• Bert van Leeuwen (10/10) Sep 03 2010 I'm a D n00b, so excuse my question if it is silly. I've cursorily "foll...
• Nick Sabalausky (20/39) Sep 03 2010 Welcome! :)
• Bert van Leeuwen (2/62) Sep 03 2010
• Pelle (7/8) Sep 03 2010 I don't know if this is intended to be supported, but at least for now
• Bert van Leeuwen (2/15) Sep 03 2010 Interesting. For large arrays, that seems to take almost double the time...
• Denis Koroskin (14/34) Sep 03 2010 I don't think you are right.
• Pelle (4/19) Sep 03 2010 Try it again with -inline. For me it's within 10% with inline, and gains...
• Bert van Leeuwen (2/27) Sep 03 2010 Awesome, that made it run about 4x faster :) thanks!

Bert van Leeuwen <bert e.co.za> writes:

I'm a D n00b, so excuse my question if it is silly. I've cursorily "followed" D
for a few years, but only now bought "The D Programming Language" (great book,
very nicely written!) and started to really play with it.

I've run into two questions which I have not been able to find the answers to
online.

1) I have an int array which I want to replace elements of with compile-time
string expression, e.g.

i=new int[100];
auto b=map!("(a==0)?42:a")(i);
writeln(b);

Cool, that works. But now I want to get at the resulting array. If I replace
"auto b" with:
int[] b = map ...
that does not work ("cannot implicitly convert expression (map(i)) of type
Map!(result,int[]) to int[]")... fine, but how do I get to the int[] ?

2) Related to above, I want to do something like map, but not return a new
array, I want to modify elements in-place in the array. How do I do that?
(without explicitly iterating with foreach etc.)

"Nick Sabalausky" <a a.a> writes:

"Bert van Leeuwen" <bert e.co.za> wrote in message 
news:i5qftb$2a25$1 digitalmars.com...
 I'm a D n00b, so excuse my question if it is silly. I've cursorily 
 "followed" D for a few years, but only now bought "The D Programming 
 Language" (great book, very nicely written!) and started to really play 
 with it.

Welcome! :)

 I've run into two questions which I have not been able to find the answers 
 to online.

 1) I have an int array which I want to replace elements of with 
 compile-time string expression, e.g.

 i=new int[100];
 auto b=map!("(a==0)?42:a")(i);
 writeln(b);

 Cool, that works. But now I want to get at the resulting array. If I 
 replace "auto b" with:
 int[] b = map ...
 that does not work ("cannot implicitly convert expression (map(i)) of type 
 Map!(result,int[]) to int[]")... fine, but how do I get to the int[] ?

Instead of returning an array, map returns a range that computes the values 
lazily (on-demand when they're needed, instead of always computing all of 
them right away). This is done for performance reasons. But you can get an 
array with the array() function from the "std.array" module:

auto b = array( map!("(a==0)?42:a")(i) );

or

auto b=map!("(a==0)?42:a")(i);
auto ba = array(b);

BTW, instead of:

map!("(a==0)?42:a")(i)

You can do:

map!"(a==0)?42:a"(i)

A nifty little bit of syntax sugar for when there's only one template 
parameter.

 2) Related to above, I want to do something like map, but not return a new 
 array, I want to modify elements in-place in the array. How do I do that? 
 (without explicitly iterating with foreach etc.)

I'll leave this one for someone else to answer, as I don't know whether or 
not something like this is already in phobos. If not, such a function can 
definitely be made, and maybe someone has already done so...?

Bert van Leeuwen <bert e.co.za> writes:

Thanks!

Nick Sabalausky Wrote:

 "Bert van Leeuwen" <bert e.co.za> wrote in message 
 news:i5qftb$2a25$1 digitalmars.com...
 I'm a D n00b, so excuse my question if it is silly. I've cursorily 
 "followed" D for a few years, but only now bought "The D Programming 
 Language" (great book, very nicely written!) and started to really play 
 with it.

 
 Welcome! :)
 
 I've run into two questions which I have not been able to find the answers 
 to online.

 1) I have an int array which I want to replace elements of with 
 compile-time string expression, e.g.

 i=new int[100];
 auto b=map!("(a==0)?42:a")(i);
 writeln(b);

 Cool, that works. But now I want to get at the resulting array. If I 
 replace "auto b" with:
 int[] b = map ...
 that does not work ("cannot implicitly convert expression (map(i)) of type 
 Map!(result,int[]) to int[]")... fine, but how do I get to the int[] ?

 
 Instead of returning an array, map returns a range that computes the values 
 lazily (on-demand when they're needed, instead of always computing all of 
 them right away). This is done for performance reasons. But you can get an 
 array with the array() function from the "std.array" module:
 
 auto b = array( map!("(a==0)?42:a")(i) );
 
 or
 
 auto b=map!("(a==0)?42:a")(i);
 auto ba = array(b);
 
 BTW, instead of:
 
 map!("(a==0)?42:a")(i)
 
 You can do:
 
 map!"(a==0)?42:a"(i)
 
 A nifty little bit of syntax sugar for when there's only one template 
 parameter.
 
 2) Related to above, I want to do something like map, but not return a new 
 array, I want to modify elements in-place in the array. How do I do that? 
 (without explicitly iterating with foreach etc.)

 
 I'll leave this one for someone else to answer, as I don't know whether or 
 not something like this is already in phobos. If not, such a function can 
 definitely be made, and maybe someone has already done so...?
 
 
 

Pelle <pelle.mansson gmail.com> writes:

On 09/03/2010 11:42 AM, Bert van Leeuwen wrote:

 2) Related to above, I want to do something like map, but not return a new
array, I want to modify elements in-place in the array. How do I do that?
(without explicitly iterating with foreach etc.)

I don't know if this is intended to be supported, but at least for now 
this works:

     int[] xs = [1,2,3,4,5,6,7];
     copy(map!`a*a`(xs), xs);

     writeln(xs);

[1, 4, 9, 16, 25, 36, 49]

Bert van Leeuwen <bert e.co.za> writes:

Pelle Wrote:

 On 09/03/2010 11:42 AM, Bert van Leeuwen wrote:
 
 2) Related to above, I want to do something like map, but not return a new
array, I want to modify elements in-place in the array. How do I do that?
(without explicitly iterating with foreach etc.)

 
 I don't know if this is intended to be supported, but at least for now 
 this works:
 
      int[] xs = [1,2,3,4,5,6,7];
      copy(map!`a*a`(xs), xs);
 
      writeln(xs);
 
 [1, 4, 9, 16, 25, 36, 49]


Interesting. For large arrays, that seems to take almost double the time of
creating a new one with array() (as in Nick's reply). So copy() obviously
doesn't do it in place (not surprising given its name), and presumably makes a
temporary array first from which to copy to a.

"Denis Koroskin" <2korden gmail.com> writes:

On Fri, 03 Sep 2010 14:32:47 +0400, Bert van Leeuwen <bert e.co.za> wrote:

 Pelle Wrote:

 On 09/03/2010 11:42 AM, Bert van Leeuwen wrote:

 2) Related to above, I want to do something like map, but not return  

 a new array, I want to modify elements in-place in the array. How do I  
 do that? (without explicitly iterating with foreach etc.)

 I don't know if this is intended to be supported, but at least for now
 this works:

      int[] xs = [1,2,3,4,5,6,7];
      copy(map!`a*a`(xs), xs);

      writeln(xs);

 [1, 4, 9, 16, 25, 36, 49]


 Interesting. For large arrays, that seems to take almost double the time  
 of creating a new one with array() (as in Nick's reply). So copy()  
 obviously doesn't do it in place (not surprising given its name), and  
 presumably makes a temporary array first from which to copy to a.

I don't think you are right.
This is how copy is defined in std.algorithm[1]:

Range2 copy(Range1, Range2)(Range1 source, Range2 target)
if (isInputRange!Range1 && isOutputRange!(Range2, ElementType!Range1))
{
     for (; !source.empty; source.popFront())
     {
         put(target, source.front);
     }
     return target;
}

[1]  
http://dsource.org/projects/phobos/browser/trunk/phobos/std/algorithm.d#L4054

Pelle <pelle.mansson gmail.com> writes:

On 09/03/2010 12:32 PM, Bert van Leeuwen wrote:
 Pelle Wrote:

 On 09/03/2010 11:42 AM, Bert van Leeuwen wrote:

 2) Related to above, I want to do something like map, but not return a new
array, I want to modify elements in-place in the array. How do I do that?
(without explicitly iterating with foreach etc.)

 I don't know if this is intended to be supported, but at least for now
 this works:

       int[] xs = [1,2,3,4,5,6,7];
       copy(map!`a*a`(xs), xs);

       writeln(xs);

 [1, 4, 9, 16, 25, 36, 49]


 Interesting. For large arrays, that seems to take almost double the time of
creating a new one with array() (as in Nick's reply). So copy() obviously
doesn't do it in place (not surprising given its name), and presumably makes a
temporary array first from which to copy to a.

Try it again with -inline. For me it's within 10% with inline, and gains 
even more if you add -O and -release.

It also doesn't allocate, which is just a bonus. :-)

Bert van Leeuwen <bert e.co.za> writes:

Pelle Wrote:

 On 09/03/2010 12:32 PM, Bert van Leeuwen wrote:
 Pelle Wrote:

 On 09/03/2010 11:42 AM, Bert van Leeuwen wrote:

 2) Related to above, I want to do something like map, but not return a new
array, I want to modify elements in-place in the array. How do I do that?
(without explicitly iterating with foreach etc.)

 I don't know if this is intended to be supported, but at least for now
 this works:

       int[] xs = [1,2,3,4,5,6,7];
       copy(map!`a*a`(xs), xs);

       writeln(xs);

 [1, 4, 9, 16, 25, 36, 49]


 Interesting. For large arrays, that seems to take almost double the time of
creating a new one with array() (as in Nick's reply). So copy() obviously
doesn't do it in place (not surprising given its name), and presumably makes a
temporary array first from which to copy to a.

 
 Try it again with -inline. For me it's within 10% with inline, and gains 
 even more if you add -O and -release.
 
 It also doesn't allocate, which is just a bonus. :-)

Awesome, that made it run about 4x faster :) thanks!