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>> [MUSIC PLAYING]

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>> ROB BOWDEN: It's me, Rob.

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Let's walk through how
to implement Mario.

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So the first thing we need to do is
prompt the user for the input.

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>> We need to ask them exactly how
tall the pyramid should be.

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So here, we see we're doing
height = GetInt.

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An And remember that the GetInt function
is implemented in the CS50

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library, so up top, we need to
remember to #include cs50.h.

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>> So why do we have this wrapped
in a do-while loop?

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Well, we also need to remember that
the user's input has to be valid.

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What is an invalid input?

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Well the pset spec specifically says
that a height less than 0 or greater

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than 23 is invalid.

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>> So up here, we see that we're defining
a constant called MAX_HEIGHT with the

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value of 23.

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This do-while loop will continue while
height is less than 0 or MAX_HEIGHT is

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less than height, which means that
23 is less than the height.

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So if height is 24 or greater, we're
going to continue looping.

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Remember that do-while loops are pretty
helpful whenever we want to get

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user input and then validate it, since
we inevitably need to ask the user at

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least once for the value they want.

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>> So once we have their input, we
can now build the pyramid.

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One of the tricks of this problem set is
that we have to start at the top of

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the pyramid.

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You can't printf the bottom of the
pyramid and then build your way up.

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So let's look at the example
from the pset spec.

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>> We see here that when we enter a height
of 8, the very bottom of the

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pyramid prints nine hashes.

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One level up from that prints
one space and eight hashes.

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One level up from that is two spaces and
seven hashes, all the way until we

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get to the top of the pyramid, which is
eight levels up, which prints seven

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spaces and two hashes.

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So remember that we have to
do this top level first.

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>> Here we're iterating from the
top level, row 8, continuing

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until row reaches 0.

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So how many spaces did we need
to print in that top row?

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We printed seven spaces
and two hashes.

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So the number of spaces we want is
the row that were on minus 1.

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>> If the top row is 8, 8 minus
1 gives us seven spaces.

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Then we have a loop that will print
out each space one at a time.

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So when spaces is 7, this loop
seven times, printing

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seven individual spaces.

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>> So now we need to print these hashes
at the end of the pyramid.

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So here, we need to calculate
the number of hashes.

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We see that we're doing height
minus row plus 2.

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So how did we get that?

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>> Remember that the top of the pyramid
is row 8, and the height is 8.

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And we still printed two hashes.

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So at the very least, 8 minus 8 plus
2 gives us the right answer.

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And then consider the bottom
of the pyramid, row 1.

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Height minus row will give us 7, and
then plus 2 gives us nine hashes,

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which is exactly the number
of hashes that we printed.

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So this is the formula we want to
use to calculate the number

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hashes in each row.

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>> Using that number, we then have another
for loop, very similar to the

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for loop that we used for the spaces,
that iterates number of hashes times

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printing a single hash each time.

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On the top row, that'll
print two hashes.

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On the bottom row, that'll
print nine hashes.

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And each other row will print every
number of hashes in between.

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>> And then at the very end, we need to
print our new line to go to the next

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row in the pyramid.

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Finally, we need to print the new line
at the end of the row in order to

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continue to the next
row of the pyramid.

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And at the end of our program,
we have the return 0.

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>> As per the pset spec, the return
0 isn't strictly necessary.

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But it does signify that main is done.

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My name is Rob, and this was Mario.

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>> [MUSIC PLAYING]

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