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Welcome back.

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The accumulator pattern is a common
programming pattern where you iterate

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through the contents of a list,
and you accumulate a single value,

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such as the sum of all of
the items in that list.

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It always involves
an accumulator variable.

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Which you'll have to initialize,
usually you'll set it to be 0 or

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some initial value that you're going to
start with and then you'll iterate.

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You'll have an iterator variable,
That iterates through some sequence and

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in each time, on each iteration,
you'll update the accumulator variable.

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When you've made it through the whole
list, updating the accumulator for

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each item in that list, at the end you're

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going to look at the accumulator variable,
and it's going to have your total value.

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So the code on lines one
through five is doing that.

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We have a sequence on line one.

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Accum is the name I've given
to my accumulator variable,

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naturally enough and now I'm
iterating through the sequence nums.

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W is going to be my iterator variable.

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Accum was the name of my
accumulator variable.

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And then on line five,
I'm going to update the accumulator.

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For each item, I'm going to add that value
to whatever's in the accumulator so far.

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At the end, I print it out and

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I should have the sum of
all the values in the list.

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So let's just run that, and we'll
demonstrate that it really does produce

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the sum of the numbers 1 through 10.

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But now let me show it to you in CodeLens.

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So in CodeLens,
we can see what's happening at each step.

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I have a sequence.

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I set my accumulator value to
have an initial value of zero.

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Then I start iterating
through the sequence.

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W is bound to 1,
the first element of the list.

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And now I do an update, accum = accum + w.

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So it used to the value 0 and
now it's going to have 0 plus 1.

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And we're on to the next item in the list.

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Now, w is going to be two.

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And the accumulator is going to get
updated this time to be one plus two.

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So it's sort of keeping a running sum for
us.

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Now it's three.

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The next time we'll add in three,
so it'll get to be six.

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We'll add in 4, and so it'll get to be 10.

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And we keep going and going and going till
we get through all the items in the list.

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Eventually the accumulator has the running
sum of all of the items in the list.

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And we can print it out.

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So I'm going to hide Codelens now and give
you a little challenge to predict what

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would we get if instead of having,
on line five the print where it is,

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what if I indented it so
that it's part of the for loop?

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Now before I run this,
why don't you try and

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make a prediction of
what you think it'll do.

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Here we go.

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So the difference is that now the print
is happening inside the for loop.

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That means on each iteration we're going
to print the value of the accumulator,

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the running sum.

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So after the first iteration it had
the value one, just zero plus one.

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Then we added in two and we had three, and

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we added in three and
we got six, and so on.

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So that's the basics of
the accumulator pattern.

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You'll be seeing that
a lot in this course.

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You start with an accumulator value that
gets initialized to some value like zero.

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You iterate through a sequence, and

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your iterator variable is used on each
iteration to update the accumulator.

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You update the accumulator each time and
at the end,

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after you've gone through all of the items
in the sequence, the accumulator value.

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After you've gone through all
the items in the sequence,

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the accumulated variable will have
the accumulated value from the whole list.

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In this case,
the sum of all of the items in the list.

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We’ll see you next time.