Question on "if" statements...

[David Cleaver]

Hello all,

I was wondering if there were some sort of limitations on the "if"
statement? I'm writing a program which needs to check a bunch of
conditions all at the same time (basically). And I'm pretty sure the
rest of the program is working just fine. The only thing I could think
might be wrong is that the if statement can only hold so many values in
itself? Let me show what I'm doing:

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

My problem is that "do_stuff" is never executed. There are 30 different
tables of int's I'm checking against up there. I could use more in the
future, and for the purposes of my program, I don't want to use less. I
know that do_stuff should be executed because I have run test cases
where it should have been run. But it just keeps goin to the "else"
statement. I'm probably way off base thinking C is the problem. If you
want to know about my setup here it is:
WinXP, P4 1.8, Compiler: DJGPP using gcc v3.0.3

So, am I expecting too much out of the if statement? Am I using the if
statement incorrectly up there? Any advice or help in this regard is
greatly appreciated.

-David C.

[Arthur J. O'Dwyer]

On Mon, 26 Apr 2004, David Cleaver wrote:

I was wondering if there were some sort of limitations on the "if"
statement? I'm writing a program which needs to check a bunch of
conditions all at the same time (basically). And I'm pretty sure the
rest of the program is working just fine. The only thing I could think
might be wrong is that the if statement can only hold so many values in
itself?
There is some limitation given by the official standard in that
it specifies a "minimal limit" for the depth of expressions (if I
recall correctly). That is, the Standard says how many "ands" an
'if' statement *must* be able to hold; but any decent implementation
will be able to handle as many as you can throw at it (up to the
available memory on your machine).
Let me show what I'm doing:

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

My problem is that "do_stuff" is never executed.

Have you considered the possibility that the body of the 'if' is
never executed because the big complicated condition is, in fact,
*false*? Have you tried running the 'if' statement in a program
in which "table001.. 030" and "b" are entirely filled with 0x1f
values? What values are stored in those tables during the run of
your current program?

[OT: And why do you need so many tables anyway, and why not use
a "for" loop to check all of them, rather than hard-code the
number of tables into your program?]

-Arthur

[Ralf Damaschke]

David Cleaver wrote:

Let me show what I'm doing:

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&

[...]

The formatting is a bit misleading. This is parsed as:

if (table001[n[0]][x[0]>>5] & (b[x[0]&0x1f] != 0) &&
....

Ralf

[William L. Bahn]

"David Cleaver" <wr*****@morphe us.net> wrote in message
news:40******** *******@morpheu s.net...

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

Are you REALLY sure that this is the test you want to be performing?

You have a really long expression containing three different operators (&,
!=, &&) used many times each. Since you have not told the compiler how to
group them, you are basically choosing to let the compiler make the decision
for you. This is frequently not a good idea.

[Stephen Sprunk]

"David Cleaver" <wr*****@morphe us.net> wrote in message
news:40******** *******@morpheu s.net...

I was wondering if there were some sort of limitations on the "if"
statement? I'm writing a program which needs to check a bunch of
conditions all at the same time (basically). And I'm pretty sure the
rest of the program is working just fine. The only thing I could think
might be wrong is that the if statement can only hold so many values in
itself? Let me show what I'm doing:

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

First, the order of operations isn't clear in your expression. As a rule of
thumb, if the reader needs to look up the rules to figure out what an
expression does, you should add parens to make it clear what you intended.
Second, judicious use of spaces makes complex expressions more readable.
Third, the test against zero is unnecessary since that's implied with
logical operators.

if ( ( table001[n[0]][x[0]>>5] & b[x[0]&0x1f] ) &&
( table002[n[1]][x[1]>>5] & b[x[1]&0x1f] ) &&
( table003[n[2]][x[2]>>5] & b[x[2]&0x1f] ) &&
... &&
( table030[n[29]][x[29]>>5] & b[x[29]&0x1f] ) )
{
do_stuff();
}//end if
else do_other_stuff( );

I don't know if this code is any more correct, but it's a lot easier to read
and debug, which others (and you, later on) will appreciate.

S

--
Stephen Sprunk "Stupid people surround themselves with smart
CCIE #3723 people. Smart people surround themselves with
K5SSS smart people who disagree with them." --Aaron Sorkin

[Chris Dollin]

William L. Bahn wrote:

"David Cleaver" <wr*****@morphe us.net> wrote in message
news:40******** *******@morpheu s.net...

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

Are you REALLY sure that this is the test you want to be performing?

You have a really long expression containing three different operators (&,
!=, &&) used many times each. Since you have not told the compiler how to
group them, you are basically choosing to let the compiler make the
decision for you.

The OP is have the *language* make the decision for them - C has a
defined "order of precedence" [1]. Having lots of conditions separated
by && should be no problem at all.

On the other hand, I'm pretty sure [2] that "X & Y != 0", applying that
order of precedence, groups as "X & (Y != 0)". <fx:flappingPag es/> Yes.

In this case, I'd just write the expression as "X & Y". [Well, actually,
I wouldn't write a monster expression with 30 lines in it embedded in
an if-expression at all, but that's a separate issue.]

[1] defined implicitly by the grammar in the Standard, but expressible
as precedence order.

[2] certain.

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html

[Richard Bos]

Chris Dollin <ke**@hpl.hp.co m> wrote:

On the other hand, I'm pretty sure [2] that "X & Y != 0", applying that
order of precedence, groups as "X & (Y != 0)". <fx:flappingPag es/> Yes.

In this case, I'd just write the expression as "X & Y".

No, you wouldn't; those two expressions can have different values if Y
is non-zero, but not equal to 1. For example, if X==3 and Y==2, the
first expression has the value 1, and the second the value 2. More
importantly to the OP, if X==Y==2, the second expression still has the
value 2, but the first expression now is 0...

Richard

[Chris Dollin]

Richard Bos wrote:

Chris Dollin <ke**@hpl.hp.co m> wrote:

On the other hand, I'm pretty sure [2] that "X & Y != 0", applying that
order of precedence, groups as "X & (Y != 0)". <fx:flappingPag es/> Yes.

In this case, I'd just write the expression as "X & Y".

No, you wouldn't; those two expressions can have different values if Y
is non-zero, but not equal to 1. For example, if X==3 and Y==2, the
first expression has the value 1, and the second the value 2. More
importantly to the OP, if X==Y==2, the second expression still has the
value 2, but the first expression now is 0...

That's the point - I believe that the OP wrote "X & Y != 0" and *meant*
"(X & Y) != 0", which can be written as "X & Y" in conditional context
with the same effect, where it avoids all questions of the relative
precedence of & and !=. Hence "in this case".

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html

[Chris Dollin]

Chris Dollin (== me) wrote:

The OP is have the *language* make the decision for them - C has a
defined "order of precedence" [1]. Having lots of conditions separated
by && should be no problem at all.

Erm, in my haste I did not make it clear that one should still
write expressions for clarity - there's a difference between knowing
the C precedence levels, assuming the reader will know them, and
writing for the IOCC.

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html

[David Cleaver]

Chris Dollin wrote:

William L. Bahn wrote:

David Cleaver wrote:

if (table001[n[0]][x[0]>>5]&b[x[0]&0x1f] != 0 &&
table002[n[1]][x[1]>>5]&b[x[1]&0x1f] != 0 &&
table003[n[2]][x[2]>>5]&b[x[2]&0x1f] != 0 &&
... &&
table030[n[29]][x[29]>>5]&b[x[29]&0x1f] != 0)
{
do_stuff();
}//end if
else do_other_stuff( );

Are you REALLY sure that this is the test you want to be performing?

You have a really long expression containing three different operators (&,
!=, &&) used many times each. Since you have not told the compiler how to
group them, you are basically choosing to let the compiler make the
decision for you.

The OP is have the *language* make the decision for them - C has a
defined "order of precedence" [1]. Having lots of conditions separated
by && should be no problem at all.

On the other hand, I'm pretty sure [2] that "X & Y != 0", applying that
order of precedence, groups as "X & (Y != 0)". <fx:flappingPag es/> Yes.

In this case, I'd just write the expression as "X & Y". [Well, actually,
I wouldn't write a monster expression with 30 lines in it embedded in
an if-expression at all, but that's a separate issue.]

Thank you very much. I never thought about there being a precedence
issue. I've fixed my code to better reflect what I was doing. [And I
was wanting (x & y), not x & (y != 0), I've changed it all to be of the
form (x & y) && ...]

May I ask, without making those tables 3-dimensional, how could I make
that code segment look better? Someone earlier in this thread mentioned
putting it into a for loop, but I can't see how that'd be possible with
30 different tables.

Anyway, I wanted to thank everyone for their suggestions so far. I'm
really glad a place like this exists where people can come for help.
Thanks again.

-David C.

[1] defined implicitly by the grammar in the Standard, but expressible
as precedence order.

[2] certain.

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html