It maybe trivial to most of you but I don't know how do this avoiding string duplications. Can someone help me?

char *str = "foobaa";
char *p = str;
char *prev = NULL;

int go(void)
{
   int ch;

   ch = top();
   move(1);
   return ch;
}

int top(void)
{
  retun *p;
}

void back(void)
{
   p = prev;
}

void move(int n)
{
   prev = p;
   p += n;
}

in D, I can wrote as following:

string str = "foobaa";
string prev;

dchar go()
{
  dchar ch = top();
  move(1);
  return ch;
}

dchar top()
{
  return str.front;
}

void move(int n)
{
  prev = str; // <- I want to avoid this string duplication did in every move() call
  foreach(i; 0 .. n) { // <- improve this is welcome too.
      str.popFront();
   }
}

void back()
{
  str = prev;
}


Here's (for better code reading readability) C and D codes on pastebin,respectively :

http://pastebin.com/J3vzP3UK

http://pastebin.com/C0NyNjY0

On Tue, Aug 26, 2014 at 08:58:13PM +0000, MacAsm via Digitalmars-d wrote:
> It maybe trivial to most of you but I don't know how do this avoiding string duplications. Can someone help me?
[...]
> in D, I can wrote as following:
[...]
> void move(int n)
> {
>   prev = str; // <- I want to avoid this string duplication did in every move() call

This does not duplicate the string. You may find this article helpful:

	http://dlang.org/d-array-article.html


T

-- 
Genius may have its limitations, but stupidity is not thus handicapped. -- Elbert Hubbard

On Tuesday, 26 August 2014 at 20:58:15 UTC, MacAsm wrote:
> string str = "foobaa";
> string prev;
[...]
>   prev = str; // <- I want to avoid this string duplication did in every move() call

If you're worried that this copies the string data, it doesn't. A dynamic array (which string is) is a pair of a pointer and a length, and only those are copied.

>   foreach(i; 0 .. n) { // <- improve this is welcome too.
>       str.popFront();
>    }

You can use std.range.popFrontN: str.popFrontN(n);

Also, there are two (maybe more) subtle differences between the D
code and the C code, of which you may not be aware:
1) The D version assumes UTF8. A char is a UTF8 code unit in D,
whereas in C it's just a byte. In D that would be (u)byte. But
char is fine when you're indeed working on UTF8.
2) The D version decodes to code points (dchar). This means e.g.,
that the D version potentially reaches the end of the string in
fewer `go` calls than the C version.

Finally, the "learn" board [1] would have been a better place for this post.  No turning back now, and it's not a big deal, but next time maybe post there.

[1] http://forum.dlang.org/group/digitalmars.D.learn

On Tuesday, 26 August 2014 at 21:29:04 UTC, H. S. Teoh via Digitalmars-d wrote:
> On Tue, Aug 26, 2014 at 08:58:13PM +0000, MacAsm via Digitalmars-d wrote:
>> It maybe trivial to most of you but I don't know how do this avoiding
>> string duplications. Can someone help me?
> [...]
>> in D, I can wrote as following:
> [...]
>> void move(int n)
>> {
>>   prev = str; // <- I want to avoid this string duplication did in every move() call
>
> This does not duplicate the string. You may find this article helpful:
>
> 	http://dlang.org/d-array-article.html
>
>
>

Thanks. This is exactly what I was worried about. If no string duplciation is done I think this D code is just fine...