Hw10 题解 (UCB CS61A@2021 Fall)

BNF

Q1: Grouping and Pipes

In this question, you will add support for grouping and piping.

Recall that grouping allows for an entire regular expression to be treated as a single unit, and piping allows for a pattern to match an expression on either side. Combined, these will let us create patterns which match multiple strings!

Define the group and pipe expressions in your grammar.

1. A group consists of any regex expression surrounded by parentheses (()).
2. A pipe operator consists of a regex expression, followed by a pipe (|) character, and lastly followed by another regex expression.

For example, r"apples" would match exactly the phrase “apples” in an input. If we wanted our pattern from before to match “oranges” as well, we could expand our rstring to do so using groupings and pipes: r"(apples)|(oranges)".

Hint: note that groups and pipes are valid regex expressions on their own! You may need to update a previously defined expression.

给本来用来匹配正则表达式的 BNF 添加功能: group 和 pipe. 其实怎么实现这一写已经在描述里面给出了, 我这里就提一点: group 和 pipe 本身也是 ?regex 的一部分, 所以要把他们加到 ?regex 里面

?start: rstring
rstring: "r\"" regex* "\""
?regex: character | word | group | pipe

group: "(" regex* ")"
pipe: regex "|" regex

character: LETTER | NUMBER
word: WORD

%ignore /\s+/
%import common.LETTER
%import common.NUMBER
%import common.WORD

Q2: Classes

Now, we will add support for character classes.

Recall that character classes allow for the pattern to match any singular character defined within the class. The class itself consists either of individual characters, or ranges of characters.

Specifically, we define the following:

1. A range consists of either NUMBERs or LETTERs separated by a hyphen (-).
2. A class expression consists of any number of characters or character ranges surrounded by square brackets ([]).

Note that for this question, a range may only consist of either NUMBERs or LETTERs; this means that while [0-9] and [A-Z] are valid ranges, [0-Z] would not be a valid range. In addition, the characters and ranges in a class may appear in any order and any number of times. For example, [ad-fc0-9], [ad-f0-9c], [a0-9d-fc], and [0-9ad-fc] are all valid classes.

这次要加上的功能是 ?-? 和 [] 的功能, 具体怎么做同样在问题描述里面给出了. 这里讲一下注意的点:

• range 的 - 左右两边要是对应的, 所以 character "-" character 这种写法是错的
• 在 [] 里, 要么是 range, 要么是 character, 可以用 (range|character) 表示，后面用 * 表示 any number
• [] 本身也是合法的正则表达式, 所以要放在 ?regex 里面

?start: rstring
rstring: "r\"" regex* "\""
?regex: character | word | group | pipe | class

group: "(" regex* ")"
pipe: regex "|" regex

range: NUMBER "-" NUMBER | LETTER "-" LETTER
class: "[" (range|character)* "]"

character: LETTER | NUMBER
word: WORD

%ignore /\s+/
%import common.LETTER
%import common.NUMBER
%import common.WORD

Q3: Quantifiers

Lastly, we will add support for quantifiers.

Recall that quantifiers allow for a pattern to match a specified number of a unit.

Specifically, we define the following:

1. A plus_quant expression consists of a group, a character, or a class, followed by a plus symbol (+).
2. A star_quant expression consists of a group, a character, or a class, followed by a star symbol (*).
3. A num_quant expression consists of either a group, a character, or a class, followed by one of the following:
   1. a NUMBER enclosed in curly braces ({});
   2. a range of NUMBERs (separated by a comma (,), which may potentially be open on only one side. For example, {2,7}, {2,}, and {,7} are valid numeric quantifiers. {,} is not valid.

Hint: these three quantifiers share many similarities. Consider defining additional expressions in this question!

可以发现这三个 _quant 都是 group 或者 character 或者 class开头, 所以这里可以定义一个 ?tmp: class | group | character 避免重复. 同时定义 ?quants: plus_quant | star_quant | num_quant, 放在 ?regex 里

这里 num_quant 需要枚举下面这几种情况

• {NUMBER, NUMBER} or {NUMBER} -> (NUMBER (",", NUMBER)?)
• {NUMBER,} -> (NUMBER ",")
• {,NUMBER} -> ("," NUMBER)

rstring: "r\"" regex* "\""
?regex: character | word | group | pipe | class | quants

group: "(" regex* ")"
pipe: regex "|" regex

range: NUMBER "-" NUMBER | LETTER "-" LETTER
class: "[" range* character* range* character* "]"

?tmp: class | group | character
plus_quant: tmp "+"
star_quant: tmp "*"
num_quant: tmp "{" ((NUMBER ("," NUMBER)?) | (NUMBER ",") | ("," NUMBER)) "}"
?quants: plus_quant | star_quant | num_quant

character: LETTER | NUMBER
word: WORD

%ignore /\s+/
%import common.LETTER
%import common.NUMBER
%import common.WORD

SQL

Q4: Size of Dogs

The Fédération Cynologique Internationale classifies a standard poodle as over 45 cm and up to 60 cm. The sizes table describes this and other such classifications, where a dog must be over the min and less than or equal to the max in height to qualify as a size.

Create a size_of_dogs table with two columns, one for each dog’s name and another for its size.

要根据狗的体型大小来决定它的 size. 这个就是一个基本的 select ... from ... where 的写法. 我们这里可以把两个表连接起来得到所有可能的结果, 然后用 where 来选出符合条件的.

注意这里用 alias 会比较简洁

CREATE TABLE size_of_dogs AS
  SELECT d.name, s.size 
  FROM dogs as d, sizes as s
  where d.height <= s.max and d.height > s.min;

Q5: By Parent Height

Create a table by_parent_height that has a column of the names of all dogs that have a parent, ordered by the height of the parent from tallest parent to shortest parent.

用 ORDER BY 来进行排序, 注意是降序所以用 DESC

CREATE TABLE siblings AS
  SELECT p1.child AS dogone, p2.child AS dogtwo, s1.size AS dogonesize, s2.size AS dogtwosize
  FROM parents AS p1, parents AS p2, size_of_dogs AS s1, size_of_dogs AS s2
  WHERE p1.parent = p2.parent AND p1.child < p2.child AND p1.child = s1.name AND p2.child = s2.name;
-- Use `<` to filter the result
-- `!=` is not enough, you will get `barack clinton` and `clinton barack` in the same time

Q6: Sentences

There are two pairs of siblings that have the same size. Create a table that contains a row with a string for each of these pairs. Each string should be a sentence describing the siblings by their size.

Each sibling pair should appear only once in the output, and siblings should be listed in alphabetical order (e.g. "barack plus clinton..." instead of "clinton plus barack..."), as follows:

Hint: First, create a helper table containing each pair of siblings. This will make comparing the sizes of siblings when constructing the main table easier.

Hint: If you join a table with itself, use AS within the FROM clause to give each table an alias.

Hint: In order to concatenate two strings into one, use the || operator.

完成了 Q5 之后这一题就很简单了.

CREATE TABLE sentences AS
    SELECT "The two siblings, " || dogone || " plus " || dogtwo || " have the same size: " || dogonesize
    FROM siblings
    WHERE dogonesize = dogtwosize AND dogone < dogtwo;