The Role of the Lexical Analyzer

The Role of the Lexical Analyzer

1 Lexical Analysis Versus Parsing

2 Tokens, Patterns, and Lexemes

3 Attributes for Tokens

4 Lexical Errors

5 Exercises for Section 3.1

As the first phase of a compiler, the main task of the lexical analyzer is to read the input characters of the source program, group them into lexemes, and produce as output a sequence of tokens for each lexeme in the source program. The stream of tokens is sent to the parser for syntax analysis. It is common for the lexical analyzer to interact with the symbol table as well. When the lexical analyzer discovers a lexeme constituting an identifier, it needs to enter that lexeme into the symbol table. In some cases, information regarding the kind of identifier may be read from the symbol table by the lexical analyzer to assist it in determining the proper token it must pass to the parser.

These interactions are suggested in Fig. 3.1. Commonly, the interaction is implemented by having the parser call the lexical analyzer. The call, suggested by the getNextToken command, causes the lexical analyzer to read characters from its input until it can identify the next lexeme and produce for it the next token, which it returns to the parser.

Since the lexical analyzer is the part of the compiler that reads the source text, it may perform certain other tasks besides identification of lexemes. One such task is stripping out comments and whitespace (blank, newline, tab, and perhaps other characters that are used to separate tokens in the input). Another task is correlating error messages generated by the compiler with the source program. For instance, the lexical analyzer may keep track of the number of newline characters seen, so it can associate a line number with each error message. In some compilers, the lexical analyzer makes a copy of the source program with the error messages inserted at the appropriate positions. If the source program uses a macro-preprocessor, the expansion of macros may also be performed by the lexical analyzer.

Sometimes, lexical analyzers are divided into a cascade of two processes:

         Scanning consists of the simple processes that do not require tokenization of the input, such as deletion of comments and compaction of consecutive whitespace characters into one.

         Lexical analysis proper is the more complex portion, where the scanner produces the sequence of tokens as output.

1. Lexical Analysis Versus Parsing

There are a number of reasons why the analysis portion of a compiler is normally separated into lexical analysis and parsing (syntax analysis) phases.

1. Simplicity of design is the most important consideration. The separation of lexical and syntactic analysis often allows us to simplify at least one of these tasks. For example, a parser that had to deal with comments and whitespace as syntactic units would be considerably more complex than one that can assume comments and whitespace have already been removed by the lexical analyzer. If we are designing a new language, separating lexical and syntactic concerns can lead to a cleaner overall language design.

2. Compiler efficiency is improved. A separate lexical analyzer allows us to apply specialized techniques that serve only the lexical task, not the job of parsing. In addition, specialized buffering techniques for reading input characters can speed up the compiler significantly.

         Compiler portability is enhanced. Input-device-specific peculiarities can be restricted to the lexical analyzer.

2. Tokens, Patterns, and Lexemes

When discussing lexical analysis, we use three related but distinct terms:

       A token is a pair consisting of a token name and an optional attribute value. The token name is an abstract symbol representing a kind of lexical unit, e.g., a particular keyword, or a sequence of input characters denoting an identifier. The token names are the input symbols that the parser processes. In what follows, we shall generally write the name of a

token in boldface.  We will often refer to a token by its token name.

• A pattern is a description of the form that the lexemes of a token may take. In the case of a keyword as a token, the pattern is just the sequence of characters that form the keyword. For identifiers and some other tokens, the pattern is a more complex structure that is matched by many strings.

       A lexeme is a sequence of characters in the source program that matches the pattern for a token and is identified by the lexical analyzer as an instance of that token.

E x a m p l e 3 . 1 : Figure 3.2 gives some typical tokens, their informally described patterns, and some sample lexemes. To see how these concepts are used in practice, in the C statement

printf ( " Total =  %d\n",  score ) ;   

both printf  and score are lexemes  matching the pattern for token id, and

" Total  =  °/,d\n" is a lexeme matching literal .   •

In many programming languages, the following classes cover most or all of the tokens:

   One token for each keyword. The pattern for a keyword is the same as the keyword itself.

         Tokens for the¹ operators, either individually or in classes such as the token comparison rfientioned in Fig. 3.2.

         One token representing all identifiers.

         One or more tokens representing constants, such as numbers and literal strings.

        Tokens for each punctuation symbol, such as left and right parentheses, comma, and semicolon.

3. Attributes for Tokens

When more than one lexeme can match a pattern, the lexical analyzer must provide the subsequent compiler phases additional information about the par-ticular lexeme that matched. For example, the pattern for token number matches both 0 and 1, but it is extremely important for the code generator to know which lexeme was found in the source program. Thus, in many cases the lexical analyzer returns to the parser not only a token name, but an attribute value that describes the lexeme represented by the token; the token name in-fluences parsing decisions, while the attribute value influences translation of tokens after the parse.

We shall assume that tokens have at most one associated attribute, although this attribute may have a structure that combines several pieces of information. The most important example is the token id, where we need to associate with the token a great deal of information. Normally, information about an identi-fier — e.g., its lexeme, its type, and the location at which it is first found (in case an error message about that identifier must be issued) — is kept in the symbol table. Thus, the appropriate attribute value for an identifier is a pointer to the symbol-table entry for that identifier.

Tricky Problems When Recognizing Tokens

Usually, given the pattern describing the lexemes of a token, it is relatively simple to recognize matching lexemes when they occur on the input. How-ever, in some languages it is not immediately apparent when we have seen an instance of a lexeme corresponding to a token. The following example is taken from Fortran, in the fixed-format still allowed in Fortran 90. In the statement

DO  5  I  =  1.25

it is not apparent that the first lexeme is D05I, an instance of the identifier token, until we see the dot following the 1. Note that blanks in fixed-format Fortran are ignored (an archaic convention). Had we seen a comma instead of the dot, we would have had a do-statement

DO  5  I  =  1,25

in which the first lexeme is the keyword DO.

E x a m p l e 3 . 2 : The token names and associated attribute values for the For-tran statement

E = M * C ** 2

are written below as a sequence of pairs.

<id, pointer to symbol-table entry for E>

<assign _ op>

<id, pointer to symbol-table entry for M>

<mult _ op>

<id, pointer to symbol-table entry for C>

<exp _ op>

<number, integer value 2>

Note that in certain pairs, especially operators, punctuation, and keywords, there is no need for an attribute value. In this example, the token number has been given an integer-valued attribute. In practice, a typical compiler would instead store a character string representing the constant and use as an attribute value for number a pointer to that string. •

4. Lexical Errors

It is hard for a lexical analyzer to tell, without the aid of other components, that there is a source-code error. For instance, if the string f i is encountered for the first time in a C program in the context:

fi ( a == f ( x ) ) . . .

a lexical analyzer cannot tell whether f i is a misspelling of the keyword if or an undeclared function identifier. Since f i is a valid lexeme for the token id, the lexical analyzer must return the token id to the parser and let some other phase of the compiler — probably the parser in this case — handle an error due to transposition of the letters.

However, suppose a situation arises in which the lexical analyzer is unable to proceed because none of the patterns for tokens matches any prefix of the remaining input. The simplest recovery strategy is "panic mode" recovery. We delete successive characters from the remaining input, until the lexical analyzer can find a well-formed token at the beginning of what input is left. This recovery technique may confuse the parser, but in an interactive computing environment it may be quite adequate.

Other possible error-recovery actions are:

   Delete one character from the remaining input.

         Insert a missing character into the remaining input.

         Replace a character by another character.

         Transpose two adjacent characters.

Transformations like these may be tried in an attempt to repair the input. The simplest such strategy is to see whether a prefix of the remaining input can be transformed into a valid lexeme by a single transformation. This strategy makes sense, since in practice most lexical errors involve a single character. A more general correction strategy is to find the smallest number of transforma-tions needed to convert the source program into one that consists only of valid lexemes, but this approach is considered too expensive in practice to be worth the effort.

5. Exercises for Section 3.1

Exercise 3 . 1 . 1 :  Divide the following C + + program:

float        limited Square ( x ) float    x        {

/* return s x - s q u a r e d , but never more t h a n 100 */

return ( x < = - 1 0 . 0 | | x > = 1 0 . 0 ) ? 1 0 0 : x * x ;

}

into appropriate lexemes, using the discussion of Section 3.1.2 as a guide. Which lexemes should get associated lexical values? What should those values be?

! Exercise 3 . 1 . 2 : Tagged languages like HTML or XML are different from con-ventional programming languages in that the punctuation (tags) are either very numerous (as in HTML) or a user-definable set (as in XML). Further, tags can often have parameters. Suggest how to divide the following HTML document:

Here is a photo of <B>my house</B>:

<PXIMG SRC = "house.gif"><BR>

See <A HREF = "morePix.html">More Pictures</A> if you liked that one.<P>

into appropriate lexemes. Which lexemes should get associated lexical values, and what should those values be?