Boolean Logic (11 mins)

In this section, I wanna show you can take
a simple test, like startsWith("A"), and

use, the notion of AND, and OR, to combine
it with other tests to sort of put

together a more complicated test. So this
is called Boolean logic. So the way this

is gonna work, is, in code, the notion of
AND is the, the symbol for that two

ampersands run together (&&). And the, the
symbol for OR is two vertical bars (||),

put together. So let me show you what that
looks like. So here's some code. And it

has, the first test just says if the name
starts with "A", just as we've seen before,

and that test is, it's complete and
functional. So that is not changed by

adding the notion of Boolean logic. So for
Boolean logic, what we do is we take that

test and we follow it with two ampersands.
So you would pronounce that as, AND. And

then it's followed by a second test. The
second test is also complete and makes

sense on its own. So, what this does is it
just takes two tests and it puts them

together, and it says, well. For this
overall if-test to be true, both of these

subparts have to be true. So the
components are, well here, I'll, I'll run

it. So what this says is, names that begin
with "A". That has to be true. And the name

has to end with "y". So if I run it, we just
get this, you know, kinda shorter list of

names that begin with "A" and end with "y".
Syntactically like I said it has the two

tests, each of which is complete. They're
joined with AND or, OR as we'll see in

a minute. And then finally there is still
this one set of parentheses, a left most

parentheses and a right most, a right most
parenthesis, around the entire thing. So

I'll do a bunch of examples, like this. A
couple other, one other thing to point out

about this, in this case what happened is,
the test is kind of long, right? I have

this whole startsWith part, and the
ampersand, so if I did it all on one line,

it gets a little, a little long. So what's
happened in this case is I, actually I hit

return after the ampersand. I went down
here and I hit the space bar, to get the

row over here and kind of line it up. So I
wrote the second test on a second line.

That's optional but you can do it. Putting
in spaces and stuff like that doesn't

upset the code. So, I'm, always my
examples I'll sort of neaten up in this

way, Where I'll tend to write one test per
line, and then I'll align them this way.

So here's the second test. The only thing
that's a little confusing is to note that

there are two parentheses are required
here. This, this first one just balances

the parentheses for the endsWith. And then
the second one, is the one that covers the

entire test, so it matches up to the, that
first one. So if you leave that out,

actually I can illustrate this, so it's a
pretty easy error to leave that one out,

'cuz you have to kinda realize alright I
need two there. So in that case, not

always, but the run button for the, just
for this class, will try to give you an

error message, like oh, it looks like
there's a missing parentheses there. So

now form it that way we'll get them,
we're still good. As I said before each of

these tests is complete and stand on its
own. So just syntactly just the way we did

things before. There's this one form of
the code that looks right but is wrong.

And so I'll create that here. So
if you just write it this way. So I'll

just sort of pronounce it. row.getField("name"), that startsWith("a") AND

endsWith("y"). So just
to the ear to a human, they would know

what that means, but this code does not
work. The problem is the second term. On

its own, doesn't make sense. Right?
Doesn't just stand on its own like ends

with, well it doesn't make sense. Usually
what we need is, we're gonna put in a

cursor here, we would need row.getField("name")
or "rank" or whatever it's gonna

be .endsWith. So this is not working.
Each, each, to the left and the right of

the ampersand, each task has to be
complete. So in this case, I'll just sort

of hit undo in Firefox here, the, if
we wanna be talking about name, like we

spell it out for the first test, and then
we also just completely spell it out for

the second test. So each test makes sense
to the computer. Alrighty, So, I'm gonna

try another example here. Let's see. So
right now, if I run this, it just shows

all the "A" names, which is quite a lot.
What the problem statement says is, change

the code so it prints the names that start
with "A" and the rank is less than 50. So,

oh here's, here's the first test, and I'll
add two ampersands, and return let's

say row, that starts, that startsWith("A")
is one test and now we're gonna add, I'm

gonna combine it with an AND, with the
second test, where I'll say rank, oh let's

say less than or equal to 50. So I
think I'll try it, yeah so that works. So

we still get "A" names but now we're just
getting up to rank 50. I'm just gonna try

a third example here. Alright, now we'll
do an example with OR. Change the code

below, so it prints rows where the
following is true. Name starts with "X", or

the name stars with "Y", or the name stars
with "C". So if I just run, run it right

now, the code that's there just does "X". So
I wanna use OR. And we'll copy this.

So by using an OR-test. You make
multiple ways for the overall if-test to

be true. It's like, well, the if-test were
true if this one is true, or if this other

thing is true. So it's kind of a widening.
So here I'll say, OR row.getField - "Y".

So, for the previous run, we only got "X".
Alright? So I'm gonna run this. Oh, so now

we get "X" and "Y". So there's "Yusuf" and
"Yeritza". Notice that I, my goal

ultimately was to show "X" or "Y" or "Z", but I
think it is kinda nice if you get the code

into a kind of half built state, but where
it makes sense. You know, just try running

it and kind of verify, oh that works
before going on to do the whole thing.

That's kind of a classic computer code
rule of thumb; don't try and do the entire

thing in one step, OR startsWith("Z"). So, I
guess partly what I'm showing here is. You

can have multiple ORs and multiple ANDs and
string things together. So let's

try that, There it goes, Quite a few "Z"
names. So there we have it, "X" or "Y" or" "Z,

so this shows the two vertical bars, and
then in this case I have strung together

three things. For this class, I will only,
I'll use either a bunch of things

connected with AND or a bunch of things
connected with OR. I won't combine them.

Combine them just brings up some other
cases that are kind of interesting but I

not, I don't really want to get into. They
don't really help a lot. Other thing I

should mention about this, alright so I
should just apologize to this. This

ampersand business is sort of a historical
accident, or the vertical bar business.

Language, influencial language chose this
in, like the mid'70s. And once one

language had chosen this symbol to mean
OR, some other languages thought, oh, well

we should just use that convention. And so
it just kinda snowballed, where now,

that's a very common convention. So I felt
like, well, it's kind of obscure, but.

That's the convention so we should just go
ahead and learn that one. Alright, So,

let's scroll down here a little bit. So
actually, what I have set up is just a

large number of examples here. And there's
a show solution button here, so if you

want practice or to review these examples
I've done, you could come back and it's

all sorted here. What I'll is I'll just
try the, the first six. So, I'll type in

the code for those and I'll leave the
others as kind of extra practice for

people who want it. Okay, so the first one
says, name starts with "Ab", or starts with

"Ac". So I'll say there's some code here
that we start with already. So it starts

with "Ab". And what was it? OR starts with
"Ac". So this one starts with here, So this

should be an OR, Two different or's. Let's
try that one. So we got "A", "Ac" Where's

the "Ac" name? Oh. Look I made a bug. I typed
"Ab" twice. Okay. There we go.

Ok, there's an "Ac". That showed that interesting
example of a bug, where sometimes bugs are

syntax errors where you hit the run button
and it, it just, you know, it, it crashes

and you get some red output. But
sometimes a bug is just that. I typed

something that does mean something to the
computer, but it's not what I intended.

And so then the output is not what I
expected, then I'm a little bit confused.

So that's the more higher level kind of
bug in this course. It's fairly

common with computers. Okay, let's see,
next one. Name starts with "Ab", or

"Ac", or "Al". Oh, I see. So this just
extends. This just shows that you can have

three. So I'll say vertical bar.
I'll paste this in. So it was "Ac",

so "Ab" and "Ac" doesn't give us a lot but
with "Al" you know it's quite a lot.

Okay. So the next one was, name starts
with O and ends with A. So I'll say

starts, I'll just translate it. Starts
with "O" AND ends with "a". Alright. Let's

try that. Oh, there's only one, "Olivia". How
bout, starts with O and gender is girl?

So, we'll just get gender right here, and
then we test that is not with startsWith

endsWith, but just with ==.
So, O AND girl. Oh, there's only two.

So we get Olive. Okay,
name ends with "a" and gender is what?

Change this to name.endsWith("a"), and
per my previous example, my previous claim, I'll just cut that out.

Now it's half built right, name ends with
"a". But I could run that and sorta verify,

all rigt that's working. And then, take
your working thing, and extend it. What

did I want? In this case I wanted oh,
names ends with "a" and gender is blank.

Okay. Jesus, Lyle. Girl names neither.
Boy. Oh, well, there's a few, Joshua,

Ezra. Okay, I'll do this last one, number
six Rank is less than ten, and gender is

girl. So I'll change this to "rank". Instead
of endsWith. I'll say, less than or equal to ten.

And, so, Now I'm not using startsWith, or
endsWith for either of these but just

notice each one of those is a complete and
correct test as we discussed before. That

one is, and that one is and I'm just
joining them from there. So rank is less

than ten, and gender in this case is girl.
Let's try that. So that makes sense. We

just get one, two, three, four of them.
One through ten girl names. This one's

kind of interesting in that this, this I
think has a sensible English translation.

What this is, is. What this says is top
ten girl names, And then, We can phrase

that as this AND thing. Where rank is less
than or equal to ten and gender is girl. Or indeed

ultimately we can phrase it as code. So
just get it down to, to where the computer

can actually do it. So there's a few more
problems here and the solutions all

available. So this is a good opportunity
to come and review what I did or try a

little bit of practice before we do the
exercises. Cuz these, in some sense, are -

computer languages have now gotten big
enough that I can really we can have a lot

of different [inaudible] alright.