Problem: Old Friends

Improve upon older problems.

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

Here’s some good news for you right off the bat: you’ve seen almost everything we’re going to talk about in this problem once before. We’re going to revisit three old programs and rewrite them so that instead of taking input from the user while the program is running, they will instead accept input from the user at the command-line, before the program is even run.

Even better, to ensure everyone is on a level playing field while solving this problem, you’ll be downloading some "distribution code" (otherwise known as a "distro"), written by us, and make modifications to it. You’re welcome of course to use the code you wrote for a prior problem (if it worked!) and re-work it for this problem, but if you struggled with the problems we’ll be reimplementing this time around, know that we will otherwise be supplying you with fully-functional code. All you have to do is change the way that the user inputs data.

Here’s how to download this problem’s "distribution code" (i.e., starter code) into your own CS50 IDE. Log into CS50 IDE and then, in a terminal window, execute each of the below.

Lastly, have a look at Doug’s short video on command-line arguments. Since we’ll be converting all three of the programs listed above to accept command-line arguments (none of them currently do!), this video should come in handy.

If you happen to see (and are confused by!) char * in this and other shorts, know for now that char * simply means string. But more on that soon!

In Hello you were asked to write a program that very simply printed the message hello, world\n to the screen when run. It’s not too much of a leap to extend this program to say hello to a specific person by asking for the user to type a name at the prompt instead, so the program behaves like this.

In fact, the distro you downloaded contains a file, hello.c, with exactly this behavior. What we want, though, is a program that has this behavior instead:

See the slight difference? Instead of prompting the user for information after the program has started running, we collect the desired information from the user before they run the program, and then use that information once the program has started. How do we do so?

Recall that our programs are capable of knowing information about what the user typed at the command line by modifying the way we write the start of our main function. Instead of

if we start main off by typing

we then have access to two special variables that we can use inside of main. First is argc, which is an integer variable that tells us how many things the user typed in at the command line, and second is argv, which is an array of strings representing exactly what the user typed.

Knowing this, and from the information in Doug’s short, can you now modify hello.c so that it prints out the name provided at the command line, instead of collecting a string from the user after the program has started?

One more wrinkle. How do you make sure the user in fact did provide you with one (and only one) additional argument, so that you can print it out? Well remember that’s what our new friend argc can manage for us. If the user doesn’t supply a command-line argument, best to terminate the program and have them try again. One way to accomplish this might be to have this near the top of our code:

Note what this accomplishes? We check to make sure that the user has supplied the proper number of command-line arguments (2). If not, we tell the user how they should run the program, and then we return 1;, which is our way of indicating that our program finished running, but not successfully. We use nonzero return values from main, also known as exit codes, to report back to the system that something went awry.

If you’d like to play with the staff’s own implementation of hello, you may execute the below.

In Fahrenheit you were asked to write a program that asked the user for a temperature in Celsius (which they provided via get_float) and which then printed out the equivalent temperature on the Fahrenheit scale.

In fahrenheit.c you will find a fully-functioning version of the code you were tasked with writing in that problem. Convert that program so that it accepts the Celsius temperature from the command line instead.

There’s a catch, though.

Just because the user types a real number at the prompt, that doesn’t mean their input will be automatically stored in a float. Actually, it will be stored as a string that just so happens to look like an float; after all, remember the data type of argv? It’s an array where each element is a string! And so you’ll need to convert that string to an actual float. As luck would have it, a function, atof, exists for exactly that purpose! Here’s how you might use it:

Notice, this time, we’ve declared celsius as an actual float so that you can do some arithmetic with it. Incidentally, you can assume that the user will only type real numbers at the command line.

Because atof is declared in stdlib.h, you’ll want to #include that header file atop your own code. And, as with hello.c earlier, you’ll want to make sure the user provides exactly the correct number of command-line arguments to your program before doing any calculations, returning 1 should they fail to.

If you’d like to play with the staff’s own implementation of fahrenheit, you may execute the below.

In Pennies you were asked to write a program that demonstrated the power of exponentiation by showing how much money you would have if a person gave you x pennies on a particular day, and then doubled the amount they gave you every day for a period of y days.

As you might expect, we’d now like the program to work as follows.

Notice a few differences with this program from the previous two. How many command line arguments does this one accept? Be sure to modify your check against argc accordingly. And also know that in stdlib.h alongside of the function atof exists another, atoi, that converts a string to an int in much the same way that atof converts a string to a float.

Incidentally, you can assume that the user will only type integers at the command line; there’s no need for you to anticipate a rogue user this time around!

If you’d like to play with the staff’s own implementation of pennies, you may execute the below.