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[MUSIC PLAYING]

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SPEAKER: Well, hello, one and all, and
welcome to our short on while loops.

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Now life is full of surprises.

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And one surprise for me recently was
taking care of this particular plant

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here.

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Now, it turns out, this plant is pretty
picky about the kind of water it gets,

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and when it gets watered.

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I'm supposed to water this plant when
the soil is dry, but not too dry.

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And so I check every day, what the
moisture level is before I actually

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know, should I water this plant or not?

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So I thought I'd maybe write
a program to remind myself

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when to water this plant,
depending on how dry the soil is.

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And one thing I've learned
is that you can actually

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measure the moisture content
of soil in percent water out

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of all the entire soil you might have.

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And soil is considered dry, supposedly,
when it reaches a moisture level of 20%,

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or a little bit less.

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So ideally here, I could write a program
to maybe check for me once a day,

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let's just say, what the moisture
content of this plant's soil is.

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And if it's 20 or less,
go ahead and alert me

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that I should be watering
this particular plant.

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So here, I have a
program called water.pi.

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And this program actually
uses a function called Sample

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that I've written elsewhere, but I'm
going to just import it and use it

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in this program.

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No need to worry about how
that's implemented in this case.

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The purpose of Sample is
really, to just sample

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my soil, hypothetically in this case,
and maybe return to me, the percent

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moisture that it has sampled from
the soil on this particular day.

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So I program in main here is as
follows, I'm going to sample my soil,

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store the result in moisture, and
then print out the current moisture

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percentage, or the water content
at least of this particular soil

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on this particular day.

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So go ahead and run Python of water.pi.

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And what do we see?

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Well today, the moisture is 28%,
so not quite dry enough to water.

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But what should I do now?

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This program only checks once for me.

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I'd love for it to maybe keep
checking, keep checking, keep

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checking while the soil is still wet.

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And I'd love for it to
stop when the soil is dry

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and alert me that it's time to
water this particular plant.

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So notice how I said there,
I wanted to do something

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while some condition is true.

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I want to keep checking the soil,
sampling while the soil is wet.

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So I could use, let's say, a while loop.

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A while loop is great when I'm not quite
sure how many times I want to loop,

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but I do want to loop while
some condition is true.

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Why don't I go ahead
here and maybe update

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this program to be a little
more dynamic and keep

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looping while some condition is true.

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I'll say maybe while the moisture
content, moisture in this case,

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is greater than 20.

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That is, the soil is still wet,
I want to do the following.

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I want to sample again.

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Maybe it's another day,
I'll sample the soil again,

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and I'll then report on the
actual moisture content, the water

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content of this soil here.

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So now, what we've done
is updated our program.

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And from top to bottom,
it reads like this.

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I'll first sample the soil, perhaps,
immediately after I water it, let's say.

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I'll start running this program.

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It will then report to me
the moisture in that soil.

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And I'll then, let's say while
the moisture is greater than 20,

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while the soil is still
wet, I will sample,

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let's say, once per day, the
moisture, and then report to myself

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the moisture content
as it currently stands.

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Now this while loop, the code
is indented inside of it, again,

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will keep running while this
Boolean expression is true.

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But once it's no longer true, we'll
actually exit out of this loop,

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and in this case, end our program.

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But before we do, maybe I'll do this.

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I'll say, time to water.

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Because I know that if moisture is not
greater than 20, well, it's equal to 20,

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or less.

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So I'll go ahead and
say it's time to water.

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Your soil is getting too dry.

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So why don't I go ahead and
maybe try this program now.

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I'll run Python of
water.pi, and I'll see

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that we could maybe, just
for the sake of the magic

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here, assume this happens once a day.

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So the first day, 34% moisture.

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Next day, 31%, then 27, 23, 22, and 18.

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Now 18 is definitely
not greater than 20.

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So we would then exit our
loop and say, time to water.

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But as long as moisture was greater
than 20, we were, as you notice,

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continuing to loop, continuing
to loop as we went through.

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Now let's just try this
again one more time.

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And we'll see that seem to take a little
bit more, a little bit more time here.

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I'm going to try this again.

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That seemed to take only three days.

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So really, soil moisture
content can vary based

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on the weather, the sun, et cetera.

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And it might really be
somewhat random how dry or wet

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my soil becomes over time.

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Now here, the magic of the while
loop is that no matter when

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this happens, whether it's four
days in, two days in, five days in,

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it's always going to end
when this condition is

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no longer true, in this case.

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And to make this point even
more clearly, let's go ahead

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and add in a variable called Days.

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And we'll say the day I first
start watering is days equal to 0.

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And then every time I
iterate, I'll add 1 to Days.

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So going from 0 to 1, then 1 to 2.

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Every day, I sample my
soil again and again,

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and I could actually make
it clear what day it is.

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I'll say day, days, and day, days here.

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And now, If I run my program again,
and make this just a little bit bigger

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for ourselves here, I should
hopefully see that this time,

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it took five days indexing from 0.

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This time, it took six
days indexing from 0.

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And this time, it only took three days.

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So again, while loop,
the perfect choice when

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you want to keep looping
while some condition is true,

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but you're not exactly
sure how long you're

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going to loop at the end of the day.

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Good for things like randomness,
good for things like this,

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where you have a plant,
you're not sure when to water.

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This was our short on while loops.

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We'll see you next time.

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