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digitalmars.D.learn - std.string.translate using initializing twice?

reply simendsjo <simen.endsjo pandavre.com> writes:
translate does this:
     bool[256] deltab; // this would make all values of deltab false as 
bool.init == false, right?

     deltab[] = false;

Isn't this just initializing all values of deltab to false twice..?

And my "Is this more readable?"

Original:

string translate(string s, in string transtab, in string delchars)
     in
     {
     assert(transtab.length == 256);
     }
     body
     {
     char[] r;
     int count;
     bool[256] deltab;

     deltab[] = false;
     foreach (char c; delchars)
     {
         deltab[c] = true;
     }

     count = 0;
     foreach (char c; s)
     {
         if (!deltab[c])
         count++;
         //printf("s[%d] = '%c', count = %d\n", i, s[i], count);
     }

     r = new char[count];
     count = 0;
     foreach (char c; s)
     {
         if (!deltab[c])
         {
         r[count] = transtab[c];
         count++;
         }
     }

     return assumeUnique(r);
     }



"More readable?":


string translate(string s, in string transtab, in string delchars)
in
{
     assert(transtab.length == 256);
}
body
{
     // Mark characters to delete
     bool[256] deltab;
     foreach (char c; delchars)
         deltab[c] = true;

     // Count characters to translate
     int numToTranslate;
     foreach (char c; s)
     {
         if (!deltab[c])
             numToTranslate++;
     }

     char[] result = new char[numToTranslate];

     // Translate
     int translateIndex = 0;
     foreach (char c; s)
     {
         bool mustTranslate = !deltab[c];
         if (mustTranslate)
         {
             result[translateIndex] = transtab[c];
             translateIndex++;
         }
     }

     return assumeUnique(result);
}
Aug 09 2010
parent reply Jonathan M Davis <jmdavisprog gmail.com> writes:
On Monday, August 09, 2010 17:45:07 simendsjo wrote:

 translate does this:
      bool[256] deltab; // this would make all values of deltab false as
 bool.init == false, right?
 
      deltab[] = false;
 
 Isn't this just initializing all values of deltab to false twice..?


I believe that you are correct and that the array is getting set twice.


  [snip]...


I confess that it's entirely irrational on my part given that D is smart enough 
that a post-increment where the temporary is not used should be just as
efficient 
as a pre-increment (even in the face of operator overloading - unlike C++), but 
it always makes me cringe to see post-increments where a pre-increment would 
do...

- Jonathan M Davis
Aug 09 2010
parent reply simendsjo <simen.endsjo pandavre.com> writes:
On 10.08.2010 02:59, Jonathan M Davis wrote:

 On Monday, August 09, 2010 17:45:07 simendsjo wrote:

 translate does this:
       bool[256] deltab; // this would make all values of deltab false as
 bool.init == false, right?

       deltab[] = false;

 Isn't this just initializing all values of deltab to false twice..?


 I believe that you are correct and that the array is getting set twice.


   [snip]...


 I confess that it's entirely irrational on my part given that D is smart enough
 that a post-increment where the temporary is not used should be just as
efficient
 as a pre-increment (even in the face of operator overloading - unlike C++), but
 it always makes me cringe to see post-increments where a pre-increment would
 do...

 - Jonathan M Davis


Yeah. Don't remember when, don't remember where, but I too have read 
that preincrement is faster (PS: I don't know any assembler!).

As long as I don't use it in an expression, I always use post-increment 
as it shouldn't make a difference.
Aug 09 2010
parent Jonathan M Davis <jmdavisprog gmail.com> writes:
On Monday, August 09, 2010 18:03:48 simendsjo wrote:

 Yeah. Don't remember when, don't remember where, but I too have read
 that preincrement is faster (PS: I don't know any assembler!).
 
 As long as I don't use it in an expression, I always use post-increment
 as it shouldn't make a difference.


post-increment creates a temporary. It's really doing something like

T temp = i;
++i;
//use temp in the expression with i++

The temporary is useless and pointless if i++ is by itself rather than in an 
expression. In the case of primitives, the compiler knows enough to optimize
out 
the temporary, but in the case of operator overloading in C++, because pre and 
post-increment are overloaded separately, it can't know for sure that it's safe 
to do the optimization, so it doesn't do it, and your code is less efficient. 
That's why code like

for(vector<int>::iterator iter = vec.begin(),
                                       end  = vec.end();
      iter != end;
      iter++)
{
  ...
}

is so bad. You really need to use ++iter in that case. However, in D, this
isn't 
a problem because pre-increment and post-increment are overloaded by one 
function and the compiler takes care of whether it should be pre or post when 
it's used. So, it should be able to do the optimization just fine. But I've 
programmed in C++ for so long (where it does matter), that I instinctively
react 
negatively to post-increment where a pre-increment will do even in languages 
like Java (which doesn't have operator overloading) or D where it doesn't 
matter.

I'd argue, that everyone (at least everyone programming in C++) should just be 
in the habit of using pre-increment except the cases where a post-increment is 
necessary (then you never have to worry about whether it's less efficient to
use 
post-increment in a particular case), but for whatever reason, new programmers 
are pretty much always taught post-increment first, and so that's what most 
programmers are used to using.

In reality, it's becoming less relevant as newer languages are designed in a 
manner than there is no difference in efficiency, but my natural reaction is
still 
very much that post-increment by itself is evil.

- Jonathan M Davis
Aug 09 2010