Efficiency Issue - true or false?

[rahul.batra]

This is supposed to be a very general question.
Consider the following 2 code samples:

Code Sample 1:

for (int i=0; i<n; ++i)
{
for (int j=0; j<n; ++j)
{
/* Do something here */
}
}

Code Sample 2:
(Now we declare j in advance rather than each time inside the i loop
as in code sample 1)

int j;
for (int i=0; i<n; ++i)
{
for (j=0; j<n; ++j) // Notice the differnce here, now
{
/* Do something here */
}
}
Is code sample 2 faster than code sample 1?
Plz specify the reasons too.

[Chris Jefferson]

ra*********@gmail.com wrote:

This is supposed to be a very general question.
Consider the following 2 code samples:

Code Sample 1:

for (int i=0; i<n; ++i)
{
for (int j=0; j<n; ++j)
{
/* Do something here */
}
}

Code Sample 2:
(Now we declare j in advance rather than each time inside the i loop
as in code sample 1)

int j;
for (int i=0; i<n; ++i)
{
for (j=0; j<n; ++j) // Notice the differnce here, now
{
/* Do something here */
}
}
Is code sample 2 faster than code sample 1?
Plz specify the reasons too.

No, they will produce exactly the same code. The reason is that
"creating an int" doesn't actually require any work at all, except
marking some register / piece of memory in which it will live.

You could find that if j was a complex class, the second might be
faster, although in most cases it probably wouldn't be, as "complexclass
j = 0" would be mapped to "complexclass j(0)" and in many cases
constructing a new object may be faster than assigning to an existing one.

Chris

[Ioannis Vranos]

ra*********@gmail.com wrote:

This is supposed to be a very general question.
Consider the following 2 code samples:

Code Sample 1:

for (int i=0; i<n; ++i)
{
for (int j=0; j<n; ++j)
{
/* Do something here */
}
}

Code Sample 2:
(Now we declare j in advance rather than each time inside the i loop
as in code sample 1)

int j;
for (int i=0; i<n; ++i)
{
for (j=0; j<n; ++j) // Notice the differnce here, now
{
/* Do something here */
}
}
Is code sample 2 faster than code sample 1?
Plz specify the reasons too.

In both cases two ints are created and used. In subsequent loops with
more int counters, you can't expect any additional cost either because
the compiler can perform further optimisations since it knows that the
variables are used for loop counters only, like placing them in a register.

--
Ioannis Vranos

http://www23.brinkster.com/noicys

[Julián Albo]

ra*********@gmail.com wrote:

Is code sample 2 faster than code sample 1?
No.
Plz specify the reasons too.

You must write one more line.

--
Salu2