sum = number1 + number2;
  cout << "The sum of these two integers is "
            << sum << endl;
  return 0;
}

~/$ ./add
The sum of these two integers is 25

Compile your program giving the executable the name add, and run it, making sure it prints out the right answer! Fix it if it doesn't!

Show your instructor when you finish this part.

Part 5: The "error" of your ways

• Save program files before compiling
• If an error is flagged on a line that looks perfectly fine, sometimes it is the preceding line that is messed up. Check it too!
• Check for missing or extra ; and } characters
• Compile your program in small chunks. That way you can focus on errors in a small section of code. Waiting to compile until the entire program is completed can lead to large numbers of errors.
• Messages that say "Cannot convert from ... to ..." usually mean you are trying to assign the wrong type to a variable.
• Check your typing. A common mistake is to name a variable one thing but spell it wrong or use some other name later.
• Sometimes, one simple problem can cause the compiler to find tons of errors (like forgetting to include using namespace std;. Correcting that one line can make all of those errors go away. So, if you compile and see an unusually large number of errors don't panic. Often a simple change will correct them.
• If you can't figure out what's wrong - ASK!!!

Experience and practice help a lot here. So for each of the below, purposefully make the syntax error in your current program and try to compile to see what the error message is. Correct the error before introducing the next error into your program (e.g., only add ONE ERROR at a time). Go slowly and pay attention, this will save you time later!

• remove a semi-colon.
• change a "<<" to a ">>".
• change the variable number1 to Number1.
• remove the ending }.
• remove the "#include <iostream>" line.
• change the "std" from "std::cout"

Part 6: More on Linux

#include <iostream>
using namespace std;
                     
int main() {
   cout << "Hello World!" << endl;
   cout << "This is a 1!" << endl;

   return 0;
}

Hello World!
Hello Solar System!
Hello Galaxy!
Hello Universe!
Hello ... ?
This is a 1!
This is a 2!
This is a 3!
This is a 4!
This is a 5!

Show your instructor when you finish this part.

Part 7: Submit this lab01.cpp to the submission system

Step 1: Install the submit script to your virtual machine or student (class) workstations:

1. Log on to http://submit.cs.usna.edu, returning to this lab page after loggin in
2. Make a directory called bin in your home directory. This can be done as: mkdir ~/bin
3. Click on "Download Personalized Submission Script", saving the file to the bin directory under your home directory
4. Open a terminal give the command:

   chmod 700 ~/bin/submit

Step 2: Submit the program

~/bin/submit -c=SI204 -p=Lab01 lab01.cpp

Part 8: Finishing Up

You should now be comfortable with being able to generate source code, compile it, execute the program, and correct basic syntax errors -- all in Linux. Let's clean up so you can get to your next class.