Problem: Fahrenheit

Implement a program that converts degrees in Celsius to Fahrenheit, per the below.

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.

In the past, if you wanted to know what the outdoor temperature was, you had to look around for one of these, which might have been mounted outside at your house.

Prior to that, if you wanted to know what the temperature was outside, you just stood outside, but we digress. Nowadays, it’s a fairly trivial exercise to pull open a weather app on your phone or visit a weather-reporting website to get the current temperature and the five-day forecast. No need to mess around with one of the above circular dials or its tube-of-mercury cousin^[1].

Depending on where you live in the world, your country uses one of two major temperature scales. If you live in the United States (and don’t spend most of your day working in a science lab), it’s likely that you’re familiar with the Fahrenheit scale, and so if you hear that it’s 30° outside you’re probably putting on a heavy coat and warm pants and bracing yourself for the possibility of snow, because that means it’s quite cold, given that the freezing point of water is 32°. On the other hand, it’s not until the temperature hits 212° that water boils.

In practically every other country of the world (or if you live in the United States and do in fact spend most of your day working in a science lab), you’re probably most familiar with the Celsius or centigrade scale. In that case, if you hear that it’s 30° outside you’re probably going to rummage through your closet for a swimsuit and put on some sunblock, because it’s a beautiful beach day. On the Celsius scale, 0° is the freezing point of water, and 100° is the boiling point.

We acknowledge that with most apps that tell you the weather, it’s fairly simple to press that switch that switches the temperature display from Fahrenheit to Celsius (or vice versa), but before that process was automated for us, one either had to memorize some of the more common conversion points or had to use a mathematical formula to convert from one scale to another, which is important if you plan on traveling internationally.

For this problem, we’re going to focus on converting in just one direction: from Celsius to Fahrenheit. As it happens, the formula for this conversion isn’t terribly complex. (Phew!) One simply takes the current temperature in degrees Celsius (°C), multiplies it by 9, divides by 5, and then adds 32. The result is the equivalent temperature in degrees Fahrenheit (°F). Not bad, right? For the more visually inclined, this translates to this formula:

Let’s do a quick test to make sure things work as expected. Worldwide, the commonly accepted value for normal human body temperature is 37°C. If we plug "37" into that formula where °C goes and do the math (37 multiplied by 9 is 333, 333 divided by 5 is 66.6, 66.6 + 32 is 98.6) we get 98.6°F which is what folks in the United States know as normal human body temperature. So that checks out. Similarly if we plug in 0°C (the freezing point of water) into that formula does it convert to 32°F, and 100°C (the boiling point of water) is apparently equivalent to 212°F. Seems like things are going well.

Log into your CS50 IDE account (remember how?) and, if you have it from Hello, take a look at your hello.c file, which should be in your chapter1/hello/ directory. (If you don’t have it, create it now.) The contents should look probably like this:

To make our program more adaptable, instead of having it always print the same simple message (hello, world\n) each time, we can modify this program to prompt a user to provide their name and then print out their name instead:

Notice a couple of important differences in this program. It seems that we have decided to include another header file called cs50.h. In this file, which you can have a peek at here and which we’ll also discuss in more detail later in the course (so don’t be alarmed by some unfamiliar syntax), you’ll see that that’s where we’ve declared the data type of string and declared a function called get_string.

A string in computer science essentially refers to a collection of characters—​a word, a sentence, or a phrase. Because when we ask the user for their name it probably consists of more than a single letter, which would be just a character (char), we need to use get_string to collect their input and we need to store it in a variable whose data type is string; we’ve done this, the variable called name has the data type of string.

What else has changed, though? That last line of code looks a bit strange. There’s a %s in there, and the variable name appears. What’s going on?

Essentially, %s is what’s known as a placeholder for a variable. At the time the program is compiled, we don’t know exactly what will be printed out (unlike the original program which will always print out hello, world\n), but we do expect that the program will print hello, followed by whatever the user typed. %s is how we indicate to printf that a string will be printed there.

What string will we print? Well, that’d be name! After we specify what we want printf to print, leaving as many placeholders as necessary, we specify what variables those placeholders refer to in order from left to right, separated by commas. We only have one placeholder in our modified program, a single %s, and so the variable that we’re telling printf to print in place of that %s is whatever the user typed at the prompt. For example:

There’s not just get_string in cs50.h though. There are functions to get user input of nearly all types: get_int, get_float, get_double, get_long, get_char.

Now, create a file called fahrenheit.c (remember how?), ensuring that it is inside of a subdirectory called fahrenheit inside of your chapter1 directory, and then double-click on that file in the file tree to the left. You should now have an open window in your text editor with the tab name fahrenheit.c, and you’re ready to write your first utility program—​a program that can be used by others to do something meaningful for them. (Not to knock the value of saying hello, world… but we can do so much more!)

Write a program that converts a temperature in Celsius to Fahrenheit, as per the sample output below, wherein underlined text represents some user’s input^[2].

To solve this problem, you needn’t do anything more complex than use your currently existing knowledge of C, and the information contained in this specification, including the temperature conversion formula. No matter how the user inputs the temperature in Celsius (that is, no matter to how many decimal places they choose), take care to display Fahrenheit to exactly one decimal place. No need to worry about floating-point imprecision or integer overflow, if you recall what those terms mean.^[3]

Do recall that if you include <cs50.h> atop your fahrenheit.c file, you will have access to a function called get_float, which will allow the user to input a floating-point value (a number with a decimal point in it, also known as a real number).The parameters of the get functions will display a prompt for users. So if you had a line in a file called name.c that looked like this get_string(“name please:”), it will display the following:

Incidentally, know that printf can be used to specify how many places after the decimal point you wish to display to the user. For example, assuming you’ve written the following program in a file called truncate.c:

When executed (by first compiling with make truncate and then executing with ./truncate), this program will output the value of the variable pi to exactly 2 decimal places: 3.14. Can you see why? Perhaps you can adapt that to display the converted temperature to just one decimal place?

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