Name: 在Python中對樹進行編碼 - Computerphile (Coding Trees in Python - Computerphile)
Uploaded: 2021-01-14T10:16:12.000Z
Duration: 21 min 27 s
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現在進行式

Yeah, we're looking at trees like those, actually, trees and computer science a little bit different.

You know, the wood is on top and the leaves on bottom.

to 不定詞

That's a rotten thing to remember about three.

Andi, I want to show how Tow whatever, Chris trees in using python and using objects.

No, Uh, please are very, very versatile data structure, and I think they're often but under use.

Some people always want to your strings for everything.

And I think a good program a really needs their trees, you know?

So the example we're going to look a TTE a syntax trees, And, uh so let me just start us An example S So we're looking at a mathematical expression, something like we times by plus X.

Now people may think it's it's It's like a sequence like a string on dhe.

Actually, I think this goes some more, general.

I mean, and you and you learn mathematics.

My impression is that lots of people get confused by thinking off arithmetic expression itself like a one dimensional structure from left to right.

And then there's his operations in this strange things, like brackets on the new you learn some marriageable on and some rules so times good before plus.

But really, these expressions is one dimensional.

Structures really represent the two dimensions faction in the A tree.

So if you look at expression exists is the one question is, what is the top level operation, right?

Plus it's yeah, because they look at the expression on the top.

We should start the star, and then we have to sup expression and left and from the right from the left.

You get this one here, the plus, and then we can go further doll like this and you can play the same game here.

So what was the top level expression here?

Yeah, could thinks about the baby because we don't have the brackets.

The first thing we look at it it's a plus here, right?

And then if you go to the left, we have the star and you're good to the violet.

We have sex, and here, if you go to the left, we have the three.

And really, the brackets are used to make clear what, what three We mean the blanket that that's a little bit off trick to represent ity every really when we thought, Think about expressions.

We should really think about the please and not about the one dimensional structure.

And yes, we have this convention like Here's At Star by its more than class.

You can avoid having to put brackets here because, as you know, there is only a a limited number of black it in the world and to be tried to safe breakers.

But really, if you look at the trees, the brackets completely disappear and we really see is a structure that you mean if he evaluates them, if he assigned values to vie and two X and then we very it's something we get different answers for these two expressions for those two trees.

What we're going to do is now is to represent these trees as objects in Python.

Okay, so I'm going to use my Jupiter system, and I'm going to define a class of expressions.

So in that class expressions and I'm not going to put anything into hears I was past.

That's actually mainly for documentation on, well, nobody finding sap classes off expressions.

What's an example, for example, operators.

So that's a class times subclass off expression.

Just different in the moment with another one is plus front.

What is it with the numbers, right, The numbers.

Oh, that's I'm gonna hate myself for saying this.

Yeah, the blankets we don't need because here's a simple in the trees.

These things, they're not really I mean, but one thing is missing.

Okay, So now we have defined structure, class structure.

But now let's put something in the first thing we put in his message to create these trees in python.

Every method in person always gets referenced to the object which cuts left and a slippers may not with self.

Okay, If you produce a product, a times, we need two expressions.

I mean, one to the left and one to the right.

So it's called them left for l and R Shorter.

And all I'm doing in my inner method ISS I'm initializing some instance variables which has the same name.

That looks very tautological, but really, what's happening here is you create an object.

And now, inside the spokes, there is very about instance very well.

One is called el, and one was called our and they're going to be filled in with some other rifle and sisters of mother objects will be created.

I'm using the same name for the perimeter and for instance, variables.

So the same game I'm going to play for plus, so I'm not going to do something naughty.

So that's really evil, all right, because one of the holy rule off programming has never copy court.

The professors can copy court in general.

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在Python中對樹進行編碼 - Computerphile (Coding Trees in Python - Computerphile)

distraction

constant

expression

structure