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BRIAN YU: Let's start by
taking a look at lines.

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In this part of the problem,
here's what you'll have to do.

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First, the lines function
is a function defined

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in helpers.pi which takes in inputs
a and b, each of which are strings.

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The first thing you'll have to
do is take each of those strings

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and split them up into individual lines.

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Then you'll need to figure out which
lines appear both in a and in b.

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And then finally, you'll
want to return a list that

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contains all of the lines that
are present in both string

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a and also string b.

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So let's take a look at what
that would actually look like.

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A string might look something like this,
line one backslash n, line 2 backslash

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n, line 3 where the backslash
n stands for new lines.

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If we want to take a string
like this and split it up

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into lines, what that
means is that we want

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to convert this string into separate
lines, line one, line two, and line

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three.

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That way we can compare
between file a and file

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b, which lines are present
in both of those files.

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Know that Python strings
do support methods, which

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may help you in the process
of trying to take a string

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and extract out all of the lines
that appear in that string.

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And you can go to this URL,
part of Python's documentation,

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to look at the methods that you
can use with Python strings.

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And some of them may help you to
figure out how to take a string

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and split it up into
its individual lines.

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After you've taken both a and b
and split them up into their lines,

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the next step is going
to be to figure out

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which lines appear in both the
lines of a and also the lines of b.

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So how do we find the lines
that are going to be in common?

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Well, you'll probably want to
consider some sort of data structure

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to keep track of which
lines are present in both.

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You might consider a list which
just stores elements in order

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Or you might want to consider a set
which stores elements not in order

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and just stores a collection of them.

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Or you might want to consider some
other data structure altogether.

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It's up to you.

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But you might want to take a look at
this part of Python's documentation

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which explains some of the common
Python data structures that

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might prove helpful to you as you
think about what algorithm to use

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and what data structure
to consider, as you

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think about how to find which
lines are present in both a and b.

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Key things to make sure of are that
you are avoiding duplicate lines.

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If, for example, the same line appears
twice in file a and three times in file

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b, you only want that to appear
one time in your resulting list.

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Because the list that you
return at the end of the lines

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function should be a
list of all of the lines

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that the two files have in common.

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But each one of those lines in common
should only appear once at maximum.

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So be sure to avoid
duplicates there and think

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about how you might make
sure your algorithm is

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careful to avoid that situation.

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