Programming Praxis

In the old days, when programs were written on punch cards or paper tape and editors didn’t provide regular-expression searches, most compilers provided a cross-reference utility that showed each use of each identifier in the program. You don’t often see cross-referencers any more, and that’s too bad, because a cross-referencer can provide good clues about the structure of a program; looking at a cross-reference listing can be a good way to start examining an unfamiliar program. Here’s some sample output from a Scheme cross-referencer, with the identifier as the first word on each line followed by a list of line numbers where the identifier appears:

0 28
1 18
< 27
= 30
?!.+-*/<=>:$%^&_~@ 5
a 24 25 26 27
add1 20
and 26 30
b 24 25 26 27
c 3 4 5 8 9 10 13
caar 30 32 36 37
car 25 26
cdar 30 33 34 36 37
cdr 27 31 34 37
char-alphabetic? 4
char-in-ident? 3 10
char-numeric? 4
char=? 13
cond 9 19 29
cons 11 21
cs 8 9 11 12 14
define 3 7 16 23 24
display 33 36
else 14 21 35
eof-object? 9 19
file 16 17
get-ident 7 18 20 21
identifiers 3
if 9
lambda 17
let 8 11 18 28
line 18 20 21
list->string 9 12
loop 8 11 14 18 20 21 28 31 34 37
lt? 24 28
member 5
newline 13 29 35
not 35
null? 9 29
or 4 25
pair? 12
peek-char 8 11 14
prev-line 28 30 31
prev-word 28 30 31 32 34 35
read-char 11 13 14
reverse 9 12
scheme 3
sort 28
string->list 5
string<? 25
string=? 20 26 30 32 35
w 18 19 20 21
when 35
with-input-from-file 17
ws 18 19 20 21 23 28 29 30 31 32 33 34 36 37
x 11
xref 1 16
xref-out 19 23

A quick look suggests that this program reads a text file (with-input-from-file, read-char, eof-object?) and does some kind of processing on its characters (char-alphabetic?, char-numeric?). There is some list-handling (car, cdr, string->list), loop is used conventionally, and ws seems to be some kind of key to understanding what the program does. And there is an unholy mess on line 5.

Your task is to write a cross-referencer for your chosen language. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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