• Q. Schroll (10/10) Aug 04 2017 One can do
• Stefan Koch (3/13) Aug 04 2017 you can see what the code is lowerd to with -vcg-ast
• Steven Schveighoffer (14/24) Aug 04 2017 Any foreach range statement like this:
• Stefan Koch (11/25) Aug 04 2017 Not quite,
• Q. Schroll (3/12) Aug 09 2017 That's enough to know that the foreach loop does not reuse the
• H. S. Teoh via Digitalmars-d-learn (8/22) Aug 09 2017 Note that using BigInt as iteration variable will probably cause a
• Steven Schveighoffer (5/25) Aug 09 2017 Right, but this is not a limitation of the API, just the implementation....
• Steven Schveighoffer (4/8) Aug 09 2017 Based on H.S. Teoh's comment in the bug report, this actually is invalid...

Q. Schroll <qs.il.paperinik gmail.com> writes:

One can do
   BigInt n = returnsBigInt();
   foreach (i; BigInt(0) .. n)
   { .. }
How is this implemented? The code for BigInt is very large and I 
didn't find it.
And is it more efficient than
   for (BigInt i = 0; i < n; ++i)
   { .. }
as incrementing is a costly operation?

Stefan Koch <uplink.coder googlemail.com> writes:

On Friday, 4 August 2017 at 12:49:30 UTC, Q. Schroll wrote:
 One can do
   BigInt n = returnsBigInt();
   foreach (i; BigInt(0) .. n)
   { .. }
 How is this implemented? The code for BigInt is very large and 
 I didn't find it.
 And is it more efficient than
   for (BigInt i = 0; i < n; ++i)
   { .. }
 as incrementing is a costly operation?

you can see what the code is lowerd to with -vcg-ast
The hand-written for is as effective as foreach usually.

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 8/4/17 8:49 AM, Q. Schroll wrote:
 One can do
    BigInt n = returnsBigInt();
    foreach (i; BigInt(0) .. n)
    { .. }
 How is this implemented? The code for BigInt is very large and I didn't 
 find it.
 And is it more efficient than
    for (BigInt i = 0; i < n; ++i)
    { .. }

Any foreach range statement like this:

foreach(var; A .. B)

is treated as if you wrote:

for(auto var = A; var < B; ++var)

So it's pretty much exactly like what you wrote, just the initializer is 
different but the result is the same.

 as incrementing is a costly operation?

Here is increment in bigint:
https://github.com/dlang/phobos/blob/master/std/bigint.d#L563

And addOrSubInt:
https://github.com/dlang/phobos/blob/master/std/internal/math/biguintcore.d#L508

I think there is room for improvement, as incrementing or decrementing 
by 1 is probably something that can be optimized.

-Steve

Stefan Koch <uplink.coder googlemail.com> writes:

On Friday, 4 August 2017 at 13:09:55 UTC, Steven Schveighoffer 
wrote:

 Any foreach range statement like this:

 foreach(var; A .. B)

 is treated as if you wrote:

 for(auto var = A; var < B; ++var)

 So it's pretty much exactly like what you wrote, just the 
 initializer is different but the result is the same.

 as incrementing is a costly operation?

 Here is increment in bigint:
 https://github.com/dlang/phobos/blob/master/std/bigint.d#L563

 And addOrSubInt:
 https://github.com/dlang/phobos/blob/master/std/internal/math/biguintcore.d#L508

 I think there is room for improvement, as incrementing or 
 decrementing by 1 is probably something that can be optimized.

 -Steve

Not quite,
foreach(x;A .. B)
it's lowerd to
auto limit = B;
auto key = A;
for(auto x = key;key < limit;++key)
{
  // use x
}

Q. Schroll <qs.il.paperinik gmail.com> writes:

On Friday, 4 August 2017 at 16:40:08 UTC, Stefan Koch wrote:
 [..]

 foreach(x;A .. B)
 it's lowerd to
 auto limit = B;
 auto key = A;
 for(auto x = key;key < limit;++key)
 {
  // use x
 }

That's enough to know that the foreach loop does not reuse the 
space for the iteration variable. That was what I cared about.

"H. S. Teoh via Digitalmars-d-learn" <digitalmars-d-learn puremagic.com> writes:

On Wed, Aug 09, 2017 at 09:34:26PM +0000, Q. Schroll via Digitalmars-d-learn
wrote:
 On Friday, 4 August 2017 at 16:40:08 UTC, Stefan Koch wrote:
 [..]
 
 foreach(x;A .. B)
 it's lowerd to
 auto limit = B;
 auto key = A;
 for(auto x = key;key < limit;++key)
 {
  // use x
 }

 
 That's enough to know that the foreach loop does not reuse the space
 for the iteration variable. That was what I cared about.

Note that using BigInt as iteration variable will probably cause a
BigInt allocation per iteration, because operations on BigInt in general
will allocate a new BigInt result. So `++key` will probably create a new
instance of BigInt each time it's executed.


T

-- 
To provoke is to call someone stupid; to argue is to call each other stupid.

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 8/9/17 5:40 PM, H. S. Teoh via Digitalmars-d-learn wrote:
 On Wed, Aug 09, 2017 at 09:34:26PM +0000, Q. Schroll via Digitalmars-d-learn
wrote:
 On Friday, 4 August 2017 at 16:40:08 UTC, Stefan Koch wrote:
 [..]

 foreach(x;A .. B)
 it's lowerd to
 auto limit = B;
 auto key = A;
 for(auto x = key;key < limit;++key)
 {
   // use x
 }

 That's enough to know that the foreach loop does not reuse the space
 for the iteration variable. That was what I cared about.

 
 Note that using BigInt as iteration variable will probably cause a
 BigInt allocation per iteration, because operations on BigInt in general
 will allocate a new BigInt result. So `++key` will probably create a new
 instance of BigInt each time it's executed.

Right, but this is not a limitation of the API, just the implementation. 
It could be improved.

https://issues.dlang.org/show_bug.cgi?id=17736

-Steve

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 8/9/17 9:08 PM, Steven Schveighoffer wrote:
 Right, but this is not a limitation of the API, just the implementation. 
 It could be improved.
 
 https://issues.dlang.org/show_bug.cgi?id=17736

Based on H.S. Teoh's comment in the bug report, this actually is invalid.

That's a tough requirement.

-Steve