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Welcome back.

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So you've already seen the for

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loop as a way to iterate over
every item in a sequence.

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A while loop is a much more
general way of iterating.

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So a while loop is kind of like
a hybrid between a for statement and

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an if statement.

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So a while loop looks like this.

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You say while, and
then just like an if statement,

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you have a conditional that
comes after the while.

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And then you have a piece of code that
will run if this condition is true.

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But unlike in if statement where after
you're done running this piece of code,

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if you use a while instead.

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Then by the time you get to this end
of code, then we loop back up here and

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check now is this condition is still true.

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So in the sense that it loops back up
to the top, that's kind of like a for

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statement.

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So again, when we get to
a while statement, we check,

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is this condition true?

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If the condition is true,
then we run this piece of code and

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then we go back up and
check is this condition still true.

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So, if this condition is not true,
just like in if statement,

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then we skip this block of code and
execute the next code block.

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So, let's see this in action.

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So, here I have some code that's going
to take the sum of numbers one plus two,

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plus three, plus whatever, and we pass in.

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So here, we pass in a number called aBound
as an argument to our sumTo function.

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And we're using a while
loop in order to do this.

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So on line 11,
we print out the value of sumTo(4),

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which should be one plus two, plus three,
plus four, which is going to give us ten.

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And then here we print
out the sumTo(1000),

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which is going to be a much larger number.

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So first I'm just going to run my code,

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just to make sure that it
still works correctly.

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I'm good, so I see I get I get ten here,
and I get this number from line 13.

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Okay, so now that we've done that,
let's look a little bit at how it works.

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So, you see that we
initialized two variables here.

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So we start up this sum to be zero, and

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this is going to be kind of a kin
to our accumulator variable.

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So, what it's going to
keep track of is theSum.

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So far.

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And then we have this other variable,
which we call aNumber.

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And what this is going to keep
track of is where we are.

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And that's going to start out as one,
but then we're going to set it to two,

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to three, and then so on until we get
to n, which in this case is aBound.

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So now here we have a while statement.

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And our condition is while aNumber
is less than or equal to aBound,

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again aBound here is
whatever we're adding up to.

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So as long as our aNumber variable
is less than or equal to aBound.

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Then what we do is we first say, theSum
equals its previous value plus a number.

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So when a number starts out as one,
then theSum is going to be zero plus one.

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Just going to be one.

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So then after that, what we do is we
say aNumber equals aNumber plus one.

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Again, aNumber keeps
track of where we are.

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So aNumber is going to start out as one,
and

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then it's going to two, and
then three, and then four,

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and so on until aNumber is less than or
equal to aBound.

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And again, aBound here is
the number that we're adding up to.

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So it's going to keep going up
by one until we get to aBound.

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And as we're doing that,
as we're incrementing this aNumber,

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then we're adding that number
to theSum that we have so far.

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And then by the time this
while loop is done running,

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theSum is going to have
the correct answer.

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But let's inspect this code just to
make it a little bit more clear.

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So I'm going to open up code lines, and
here I have the same piece of code,

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except I'm only printing out
sumTo when called with four.

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So, the first thing that we're going
to do is evaluate the function, so

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we can see that sumTo is the function
that we declared right here.

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And then we print out the value
of sumTo when called with four.

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So, what the means is that aBound
is going to have the value of four.

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And then we start out the theSum with
value zero, and aNumber with one.

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Remember, theSum keeps track of the sum so
far.

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And aNumber keeps track of where we are.

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And now we're at the important bit,
the while loop.

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Again, we say while aNumber is
less than or equal to aBound.

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The first time that we run this,
aNumber has the value one.

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One is less than or equal to aBound, which
is four, and so yes, we do run this code.

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So in this code, we say theSum
equals this sum plus aNumber.

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So theSum is going to go from zero to one.

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Then we say aNumber is aNumber plus one,
so it goes from one to two.

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Now, the next time we run this, we check,

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is aNumber still less than or
equal to aBound.

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So in this case, aNumber is now two.

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And we ask, is two less than or
equal to four?

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Yes it is.

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And so we run the code in here, and
we save theSum equals theSum plus aNumber.

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aNumber is now two, so theSum is
going to go from one, two, three.

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So theSum is now three, and
then aNumber gets its previous

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value plus one, so
aNumber is going to from two to three.

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And we ask again,
is aNumber less than or equal to aBound?

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Yes, three is less than or
equal to four so we run this code.

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So theSum goes from three to three
plus aNumber which is three.

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So theSum goes to six, and
aNumber is going to from three to four.

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And then we ask,
is four less than or equal to four?

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Yes, it is, so theSum is going to go
from six to six plus four, or ten.

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And aNumber is going to go from
four to four plus one or five.

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And now here's another key point.

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So now, aNumber is five.

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And when we ask is five less than or
equal to four?

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It is not, so this condition is false.

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Meaning that we're done
running our while loop.

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And we can see that the sum here has
the value that we actually want.

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So now, when we're done running our while,
we skip to line nine,

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which returns theSum, and we get ten,
which is the correct answer.

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So let's answer a few questions.

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First, true or false, you can rewrite
any for loop as a while loop.

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Well, this is true.

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A while loop is a much more
general form of iteration.

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It's capable of expressing what
you can express in a for loop and

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more, as we'll see in a bit.

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This question asks,

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which type of loop can be used to
perform the following iteration?

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You choose a positive integer at random,
and

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then print out the numbers one up to and
including the selected integer.

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In this case, we could actually use a for

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loop if we wanted to,
because we could use the range function.

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And anytime you can use a for loop,
you could also use a while loop.

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So I'm going to say here
that the answer is A.

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In this question, we're asked to write a
while loop that's initialized to zero and

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stops at 15 if the counter
is an even number.

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Append the counter to
a list called eve_nums.

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So, I'm going to say count equals zero,
and I want it to stop at 15,

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so I'll say while count is less than or
equal to 15.

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And I'm just going to say
count equals count plus one.

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So, what we're doing in this code is
we're initializing count to be zero.

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And then inside of the while loop,
as long as count is less than 15,

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we assign it to value to its
previous value plus one.

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So count is going to go from zero to one,
to two, to three, four, five,

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six, seven, eight, nine, ten,
11, 12, 13, 14, and then 15.

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And then it does get assigned to 16.

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But as soon as it's 16,
then we exit this while loop,

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because 16 is not less than or
equal to 15.

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And so by the time we're done with this
while loop, count is going to be 16.

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Now this question is asking us to do
a little bit more, it's saying if

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the counter is an even number, append
the counter to list called eve_nums.

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So I can check if the counter
is an even number

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by saying if count modulo two is zero.

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Again that's just saying if the remainder
when divided by two is zero.

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And what we want to do then is we
want to append it to this list

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which I'll call eve_nums.

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I mean, to start out as an empty list and

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if count is even,
I want to say eve_nums.append count.

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So let's test our code to
be sure that it works.

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Okay, so we can see that now
our code passes all the tasks.

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And even_nums has the values zero, two,

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four, six, eight, and so on to 14.

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This question says,
below we've provided a for

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loop that sums all of
the elements of list1.

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Write code that accomplishes the same
tasks but instead uses a while loop.

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Assign the accumulator
variable to the name accum.

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So the strategy that we're going
to take in order to do this

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is we're going to have one variable that's
going to keep track of the current index.

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So I'm going to call that variable idx.

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idx is going to be zero, and then one,
and then two, three, four, five, six.

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And then by the time it's seven, we want
it to stop because there isn't a 7th item.

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So let's write the code to
properly set idx first.

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I'm going to say IDX equals zero,

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and I'll say while idx is less

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than the length of list1,

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then idx equals idx plus one.

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Now an important point here is
that I said while idx is less than

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the length of list1, and
I initialized it to zero.

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So why did I do both of these things?

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Well, first I initialized it
to zero because again list and

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sequences are zero indexed.

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Meaning that we have to start out
at zero to get the first item.

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Now, I said less then the length of this
one, rather than less than or equal to,

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because here this one is one, two, three,
four, five, six, seven items long.

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So the length of this one is seven.

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But we only want to go
until the last item.

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And because we're zero index, we have
the last item is actually item six.

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So by the time we get to idx equals seven,
then we want to breakout of this loop.

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We only want idx to be zero, one,
two, three, four, five, six.

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So I'm going to say less than the length
of this list, not less than or equal to.

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Okay, now we don't just want to loop
through all of the indices of this list.

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We also want to assign a new variable
accum to be the sum of every item.

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So I'm going to start
out accum equals zero,

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to say that we haven't seen anything so
far.

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And then just like on line six here
where we say total equals total

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plus the value of the element.

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I want to say accum equals accum, and

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then I'm want to say,
plus the value at index idx.

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And I get that by saying list1 sub idx.

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So when idx is zero,
this is going to be eight.

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When idx is one,
this is going to be three and so on.

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So again, as idx iterates
through all of the indices.

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This is going to be
the value at that index, and

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so we add the value at that index to
the previously accumulated value.

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Now let's run our code and
be sure that it's correct.

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So we can see that our code
works as we wanted it to.

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That's all for now, until next time.