chris@umcp-cs.UUCP (Chris Torek) (11/18/85)
Darn... when I got up this evening I realized that I had not really
said what I wanted to in the first part of that article. I hate
to reopen the wound and rub in salt, but . . . here goes:
Given a C construct such as
for (init; cond; step) {
code;
if (cond2)
break;
code2;
}
one can, without changing the meaning of the code, write
init;
bool := true;
while (cond) and bool do
begin code;
if cond2 then
bool := false
else
begin code2;
step
end
end;
which is how the loop would most likely appear in Pascal. (The
expanded form if `code' or `code2' contains `continue' statements
is somewhat different, but does exist.) This expanded form can be
transformed direcly into C in the obvoius way.
These are semantically equivalent when it comes to proving the code
correct. Since they are equivalent, you may elect to use either
form. In C, I would use the `compressed' form; but this is a matter
of taste, and you probably know the old expression concerning such.
--
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251)
UUCP: seismo!umcp-cs!chris
CSNet: chris@umcp-cs ARPA: chris@mimsy.umd.educhris@umcp-cs.UUCP (Chris Torek) (11/23/85)
For those who wish to read the `original' article (I have not), here is the reference: Knuth, D. E., `Structured Programming with GOTO Statements', _Computing_Surveys_, 6, 4, 1974, 261-301. The idea is to use `goto' in a very restricted way to express what the code is supposed to *do*: For example, if you are searching for something, and need to do `x' if it is not found, but `y' if it is, then doing a `goto found' and separating the code for `y' from the code for the search is quite reasonable. Others, of course, will argue that the code for `x' and `y' should be moved to other routines; but I believe this kind of separation can be overused, just as can `goto's. [The information in the next few paragraphs prompted me to add net.lang.pascal to the Newsgroups line above. Please stop here if bored.] Knuth's usage of `goto' and even `return' may be found in the source to TeX and MetaFont, which are written in WEB, a language combining a Pascal preprocessor and a program description/documentation language. The `Tangle' processor converts a WEB file to a Pascal file; and the `Weave' processor converts a WEB to a TeX file. (Incidentally, there is an interesting bootstrap problem here, as both Tangle and Weave are also written in WEB.) This may again be more a personal preference than anything else, but I find that Knuth's programs written in WEB are considerably easier to read than many other smaller programs I have seen written directly in Pascal. Even the `return' statements---which are actually `goto's in disguise, and rather thin disguise at that---are better than the convoluted code with extra booleans found in programs that steadfastedly avoid the `goto temptation'. The WEB system is available for 4.1 and 4.2 BSD Unix as part of the Unix TeX distribution. (There is a distribution fee, around $75, I think.) Contact Pierre MacKay at the University of Washington for more information: ARPA: MacKay@WASHINGTON; UUCP: ...!uw-beaver!mackay. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 4251) UUCP: seismo!umcp-cs!chris CSNet: chris@umcp-cs ARPA: chris@mimsy.umd.edu