Count I (5 mins)

Thus far, we've written code that, where
there's an if-test. And if the test is

true, we print the row in the data. So in
this short section, I wanna show how,

instead, you would just count the number
of rows where the if-test is true. And

that's, that's more akin to what you
really wanna do with computers. So in

order to do this, we're gonna have to add
some novel code into, the code we've been

doing. So, it's described, here but I'll,
I'll point out the parts in this code

example. So there's three things that have
to be added to do counting. One is I'm

gonna introduce a new variable, Which I'll
be sure I'll call count. And I'm just

gonna set it to zero, and I'm gonna do
that before the loop starts running. So,

count equals zero. Then, inside of the
loop, inside the if-statement, where, you

know, I, I print the row as we've done
many times before. But then I'm also gonna

have this code, count = count + 1. And that's kind of a funny looking

line. I'll tell you, what that does. Is
whatever value is stored inside of count

it increases it by one. So it just bumps
it up from five to six, or ten to eleven,

Or whatever. But let me, let me explain
how this works. So, in mathematics, this

line doesn't really make any sense. Like,
you know, what value is equal to itself

plus one. But the reason this works in the
computer is that the function of the equal

sign in computer code is actually more
simple than it is in mathematics. And the

key thing to understand is that it first
evaluates the right hand side. And then

only once that has been resolved to get a
value, then that value is stored into the

variable in the left hand side. So imagine
the very first time this runs. So count is

zero and then it gets to this line. So the
first thing it's going to do is evaluate

the right hand side. So count is zero, so
zero plus one is one. So it's gonna, this

pile apart is one. So once it's figured
that out, it stores one into count so now

count is one. So you can see right there
the action where coming into the line

the count was zero and then coming out
it's one. So it bumps it up. So then the

next time it sees this line, if the if-statement is true. That little value at

the right hand side, count as one. So one
plus one is two, so having figured out

that's it's two than it stores two in the
count so now the count has two. So

ultimately, you don't need to have a total
command of the details of this thing, you

just need to know that the form, x=x+1
for some variable, it just bumps it up by

one, each time its run. So, I was saying
there is three parts. We've got to set to

zero and the count=count+1
inside the if-statement and then, finally we just, we

just print out whatever value is left in
the, in the count after all, after the

for-loop has run through all its times. So, let's just try this.
So if I run this. What we see is that the

loop runs, and the if-statement here,
checks for names beginning with "A". So, we

just see all these "A"-names. And then, down
at the end, there's this one line, "count: 258"

So what that shows is, the for-loop, you know, the count was started at

zero. The for-loop ran all its times. The
if-statement was true, apparently 258

times out of the 2,000 times. And then we
get this "count: 258". Prints it out

because of this line, So, that one runs
after the loop is done, so all that these

internal prints are done. Alright, So let
me try some experiments here, So one easy

thing to do, is like, well what if I just
remove this print that's inside the loop?

So, I'm still gonna run through the rows,
I still have the if-statement, but then the

only thing that happens inside if-statement is count=count+1.

So just bump it up by one. So, now if I
run this program. I just, I just get this

one line of output. I run it and it just
says, bam, 258. So this is beginning, this

is beginning to resemble more what you
think of as com-, you know, the form of

computer is taking in some massive data
and kind of sifting through it and giving

you kind of a, a final answer. Alright,
So, let try, try some more problems here.

And as us ual, we've got the solutions
variable. How many names start with "X" and

then compare it to how many start with "Y".
So, if I wanna count how many names start

with "X", I just change, you know, the
count, and the count equals plus one.

count=count+1 I can just
keep. So I'll just say, how many start

with "X"? So, if I run it, it says, six.
So now it says oh well what if I want

to know how many start with "Y" well we
can see all, all the structure we keep and

I just have to change that one thing to "Y".
And its seventeen, so I guess more names

start with "Y", a lot more names start with
"Y" than with "X". Then it says, for the third

one, how many girl names begin with "A",
then change to count how many boy names

begin with "A". All right so this is going
to bring in material from last time, I

wan't to use an and here. So I'll say,
startsWith("A"). I'll say and ("&&") row.getField("gender")=="girl"

So that's
the task, and then inside I just say

count=count+1. Alright, a hundred sixty
nine, So the, the follow up question is

how many boy names. So for boy I would
just change this. So this notion of the if

and the task are really, is the same for
counting. It's really just where as previously we

would have said print(row) now I just have this
count=count+1 so I can do the counting.

[inaudible] so more grow names
[inaudible]. Okay, so that's our first

look at basic counting. So let's try some
exercises.