NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/5739

Internet Message Format | 1996-08-05 | 4.3 KB

Path: newshost.cyberramp.net!news
From: [e-mail] (John Noland)
Newsgroups: comp.lang.c
Subject: Re: simple code, argc, argv, strcmp()
Date: 21 Feb 1996 00:01:41 GMT
Organization: Prose Software
Message-ID: <4gdnd5$[e-mail]>
References: <11f7cc$17261a.3b3@daprez> <[e-mail]> <4g040v$[e-mail]>
NNTP-Posting-Host: ramp2-16.cyberramp.net
X-Newsreader: WinVN 0.99.5

In article <4g040v$[e-mail]>, [e-mail] says...

>>int main (int argc, char **argv) {
>>
>>  if (!strcmp(argv[1],"-d") || !strcmp(argv[1],"-e")) {
>>    Usage();
>>  }
>...
>>
>>Can someone see what's wrong with this ?
>>am I not using strcmp() right ?
>

I need to make the following statements just to be thorough:

1. int strcmp(const char *s1, const char *s2);
strcmp returns a value that is
< 0  if s1 is less than s2
== 0 if s1 is the same as s2
> 0  if s1 is greater than s2

2. if (conditional-expression) TRUE-STATEMENT <else FALSE-STATEMENT>
The conditional-expression must be of scalar type. The expression is
evaluated. If the value is zero, we say that the conditional-expression
is FALSE; otherwise, it is TRUE.
If there is no else clause, and the conditional-expression is TRUE,
TRUE-STATEMENT is executed; otherwise, TRUE-STATEMENT is ignored.

3. In the expression E1 || E2, both operands must be of scalar type. The
result is of type int, and the result is 1 (TRUE) if either of the values
of E1 or E2 are nonzero. Otherwise, the result is 0 (FALSE).
E1 is evaluated first; if E1 is nonzero, E1 || E2 gives 1 (TRUE), and E2
is not evaluated. This so-called "short-circuit evaluation" is true in
Microsoft and Borland compilers, but is not specified by "K&R" or
ANSI. Your compiler may or may not do it this way.

4. The unary negation operator ! converts a non-zero (TRUE) operand into
zero, and a zero (FALSE) operand into 1.

5. You will need to check the language reference that came with your
compiler to verify precedence and order of evaluation. They tend to
be consistent from one to another, but they aren't specified in any
official standard, so yours could be different(BLECHHH!).

How does the statement evaluate:

if argv[1] is neither "-d" or "-e"?

1. strcmp returns an int not equal to zero for each expression.
2. The ! operator logically negates the return value of each
   strcmp, giving zero for both. Our statement is now essentially:
   <if (0 || 0)>.
3. 0 || 0 gives 0. We now have <if (0)> or <if (FALSE)>

RESULT: usage() is not called. This is an INCORRECT result!

if argv[1] is "-d" or "-e"?

1. One strcmp returns zero, the other returns non-zero.
2. The ! operator logically negates the return value of
   each strcmp, giving zero for one and one for the other.
   Our statement is now essentially: <if (1 || 0)> or
   <if (0 || 1)>.
3. 1 || 0 or 0 || 1 gives 1. We now have <if (1)>.

RESULT: usage() is called. This is an incorrect result!

if argv[1] is equal to "-d" and "-e"?

Wait a minute! Is this combination possible? Do you see the problem
with this? The same variable cannot possibly have two different
values at the same time.

To ease the illustration of this and rid strcmp from the discussion,
let's use an int in simplified conditional statements. Your statement
above would look something like this (though not exactly, I'll get to
that in a minute!):

int number;
...
       if ((number != 6) || (number != 9)) some_function();


This a common mistake in most programming languages (not just C). The
problem is that at least one of the simple conditions will always be
true; number will always either not contain a 6 or not contain a 9, or
not contain either one. Thus, the compound condition is always true.
This isn't true with the strcmp example above. Why is that, do you
reckon? Here's a hint, I think you can figure it out yourself:
    if (!(number == 6) || !(number == 9)) some_function();

The "moral" of all of this is a lesson in boolean algebra, which is:
When negating conditions separated by ||: if !(exp1 || exp2), the
stated conditions become: if (!exp1 AND !exp2)...

Hence,
   if !(strcmp(argv[1], "-d") || strcmp(argv[1], "-e") usage();
or
   if (!strcmp(argv[1], "-d") && !strcmp(argv[1], "-e") usage();

would be solutions that do what you desire.

Sorry this is such a long answer, but I've found that only the real
gurus are allowed to be terse in this group.

-John