CS 375: Lexical Analyzer

Due: February 4, 2021.

Write a lexical analyzer for Pascal. The program may be written in C or in Lisp.

Input to the Lexical Analyzer is obtained by calling functions that get input characters. getchar() returns the next character from the input and moves the character pointer; peekchar() returns the next character without moving the pointer. peek2char() returns the second character without moving the pointer. These functions are provided.

The Lexical Analyzer is called as the function gettoken() (provided in the file scanner.c); its output is one token. A token record contains the following fields (plus some pointers, to be used later):

White Space and Comments:

Blanks, tabs, ends of lines, and comments are considered to be separators; the Lexical Analyzer consumes (skips over) these, but does not return anything for them. Comments are contained between the characters { and } , or between (* and *) . The first occurrence of the terminating character(s) ends the comment; comments cannot be nested.

Operators and Delimiters:

Operators are as follows: + - * / := = <> < <= >= > ^ .

The following Reserved Words are treated as Operators: and or not div mod in

Delimiters are: , ; : ( ) [ ] ..

The result returned for an Operator or Delimiter is:

tokentype: OPERATOR (0) or DELIMITER (1)
whichval: integer denoting which operator (1..19) or delimiter (1..8).

Reserved words:

The Reserved Words of Pascal (other than Operators) are:
array       downto    function   of           repeat    until
begin       else      goto       packed       set       var  
case        end       if         procedure    then      while
const       file      label      program      to        with 
do          for       nil        record       type
The result returned for a Reserved Word is:
tokentype: RESERVED (2)
whichval: integer denoting which reserved word (1..29)


Identifiers begin with a letter, followed by any number of letters or digits. We will assume that only the first 15 characters of the identifier name are significant; longer identifiers should be truncated to 15 characters. We will assume that the case of letters is left as specified; our input will use lower-case letters.

The result returned for an Identifier is:
tokentype: IDENTIFIERTOK (3)
stringval: string (identifier name)


Strings are enclosed by ' characters. The character ' may be included within a string by doubling it, as in the example 'don''t' . For simplicity, we will assume that a string has at most 15 characters.

The result returned for a String is:
tokentype: STRINGTOK (4)
stringval: string

Unsigned Numbers:

Unsigned Numbers must begin with a digit; if there is a decimal point, it must be followed by at least one digit. A number may be followed by a signed decimal exponent, in which case it is floating-point (whether there is a decimal point or not).

The lexical analyzer must convert a number to internal numeric form. Care must be taken to ensure that the result is numerically accurate and that errors such as numeric overflow are detected. Challenging examples are included in the test data; correct handling of these examples will be a grading criterion. If there is an error, your program should print an error message and return a number token of the correct type (integer or real); the value of the number will not matter.

The result returned for a Number is:
tokentype: NUMBERTOK (5)
datatype: integer denoting type: INTEGER or REAL
tokenval: numeric value in internal form. For C, use intval and realval


Test your program on the files graph1.pas and scantst.pas .


Jensen, K. and Wirth, N., Pascal User Manual and Report, Springer-Verlag, 1976.

Notes for C:

Use the makefile in the directory /projects/cs375/. A file lex1.c containing program stubs for this assignment is provided; you may use this file as the starting point for your program. Use make lex1 to compile this program:
mkdir test
cd test
cp /projects/cs375/* .
make lex1
123 45678 12
17 444
^C              to quit
Copy lex1.c to lexanc.c and extend lexanc.c for this assignment; use make lexanc to compile your program. The file lexandr.c contains a driver program to call gettoken() and print the results. The file scanner.c contains functions peekchar and peek2char and some utility functions. The function getchar is part of the stdio.h package. Use the definitions in token.h and lexan.h, which definine the TOKEN structure, operator numbers etc. The file printtoken.c contains the function talloc, which makes a new token data structure and returns a pointer to it. You probably should not change the files scanner.c and printtoken.c. Note that the tables in printtoken.c are intended for printing and may not be what you need for the lexical analyzer; copy and modify these tables to make new ones if you wish.

Note that a string in C is an array of characters terminated by a zero '\0' character; you should terminate all strings this way. A string cannot just be assigned as in Java; it must be copied in a loop one character at a time, or you can use strcpy(to,from).

You must perform scanning at the character level; you are not allowed to use C library functions that do the work of the lexical analyzer, such as sscanf. However, you may use standard string library functions such as strcmp (string compare).

Notes for Lisp:

You should use Lisp only if you are already good at Lisp. However, there is little risk in doing the first project in Lisp.

The file scanner.lsp contains the functions getchar, peekchar, and peek2char; these return NIL at end-of-file. charclass determines the class of a character. Call the function read-file to read the input file, e.g.

(read-file "/u/cs375/graph1.pas")
After calling read-file, the call (test-scanner) will test your program on the file. The file tokendefs.lsp contains macro definitions for tokens and the functions talloc (which makes a token structure) and printtoken.

Character constants are written with a #\ prefix, e.g. the character * is written #\* ; the blank space is written #\Space .

Useful Common Lisp functions: char-code, make-string, char-downcase, char=, string=.

Other useful files: number.lsp   while.lsp

You must perform scanning at the character level; you are not allowed to use Lisp functions that do the work of the lexical analyzer, such as intern or read-from-string.