Relational Operators

[AliceB.Toklas]

In my Absolute Beginner's Guide to C book by Greg Perry
where it is instruction how relational operators work it gives the
following example:

int i = 5;

so the following statement is true:

int i == 1

Um, to me that says integer i is 5, and 5 is equal to 1.

Can someone help me understand this?
Thanks

[Richard Heathfield]

August Karlstrom said:

Ben Pfaff skrev:

>August Karlstrom <fu********@com hem.sewrites:

>>Ben Pfaff skrev:
== is not a relational operator (it is an equality operator)
Of course it is -- in the same sense as a car is a vehicle. Other
relational operators are <= and >=.

Not in C.

<proof snipped>

>
Laughable

Not at all. The relational operators test an ordering - a relation -
between two objects. == doesn't do that. It tests only whether two
objects are equal, and yields no relational information whatsoever.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[CBFalconer]

Ben Pfaff wrote:

August Karlstrom <fu********@com hem.sewrites:

>Ben Pfaff skrev:

>>== is not a relational operator (it is an equality operator)

Of course it is -- in the same sense as a car is a vehicle.
Other relational operators are <= and >=.

Not in C.

6.5.8 Relational operators
Syntax
1 relational-expression:
shift-expression
relational-expression < shift-expression
relational-expression shift-expression
relational-expression <= shift-expression
relational-expression >= shift-expression

6.5.9 Equality operators
Syntax
1 equality-expression:
relational-expression
equality-expression == relational-expression
equality-expression != relational-expression

The reason being that the two kinds need separating, since you can
use equality operators on any two pointers, but not necessarily
relational operators.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

[Eric Sosman]

Richard Heathfield wrote:

August Karlstrom said:

>Ben Pfaff skrev:

>>August Karlstrom <fu********@com hem.sewrites:

Ben Pfaff skrev:
== is not a relational operator (it is an equality operator)
Of course it is -- in the same sense as a car is a vehicle. Other
relational operators are <= and >=.
Not in C.

<proof snipped>

>Laughable

Not at all. The relational operators test an ordering - a relation -
between two objects. == doesn't do that. It tests only whether two
objects are equal, and yields no relational information whatsoever.

... and on a more practical level, the equality operators
can be used in situations where the relational operators would
produce undefined behavior:

int this;
char whatnot[97];
int that;

if (&this == &that) ... /* valid, false */
if (&this < &that) ... /* undefined behavior */

The two kinds of operators behave differently, so the
difference between them is not in the least "laughable. "

--
Eric Sosman
es*****@acm-dot-org.invalid

[August Karlstrom]

Richard Heathfield skrev:

August Karlstrom said:

>Ben Pfaff skrev:

>>August Karlstrom <fu********@com hem.sewrites:

Ben Pfaff skrev:
== is not a relational operator (it is an equality operator)
Of course it is -- in the same sense as a car is a vehicle. Other
relational operators are <= and >=.
Not in C.

<proof snipped>

>Laughable

Not at all. The relational operators

in C terminology

test an ordering - a relation -
between two objects.

Exactly, these are *order* relations.

== doesn't do that.
It tests only whether two
objects are equal, and yields no relational information whatsoever.

Correct, equality is not an order relation.

Generally, if x and y are elements of the set M, a relation between x
and y is defined as a subset of M^2 (Cartesian product). So equality is
obviously a relation.

What's laughable is using the term relation but without letting equality
be an instance of it. They should have used a different name for the
order relations and the shift expressions.
August

[Keith Thompson]

August Karlstrom <fu********@com hem.sewrites:
[...]

Generally, if x and y are elements of the set M, a relation between x
and y is defined as a subset of M^2 (Cartesian product). So equality is
obviously a relation.

What's laughable is using the term relation but without letting equality
be an instance of it. They should have used a different name for the
order relations and the shift expressions.

Yeah, and a "byte" should be exactly 8 bits, and an "object" should be
something with methods and inheritance, and a "string" should be a
data type.

The authors of the standard had to express technical concepts using
English words, introducing explicit definitions where necessary. They
may not have done a perfect job of picking the best term in each
instance, but IMHO, they did pretty well. Referring to <, <=, >, and

>=, but not == and !=, as "relational operators", just doesn't bother

me that much.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"