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>> DAVID J. MALAN: As you probably know,
temperatures are measured using

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different scales in different
parts of the world.

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For instance, 212 degrees Fahrenheit is
100 degrees Celsius and 32 degrees

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Fahrenheit is 0 degrees Celsius.

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Wouldn't it be nice if we had a computer
program that allowed us to

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convert from, say, Fahrenheit
to Celsius?

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>> Well, we can write that program.

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Let's first start with include cs50.h,
so that we can use a function called

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get float, which, like get int, gets a
number from the user, but this time a

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floating point number-- one
with a decimal point.

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>> Let's also include standard io.h
so that we have access to

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functions like print f.

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And let's also declare main
in the usual way.

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>> Let's next prompt the user for
a temperature in Fahrenheit.

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Let's now actually get that temperature
from the user, first by

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declaring a variable called
f of type float.

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A float, again, meaning a variable that
stores a floating point value,

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one with a decimal point.

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>> Let's assign it the return
value of get float.

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And then let's do a bit of arithmetic
on it, first declaring another float

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called C for Celsius and store in C the
result of some common arithmetic.

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5.0 divided by 9.0 times F minus 32.0.

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>> Let's now print the result
of this computation.

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percent f--

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signifying a placeholder for
floating point value--

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comma C to print out the
temperature in Celsius.

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Let's save my final.

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Compile with F to C. Run the program
with dot slash F to C.

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>> And let's try those common
temperatures.

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212 degrees in Fahrenheit
is 100 degrees Celsius.

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32 degrees in Fahrenheit is
0 degrees in Celsius.

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>> Now lets refine this program a little
bit by not printing quite as many 0's

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after that decimal place.

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To do this, I'm going to
go back to line 11.

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And rather than just specify percent f,
I'm going to instead specify, say,

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0.1 f, informing print f that I only
want to print a floating point value

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to one value after the decimal place.

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>> Let's resave my program.

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Recompile it with make F to C. Then
rerun it with dot slash F to C. And

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let's retry with, say, 212,
which gives me 100.0.

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>> Now it's worth noting that I did
something very deliberately in line 9.

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Notice how I wrote 5 is 5.0, 9
as 9.0, and even 32 as 32.0.

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Well, the first two of those values were
very deliberately chosen to be

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floating point values, not just because
of consistency with the rest

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of my program--

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which clearly involves floating
point values--

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but because it turns out that C, If you
divide an int by another int, the

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resulting answer you're going to get is
itself an int, even if that means

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having to throw away everything
after the decimal point.

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>> In other words, if I change this 5.0 to
5 or this 9.0 to 9 and then resave

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my program, recompile with make F to C,
and then re-run it with dot slash F

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to C and type in an input of like 212,
notice that the answer I'm going to

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get this time is actually quite wrong.

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0.0 is not the correct degree in
Celsius as 212 Fahrenheit.

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>> Well, what's going on?

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Well, in line 9, because 5 is now an
integer and because 9 is now an

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integer, the result mathematically
should be 0.5555 and so on.

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But because the result, according to
C's rules, has to be an int, that

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0.5555 gets thrown away,
leaving us with just 0.

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>> So in the end, I end up multiplying
quite accidentally 0 times f minus

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32.0, which is no matter what
always going to give me 0.

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So do keep in mind, any time using
floating point values in proximity of

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ints, you might not necessarily
get the answer so you expect.

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And so take care to use, as I did in the
first case, floating point values

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throughout to avoid any such issues.

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