I have an if statement that looks like this:
if(foo(&x) && x > y)
...
where the value of x is modified by foo, and the comparison between x
and y only makes sense after x has been modified by foo (and, of
course, if foo returns true). Am I guaranteed (assuming my compiler
generates correct code) that x > y is evaluated after foo(&x) returns?
Assuming things work the way I think they do, is it considered bad form
to write code like this? It saves me from doing something like the
following, which I think is ugly-looking:
if(foo(&x))
{
if(x > y)
{
...
}
}
Thanks,
Tim 3 1563
tconkling wrote: I have an if statement that looks like this:
if(foo(&x) && x > y) ...
where the value of x is modified by foo, and the comparison between x and y only makes sense after x has been modified by foo (and, of course, if foo returns true). Am I guaranteed (assuming my compiler generates correct code) that x > y is evaluated after foo(&x) returns?
Yes in some sense. The order of evaluation is lexicographic, so first
foo(&x)
and *IF* foo is true then is (x > y) evaluated, otherwise it goes rigth
to the else statement.
Assuming things work the way I think they do, is it considered bad form to write code like this? It saves me from doing something like the following, which I think is ugly-looking:
if(foo(&x)) { if(x > y) { ... } }
They are equivalent.
tconkling wrote: I have an if statement that looks like this:
if(foo(&x) && x > y) ...
where the value of x is modified by foo, and the comparison between x and y only makes sense after x has been modified by foo (and, of course, if foo returns true). Am I guaranteed (assuming my compiler generates correct code) that x > y is evaluated after foo(&x) returns?
Assuming things work the way I think they do, is it considered bad form to write code like this? It saves me from doing something like the following, which I think is ugly-looking:
if(foo(&x)) { if(x > y) { ... } }
Thanks, Tim
Yes, those forms are equivalent. Of course, you don't need the braces
around the inner if().
"tconkling" <tc*******@gmai l.com> writes: I have an if statement that looks like this:
if(foo(&x) && x > y) ...
where the value of x is modified by foo, and the comparison between x and y only makes sense after x has been modified by foo (and, of course, if foo returns true). Am I guaranteed (assuming my compiler generates correct code) that x > y is evaluated after foo(&x) returns?
Assuming things work the way I think they do, is it considered bad form to write code like this? It saves me from doing something like the following, which I think is ugly-looking:
if(foo(&x)) { if(x > y) { ... } }
Yes, that's a special property of the "&&" operator (also "||" and
","). The "&&" operator evaluates its left operand first, then
evaluates the right operand only of the left operand evaluted to a
non-zero value. There's also a sequence point between the evaluation
of the left and right operands.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
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