Hello

Though CS50’s lectures this summer use CS50 IDE, you will complete your problem sets this summer using a different platform called GitHub Codespaces. Codespaces is a web-based development environment that allows you to program “in the cloud,” without installing any software locally. Indeed, Codespaces provides you with your very own “workspace” (i.e., storage space) for each problem in which you can save your own files and folders (aka directories).

Logging In

• Head to GitHub and, after signing in, accept this assignment on GitHub Classroom.
• After about a minute, refresh the page and click the link to visit your personal GitHub Classroom assignment page.
• On the assignment page, click the green Code button and choose Open with Codespaces.
  • (If you don’t see an Open with Codespaces button, email heads@cs50.harvard.edu with your GitHub username!)
• Cilck New codespace and then, if prompted, Create codespace.
• Once your Codespace loads, click the + button in the bottom section of your window (next to the word “bash”). You should then see blue text appear that says /workspaces/hello-USERNAME (where USERNAME is your GitHub username).

Each problem you work on in CS50 will get its own Codespace. The first time you start working on a problem, you’ll create a new Codespace. If you’re continuing work on an existing problem, you can click on the existing Codespace to open it.

Once your Codespace loads, you should see that (by default) it’s divided into three parts. Toward the top of your Codespace is your “text editor”, where you’ll write all of your programs. Toward the bottom of is a “terminal window”, a command-line interface (CLI) that allows you to explore your workspace’s files and directories, compile code, run programs, and even install new software. And on the left is your “file browser” (aka “Explorer”), which shows you all of the files and folders currently in your workspace.

Shall we have you write your first program? In the upper left corner of the window, click the three horizontal lines to open the menu. From the File menu, click New File. Once the file is created, go back to the File menu again and click Save As. Save the file as hello.c inside of your /workspaces/hello-USERNAME/ directory (where USERNAME is your GitHub username, which should be the default).

Proceed to write your first program by typing precisely these lines into the file:

#include <stdio.h>

int main(void)
{
    printf("hello, world\n");
}

Notice how Codespaces adds “syntax highlighting” (i.e., color) as you type, though Codespaces’ choice of colors might differ from this problem set’s. Those colors aren’t actually saved inside of the file itself; they’re just added by Codespaces to make certain syntax stand out. Had you not saved the file as hello.c from the start, Codespaces wouldn’t know (per the filename’s extension) that you’re writing C code, in which case those colors would be absent.

Listing Files

Next, in your terminal window, immediately to the right of the prompt (which should resemble /workspaces/hello-USERNAME (main) $), execute

ls

by typing ls at the prompt and pressing Enter. You should see two files: README.md (a file with some instructions for startinga nd submitting this problem) and hello.c (the file you just created). That’s because you’ve just listed the files in your hello folder. In particular, you executed (i.e., ran) a command called ls, which is shorthand for “list.” (It’s such a frequently used command that its authors called it just ls to save keystrokes.) Make sense?

Compiling Programs

Now, before we can execute the hello.c program, recall that we must compile it with a compiler, translating it from source code into machine code (i.e., zeroes and ones). Execute the command below to do just that:

make hello

And then execute this one again:

ls

This time, you should see not only hello.c but hello listed as well? You’ve now translated the source code in hello.c into machine code in hello.

Now execute the program itself by executing the below.

./hello

Hello, world, indeed!

CS50 IDE is a web-based “integrated development environment” that allows you to program “in the cloud,” without installing any software locally. Indeed, CS50 IDE provides you with your very own “workspace” (i.e., storage space) in which you can save your own files and folders (aka directories).

Logging In

Head to ide.cs50.io and click “Sign in with GitHub” to access your CS50 IDE. Once your IDE loads, you should see that (by default) it’s divided into three parts. Toward the top of CS50 IDE is your “text editor”, where you’ll write all of your programs. Toward the bottom of is a “terminal window” (light blue, by default), a command-line interface (CLI) that allows you to explore your workspace’s files and directories, compile code, run programs, and even install new software. And on the left is your “file browser”, which shows you all of the files and folders currently in your IDE.

Start by clicking inside your terminal window. You should find that its “prompt” resembles the below.

~/ $

Click inside of that terminal window and then type

mkdir ~/pset1/

followed by Enter in order to make a directory (i.e., folder) called pset1 in your home directory. Take care not to overlook the space between mkdir and ~/pset1 or any other character for that matter! Keep in mind that ~ denotes your home directory and ~/pset1 denotes a directory called pset1 within ~.

Here on out, to execute (i.e., run) a command means to type it into a terminal window and then hit Enter. Commands are “case-sensitive,” so be sure not to type in uppercase when you mean lowercase or vice versa.

Now execute

cd ~/pset1/

to move yourself into (i.e., open) that directory. Your prompt should now resemble the below.

~/pset1/ $

If not, retrace your steps and see if you can determine where you went wrong.

Now execute

mkdir ~/pset1/hello

to create a new directory called hello inside of your pset1 directory. Then execute

cd ~/pset1/hello

to move yourself into that directory.

Shall we have you write your first program? From the File menu, click New File, and save it (as via the Save option in the File menu) as hello.c inside of your ~/pset1/hello directory. Proceed to write your first program by typing precisely these lines into the file:

#include <stdio.h>

int main(void)
{
    printf("hello, world\n");
}

Notice how CS50 IDE adds “syntax highlighting” (i.e., color) as you type, though CS50 IDE’s choice of colors might differ from this problem set’s. Those colors aren’t actually saved inside of the file itself; they’re just added by CS50 IDE to make certain syntax stand out. Had you not saved the file as hello.c from the start, CS50 IDE wouldn’t know (per the filename’s extension) that you’re writing C code, in which case those colors would be absent.

Listing Files

Next, in your terminal window, immediately to the right of the prompt (~/pset1/hello/ $), execute

ls

You should see just hello.c? That’s because you’ve just listed the files in your hello folder. In particular, you executed (i.e., ran) a command called ls, which is shorthand for “list.” (It’s such a frequently used command that its authors called it just ls to save keystrokes.) Make sense?

Compiling Programs

Now, before we can execute the hello.c program, recall that we must compile it with a compiler, translating it from source code into machine code (i.e., zeroes and ones). Execute the command below to do just that:

make hello

And then execute this one again:

ls

This time, you should see not only hello.c but hello listed as well? You’ve now translated the source code in hello.c into machine code in hello.

Now execute the program itself by executing the below.

./hello

Hello, world, indeed!

Harvard College students (those with an @college.harvard.edu email address) should submit this problem via GitHub, not via Gradescope.

In your Codespace, execute the below, replacing USERNAME with your actual GitHub username.

submit50 classroom50/hello USERNAME

1. Download your hello.c file by control-clicking or right-clicking on the file in CS50 IDE’s file browser and choosing Download.
2. Go to CS50’s Gradescope page.
3. Click “Problem Set 1: Hello”.
4. Drag and drop your hello.c file to the area that says “Drag & Drop”. Be sure it has the correct filename!
5. Click “Upload”.

You should see a message that says “Problem Set 1: Hello submitted successfully!” You won’t see a score just yet, but if you see the message then we’ve received your submission!