Question

附录一

执行类似事情的一般方式是什么?

Answer 1

我不敢肯定你对你们的兴趣有多大细节,但正如你希望执行教区一样。通常有两个步骤:

The lexer reads over the text and converts it to tokens. For example, it might read "2 + 3 * 2" and convert it to INTEGER PLUS INTEGER STAR INTEGER

The parserreads in the tokens and tries to line with them. 例如,你可能会有这些规则:

Expr := Sum | Product | INTEGER;
Sum := Expr PLUS Expr;
Product := Expr STAR Expr;

它读作标语,试图适用规则,使起首规则在读物时标明。在此情况下,可以:

Expr := Sum
Expr := Expr PLUS Expr
Expr := INTEGER(2) PLUS Expr
Expr := INTEGER(2) PLUS Product
Expr := INTEGER(2) PLUS Expr STAR Expr
Expr := INTEGER(2) PLUS Integer(3) STAR Expr
Expr := INTEGER(2) PLUS Integer(3) STAR Integer(2)

There are many types of parsers. In this example I read from left to right, and started from the initial expression, working down until I d replaced everything with a token, so this would be an LL parser. As it does this replacement, it can generate an abstract syntax tree that represents the data. The tree for this might look something like:

您可以看到,产品规则是《概述》规则中的一项儿童,因此最终将首先:2 + (3 2)。如果表达方式不同,我们就可能会用这一树来结束:

我们现在重新计算<代码>(2+ 3) * 2。树冠以什么方式 generate树。

If you actually want to parse expressions, odds are you don t want to write the parser by hand. There are parser generators that take a configuration (called a grammar) similar to the one I used above, and generate the actual parser code. Parser generators will let you specify which rule should take priority, so for example:

Expr := Sum | Product | INTEGER;
Sum := Expr PLUS Expr; [2]
Product := Expr STAR Expr; [1]

我将产品规则列为优先事项1,而苏姆则列为优先事项2,因此,鉴于所生成的产品名称将有利于产品。你们还可以设计图表本身,以便确定优先事项(这是比较常见的做法)。例如:

Expr := Sum | INTEGER;
Sum := Expr PLUS Product;
Product := Term STAR INTEGER;

This forces the Products to be under the Sums in the AST. Naturally this grammar is very limited (for example, it wouldn t match 2 * 3 + 2), but a comprehensive grammar can be written that still embeds an order of operations automatically

Answer 2

You would need to write a parser for your fairly simple programming language. If you want to do this in Python, start by reading Ned Batchelder s blog post Python Parsing Tools.

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