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>> ROB BOWDEN: Hi, I'm Rob.

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And when in Rome, let's program
as the Romans do.

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So, first thing we need to do is check
to make sure the user actually entered

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the number that we want to rotate each
character by at the command line.

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>> So, we check.

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Is argc equal to 2?

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If it's not, that means the user either
didn't enter the number at the

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command line or they entered too many
things at the command line, in which

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case we don't know what to do
with the rest of the things.

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So, as long as argc was two, we can
continue with the rest of the program.

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>> Now, remember that argv
1 is always a string.

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But we want to treat it as an integer.

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So, we use the atoi function that's
mentioned p set spec to convert the

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string argv 1 to an integer,
and we store that in key.

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We want to make sure that the integer
that was entered isn't negative.

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And if it was, we want to print that to
the user that they're not allowed

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to do that, and return.

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>> So, now we have a valid
number from the user.

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But we need to get a string from the
user that we want to encrypt.

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So we used to do-while loop and use get
string to get this string from the

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user, and continue getting string
until they actually

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enter a valid string.

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>> Once we have that string, we
can go on to encrypt it.

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So, this for loop is doing
that encryption.

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Notice that we're iterating from i
equals 0 t all the way up to i equals

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n, where n is defined as the length of
our message, where the message is the

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string that the user entered.

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>> So we're iterating over all
characters in the string.

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And if a character is uppercase, then
we're going to shift it using this

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formula, which we'll
get to in a second.

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And if it was lowercase, then
we'll shift it using this is

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very similar formula.

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And if it was neither an uppercase nor
a lowercase letter--for example, it

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could be a number or a symbol--then we
aren't going to do anything to it.

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>> So, what are these formulas?

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Well, how do we shift the character A?

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We want to treat the character
A as the number 0.

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And we want to treat the character
B as the number 1, and so on.

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It's the zeroth and first characters
of the alphabet and so on.

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>> So here, message i minus A is supposed
to give us the numeric place of the

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letter in the alphabet.

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So if message i is A, this
is going to give us 0.

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Now, we want to add to that the
number that the user wants to

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rotate the letter by.

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So, if they entered 3, then if message
i was A, this will give us 0 and plus

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key will give us 3.

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>> So now, we are considering the
third place in the alphabet.

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We'll see why we need to mod
by num letters in a second.

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But finally, we want to add on to the
end A again to shift us back into the

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ASCII values for these characters
instead of the numerical places in the

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alphabet of these characters.

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>> So, why do we mod by num letters?

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Remember that we want to wrap around
from the end of the alphabet to the

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beginning of the alphabet.

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So if the user had entered Z, then
subtracting A from that is going to

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give us 25.

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And if the key were 3, then
we're going to have 28.

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But Z should wrap around to ABC, so 28
mod num letters, which is hash defined

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as 26 to be expected, is going to give
us 2, where C is the second letter of

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the alphabet, if you remember that A is
the zeroth letter of the alphabet.

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>> So that's why we need to
mod by num letters.

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Now, we need to do the same exact thing
for lowercase letters, except

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remember that when we're dealing with
lowercase letters, they have different

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ASCII values.

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And so we need to subtract and add on a
to shift it down from the lowercase

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ASCII values and then back to the
original lowercase ASCII values.

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>> And finally, that'll give us
our encrypted message.

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And in the end, we just print out
that message, and that's it.

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My name is Rob, and was Caesar.

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