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ZAMYLA CHAN: Now, let's tackle vigenere.

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The to-dos are going to be as follows--

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getting the key from the
user from the command line,

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getting the plaintext prompting for
that, encyphering that plaintext,

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and then finally printing the
ciphertext back to the user.

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The key is going to be in
the command line argument,

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and we can access it as follows--

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using the argv array.

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In C, we simply called argv index 1.

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And in Python, we'll have to
add in a system.argv for that.

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Then you'll also want to check to
make sure that your key is comprised

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of all letters, all alphabetic.

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So use that isalpha function
here to check for that.

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To get the plaintext from the user,
prompt them for it using the get_string

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function found in the cd50 module.

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Now, let's talk about encyphering.

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If I wanted to iterate over all of the
letters in my name, printing those,

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then I could store my name
in a variable-- name--

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and then I would execute
the for loop as follows--

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for c in name: print(c).

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Notice here that the variable c is
just a placeholder, the iterator,

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for every character in my name.

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Now, if I ran this,
then the print method

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would include a new line
after every character.

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So make sure to check
up on how print works

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and how to omit a new
line coming automatically

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after every call to print.

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All right.

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So now, that we know
at the most basic level

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how to iterate over our plaintext,
let's look at the formula for vigenere.

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Where we have the ith letter
of the plaintext shifted

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by the jth letter of the key, and
all of that is [? moduloed ?] by 26.

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And that gives you the ith
letter of our cipher text.

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So here, you'll notice
that, as before, we have

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two indices that we're dealing with--

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i and j.

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But in the for loop
example before, we only

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had the capability for
one index or one iterator.

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So you'll have to find a way to
keep track of your second index.

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Now, we want to advance
that index in the keyword

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only if the character in
plaintext is a letter,

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so go back to the isalpha function.

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And as before, with our ciphers,
we'll want to preserve the case.

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So you might want to use the
isupper and islower functions.

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Finally, you'll want to implement
alphabetical wraparound.

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I'll give you a hint to check
out these two functions.

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And in combination with
the formula from before,

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figure out how to go back
and forth between ASCII

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values and alphabetical
indices to make sure that you

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wraparound the alphabet correctly.

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Print the ciphertext back to the
user, and you've finished the problem.

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My name is Zamyla,
and this was vigenere.

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