Python Expressions Solution - Intro to Computer Science Udacity Japanese English

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正解は1つ目です　これは数字の3です

So, the answer is the first one. This is just

この規則に従うと式から3が導き出せます

a number three. And we can get three from expression

式から開始し数字に置き換えます

by following this rule. Starting from expression, replacing with number,

数字を数字3に置き換えます　2つ目は無効です

and then replacing number with the number three. The second one

式から開始すると

is not valid. And the way to see that is

3つの選択肢しかないことが分かります

if we start from expression, we only have three choices. We

それを式、演算子、式に置き換えます

can replace it with expression operator expression. We can replace

それを数字に置き換えるか

it with number, or we can replace it with a left

（式）に置き換えることができます

paren, expression, right paren. Since the expression we're trying

合わせようとしている式が左側の丸括弧から開始しているので

to match starts with a left paren, the only

左側丸括弧を得る唯一の方法は

way to get it left paren is to eventually

規則に従い式を左側の丸括弧に置き換えることです

have this rule that replaces expression with left paren. When

この規則を用いると

we use that rule, what we get is the

左側の丸括弧に続く式を取得できます

left paren followed by expression. Now we have expression here.

この式は左右離れた丸括弧を合わせました

>From here, we've matched the far left paren and

残ったものがこれです

the far right paren. What we have left is this.

この式を（式）に置き換えます

We can replace this expression with paren expression,

この規則を使用してこの式を置き換える丸括弧があります

and we have paren that's replacing this expression using

以前持っていた2つ目の右側丸括弧があります

this rule. And then we still have the

2つの右側丸括弧を合わせる必要があり

second right paren that we had before. This isn't

これはここでは適合しません

going to match here because we need two right

この規則では丸括弧から始めたら

parens to match. And the essence of this rule

最後に必ず閉じなければいけません

is that every time we open a parenthesis we

丸括弧のバランスが取れていない場合は

have to eventually close it. So we can't product

式を成立させることができず

expressions where the parenthesis aren't balanced. So this

有効なPython式にはなりません

is not a valid Python expression. We can

3つ目は作成することができます

produce the third one, this has many nested

ネスト化した式が多く含まれています

expressions. We have three perens that are open, but

開いている3つの丸括弧はバランスを取り閉じました

we have balanced and closed perens. And the

この式を導き出すために

way we can derive this expression, we'll start

開始点になる1の式から開始します

with one expression that's our starting point. We

この規則を用いて（式）に置き換えます

use this rule to replace it paren expression, paren.

1から開始する式があるので

Now, we have an expression that starts with a one,

この式を式、演算子、式に置き換える必要があります

so we want to replace this expression with expression operator

以前持っていた外側の丸括弧が残っているので

expression. We still have the outer parens that we had

この部分を処理します

before. Now, we're working on this part. We want to to

式を数字に置き換え数字1に置き換えます

replace this expression with a number and then with the

Opを置き換えます

number one. We're going to replace the Op, the following

次の演算子は乗算ですこれは＊に置き換えられます

operator goes to times rule. That's going to be replaced

そして今度はこの式を置き換えます

with times And now we've got to replace this expression.

これらすべてを作成します

We're trying to produce this whole thing, so we're going to replace

丸括弧を使いこれを置き換え続けてこの式を式、演算子、式に置き換えます

this with, using the parentheses rule. And we can keep going, we're going to replace this expression

最終的に2 ＊を取得します

with expression operator expression. Eventually get

3 ＊ 4を得るために再び丸括弧に置き換えます

the two times and then do that replacement with parens again,

このすべてのステップを書き込むことはしません

to get the three times four. And not going to fill in

画面に収まらないからです

all the steps here because they won't fit on the screen.

置換を続ければ複雑な式を作れます

But I hope you get the basic idea that we can

この基礎的な考え方を理解してもらえると思います

keep doing the replacements, making complex expressions like this one. So the

4つ目は無効な式でした

fourth possibility is not a valid expression. There's no way

この規則を用いて前に演算子を得る方法はありません

that we can get An operator in front with this

実際は演算子を用いて式にできる規則が

rule. There actually are rules in Python that allow us

Pythonにあります

to have an expression go to operator expression. But we

しかしこれは“3”と“3”という2つの式なのです

have two expressions here. There's a three space three. If

数字33だった場合この文法で作り出したものではありませんが

it was just the number 33, this would actually be a valid Python expression. Not something that we can produce

有効なPython式になります 5つ目は有効ですが賢明ではありません

with this grammar though. And the first one is valid It's

このようなネスト化された丸括弧でも

not sensible, or necessary, to have er, nested

この規則を使い続けることができます

parentheses like this, but we could always keep using

丸括弧で最後の規則を使う式から

this rule, so we can derive that expression starting

開始する式を導くことができます

from expression using the last rule with the parens,

そして（式）と閉じられた括弧で終わりますその規則を再び使うことができます

and we're going to end up with paren, expression, paren, close paren. We can use that rule

真ん中の式を式、丸括弧、式で置き換えます

again, we're going to replace the middle expression with expression

この式を最後の規則に置き換えます

paren expression. Replacing this expression with the last rule.

丸括弧で囲まれた式が得られます

That'll give us expression surrounded by parens, and we still have

そして2つの外側の丸括弧が各側に残っています

the two outer parens on each side. So we can have

括弧は自由に持てますがあまり意味はありません

as many necessary parens as we want. It's not necessary, it

このようにコードを書く必要は決してありません

doesn't make a lot of sense. We would never actually want

それでも有効なPython式です

to write code that way. But it's still a valid Python

このような再帰的文法規則はとても強力です

expression. So recursive grammar rules like these are very powerful. We

簡単な規則だけで大きな言語を記述できます

only need a few simple rules to describe a big language,

このコースで学ぶすべてのPython言語は

and the whole Python language that we're going to learn in

このように記述できます

this course can be described this way.

すべてのPython言語は記述しませんが

We're not going to describe the entire Python language

文法がどのように機能するのかを理解していただきたいです

this way, but what I hope is that you have a good sense for how grammars

やや非形式的に構成を紹介しました

work, and you can see that, even as we introduce constructs somewhat more informally as we

このような形式な規則や配置規則に分解できます

go on, that they could be broken down into these kinds of formal or placement rules.

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