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digitalmars.D.learn - foreach annoyance

reply "Regan Heath" <regan netwin.co.nz> writes:
Simple question.

Does anyone else find themselves with this problem?

//contrived example, actual values/data not important
char[] string = "abcdefgh";
foreach(char c; string)
{
   if (c == 'e') break;
}
//at this point I'd like to do something if the item wasn't found.. how  
can I tell?


I can tell when using a for loop:

int i;
for(i = 0; i < string.length; i++)
{
   if (string[i] == 'e') break;
}
if (i == string.length) {} //not found

But I thought the idea was to use foreach so that I can change the  
container type at a later date with no other changes required.


Of course, I could add 'i' to the foreach in the same way, eg.

int i = 0;
foreach(char c; string)
{
   if (c == 'e') break;
   i++;
}
if (i == string.length) {} //not found


But it would be nice if there was a more elegant solution. Something like:

foreach(char c; string)
{
   if(c == 'e') break;
}
overflow
{
   //not found
}

However "overflow" doesn't seem like quite the right term.

Regan
Aug 04 2005
next sibling parent Derek Parnell <derek psych.ward> writes:
On Fri, 05 Aug 2005 00:27:48 +1200, Regan Heath wrote:


 Simple question.
 
 Does anyone else find themselves with this problem?
 
 //contrived example, actual values/data not important
 char[] string = "abcdefgh";
 foreach(char c; string)
 {
    if (c == 'e') break;
 }
 //at this point I'd like to do something if the item wasn't found.. how  
 can I tell?


I know this is not a real fast way to do it, but I have been know to do ...

 char[] string = "abcdefgh";
 foreach(int i, char c; string)
 {
    if (c == 'e') break;
    if (i == string.length-1) ...
 }

But in most cases this is too much of an overhead.

-- 
Derek Parnell
Melbourne, Australia
4/08/2005 10:59:11 PM
Aug 04 2005
prev sibling next sibling parent reply jicman <jicman_member pathlink.com> writes:
Regan Heath says...

Simple question.

Does anyone else find themselves with this problem?

//contrived example, actual values/data not important
char[] string = "abcdefgh";
foreach(char c; string)
{
   if (c == 'e') break;
}
//at this point I'd like to do something if the item wasn't found.. how  
can I tell?


I can tell when using a for loop:

int i;
for(i = 0; i < string.length; i++)
{
   if (string[i] == 'e') break;
}
if (i == string.length) {} //not found

But I thought the idea was to use foreach so that I can change the  
container type at a later date with no other changes required.


Of course, I could add 'i' to the foreach in the same way, eg.

int i = 0;
foreach(char c; string)
{
   if (c == 'e') break;
   i++;
}
if (i == string.length) {} //not found


But it would be nice if there was a more elegant solution. Something like:

foreach(char c; string)
{
   if(c == 'e') break;
}
overflow
{
   //not found
}

However "overflow" doesn't seem like quite the right term.


I know, this is NOT what you want, but this is the best way to tell, anyway.
Using the good ole found variable. :-)

|bit found = false;
|foreach (char c; string)
|{
|  if(c == 'e')
|  {
|    found = true;
|    break;
|  }
|}
|
|if (found)
|{
|// found it
|}

However, there is another way you could do this:

|int ii;
|foreach (int i; char c; string)
|{
|  ii = i;
|  if(c == 'e')
|    break;
|}

After the loop is processed, ii will have the value that i had at the time of
encounter or loop completion.

just a thought...

josé
Aug 04 2005
parent "Regan Heath" <regan netwin.co.nz> writes:
On Thu, 4 Aug 2005 13:26:31 +0000 (UTC), jicman  
<jicman_member pathlink.com> wrote:

 However, there is another way you could do this:

 |int ii;
 |foreach (int i; char c; string)
 |{
 |  ii = i;
 |  if(c == 'e')
 |    break;
 |}

 After the loop is processed, ii will have the value that i had at the  
 time of
 encounter or loop completion.

 just a thought...


In this code ii is string.length-1 if it finds nothing, or matches the  
last element.

Regan
Aug 04 2005
prev sibling next sibling parent reply "Ben Hinkle" <bhinkle mathworks.com> writes:
 But it would be nice if there was a more elegant solution. Something like:

 foreach(char c; string)
 {
   if(c == 'e') break;
 }
 overflow
 {
   //not found
 }


Another option is to make foreach return the value of the opApply (or the 
builtin loop if there is no opApply) and turn foreach into an expression 
instead of a statement. So for example
  int found = foreach(char c; string){
    if (c == 'e') break;
  };
  if (found) {
    // found 'e' in string
  } else {
    //. not found case
  }

Note making foreach an expression would break existing code since the 
trailing ; would be needed to turn the expression into a statement. It also 
might be confusing that people might think they could have "return" 
statements in the foreach body instead of "break". People would also 
probably want to return values from the foreach like the index at the break.

Given those problems if making foreach an expression is too wacky the result 
can be stored in an implicit variable _foreachAborted or something:
  foreach(char c; string){
    if (c == 'e') break;
  }
  if (_foreachAborted) {
    // found 'e' in string
  } else {
    //. not found case
  }
Aug 04 2005
next sibling parent reply AJG <AJG_member pathlink.com> writes:
Hi Ben,


Another option is to make foreach return the value of the opApply (or the 
builtin loop if there is no opApply) and turn foreach into an expression 
instead of a statement. So for example
  int found = foreach(char c; string){
    if (c == 'e') break;
  };
  if (found) {
    // found 'e' in string
  } else {
    //. not found case
  }


Fantastic! This would be simply awesome. It would pave the way for the
expressionisation of all constructs just like current GNU C extensions allow. A
vote for expression-foreach is a vote for the future. 


Note making foreach an expression would break existing code since the 
trailing ; would be needed to turn the expression into a statement. It also 
might be confusing that people might think they could have "return" 
statements in the foreach body instead of "break". People would also 
probably want to return values from the foreach like the index at the break.


Yeah, the trailing ; could break things, but it would be well worth it.
Alternatively, we could have both versions (expression foreach w/ the ; and
regular foreach w/o ;. Then, regular foreach could be deprecated over time).


Given those problems if making foreach an expression is too wacky the result 
can be stored in an implicit variable _foreachAborted or something:
  foreach(char c; string){
    if (c == 'e') break;
  }
  if (_foreachAborted) {
    // found 'e' in string
  } else {
    //. not found case
  } 


On the other hand, this part I don't like that much.
--AJG.
Aug 04 2005
parent reply "Ben Hinkle" <ben.hinkle gmail.com> writes:
 Fantastic! This would be simply awesome. It would pave the way for the
 expressionisation of all constructs just like current GNU C extensions 
 allow. A
 vote for expression-foreach is a vote for the future.


Holy smokes. I just read through the GNU C extension list. Those guys make 
Willy Wonka look like an amateur :P
I wonder what feedback they've gotten about some of those ideas.
Aug 04 2005
parent AJG <AJG_member pathlink.com> writes:
Hi,


 Fantastic! This would be simply awesome. It would pave the way for the
 expressionisation of all constructs just like current GNU C extensions 
 allow. A
 vote for expression-foreach is a vote for the future.


Holy smokes. I just read through the GNU C extension list. Those guys make 
Willy Wonka look like an amateur :P


Yes indeed. When I started using GNU C (and the exts) I became spoiled for life.
Going back to even C99 is a painful proposition.


I wonder what feedback they've gotten about some of those ideas. 


I suspect it's been mostly positive for two reasons:

a) They are all toggleable (specifically or all at once). So you can use them or
completely ignore them. You can also force ANSI/C89/99 compatibility if you
want.

b) They keep churning 'em out. ;)

I really, really hope Walter takes a look at them and invests time in
implementing the best of them in D. A particularly easy one I suggested a while
back was omitting the middle operand in the ternary operator.

Expression-foreach would be a great step in that direction, so I definitely
support it.

Cheers,
--AJG.
Aug 05 2005
prev sibling parent reply "Ilya Zaitseff" <sark7 mail333.com> writes:
How about foreach...else statement? I think it is an elegant solution.

On Fri, 05 Aug 2005 00:57:32 +1100, Ben Hinkle <bhinkle mathworks.com>  
wrote:

 But it would be nice if there was a more elegant solution. Something  
 like:

 foreach(char c; string)
 {
   if(c == 'e') break;
 }
 overflow
 {
   //not found
 }


 Another option is to make foreach return the value of the opApply (or the
 builtin loop if there is no opApply) and turn foreach into an expression
 instead of a statement. So for example
   int found = foreach(char c; string){
     if (c == 'e') break;
   };
   if (found) {
     // found 'e' in string
   } else {
     //. not found case
   }

 Note making foreach an expression would break existing code since the
 trailing ; would be needed to turn the expression into a statement. It  
 also
 might be confusing that people might think they could have "return"
 statements in the foreach body instead of "break". People would also
 probably want to return values from the foreach like the index at the  
 break.

 Given those problems if making foreach an expression is too wacky the  
 result
 can be stored in an implicit variable _foreachAborted or something:
   foreach(char c; string){
     if (c == 'e') break;
   }
   if (_foreachAborted) {
     // found 'e' in string
   } else {
     //. not found case
   }
Aug 08 2005
parent reply "Regan Heath" <regan netwin.co.nz> writes:
On Tue, 09 Aug 2005 13:30:23 +1100, Ilya Zaitseff <sark7 mail333.com>  
wrote:

 How about foreach...else statement? I think it is an elegant solution.


To me 'else' implies "if there is nothing in the container" whereas I want  
"if you reach the end of the container", having else do what I want would  
be confusing.. or maybe just to me :)

Regan


 On Fri, 05 Aug 2005 00:57:32 +1100, Ben Hinkle <bhinkle mathworks.com>  
 wrote:

 But it would be nice if there was a more elegant solution. Something  
 like:

 foreach(char c; string)
 {
   if(c == 'e') break;
 }
 overflow
 {
   //not found
 }


 Another option is to make foreach return the value of the opApply (or  
 the
 builtin loop if there is no opApply) and turn foreach into an expression
 instead of a statement. So for example
   int found = foreach(char c; string){
     if (c == 'e') break;
   };
   if (found) {
     // found 'e' in string
   } else {
     //. not found case
   }

 Note making foreach an expression would break existing code since the
 trailing ; would be needed to turn the expression into a statement. It  
 also
 might be confusing that people might think they could have "return"
 statements in the foreach body instead of "break". People would also
 probably want to return values from the foreach like the index at the  
 break.

 Given those problems if making foreach an expression is too wacky the  
 result
 can be stored in an implicit variable _foreachAborted or something:
   foreach(char c; string){
     if (c == 'e') break;
   }
   if (_foreachAborted) {
     // found 'e' in string
   } else {
     //. not found case
   }
Aug 08 2005
parent reply AJG <AJG_member pathlink.com> writes:
In article <opsu7ymedr23k2f5 nrage.netwin.co.nz>, Regan Heath says...

On Tue, 09 Aug 2005 13:30:23 +1100, Ilya Zaitseff <sark7 mail333.com>  
wrote:

 How about foreach...else statement? I think it is an elegant solution.


To me 'else' implies "if there is nothing in the container" whereas I want  
"if you reach the end of the container", having else do what I want would  
be confusing.. or maybe just to me :)


No, I agree. My suggestion suggestion a while ago interpreted it just like that.

foreach (item; array) 
// Array not empty.
else
// Array is empty.

Cheers,
--AJG.



Regan


 On Fri, 05 Aug 2005 00:57:32 +1100, Ben Hinkle <bhinkle mathworks.com>  
 wrote:

 But it would be nice if there was a more elegant solution. Something  
 like:

 foreach(char c; string)
 {
   if(c == 'e') break;
 }
 overflow
 {
   //not found
 }


 Another option is to make foreach return the value of the opApply (or  
 the
 builtin loop if there is no opApply) and turn foreach into an expression
 instead of a statement. So for example
   int found = foreach(char c; string){
     if (c == 'e') break;
   };
   if (found) {
     // found 'e' in string
   } else {
     //. not found case
   }

 Note making foreach an expression would break existing code since the
 trailing ; would be needed to turn the expression into a statement. It  
 also
 might be confusing that people might think they could have "return"
 statements in the foreach body instead of "break". People would also
 probably want to return values from the foreach like the index at the  
 break.

 Given those problems if making foreach an expression is too wacky the  
 result
 can be stored in an implicit variable _foreachAborted or something:
   foreach(char c; string){
     if (c == 'e') break;
   }
   if (_foreachAborted) {
     // found 'e' in string
   } else {
     //. not found case
   }
Aug 08 2005
parent reply Derek Parnell <derek psych.ward> writes:
On Tue, 9 Aug 2005 04:15:38 +0000 (UTC), AJG wrote:


 foreach (item; array) 
 // Array not empty.
 else
 // Array is empty.


This would be useful, as it saves me declaring a loop index just for
checking the first or last iteration, which is something I do frequently,
...

 foreach( <itemdecl>; <arrayref> )
   on first <statement> // First iteration only
   on last <statement>  // Final iteration only
 else <statement>    // Only when there are zero iterations

Example:

  foreach( CustRec c; CustomerList )
    on first {
       Lineno = 0;
       Totals = 0;
       Pageno = 0;
       InitializeReport();
    }

    Lineno++;
    GenPageHeading(Lineno, Pageno);
    GenDetailLine(Lineno, c, Totals);

    on last {
        GenFooter(Totals);
        CloseReport();
    }
  else
     Msg("No customers on file");
         
Not a big thing, just a nice-to-have.

-- 
Derek
Melbourne, Australia
9/08/2005 2:28:36 PM
Aug 08 2005
parent AJG <AJG_member pathlink.com> writes:
Hi,


 foreach (item; array) 
 // Array not empty.
 else
 // Array is empty.


This would be useful, as it saves me declaring a loop index just for
checking the first or last iteration, which is something I do frequently,
Not a big thing, just a nice-to-have.


Then, by all means, vote for it*:

http://www.all-technology.com/eigenpolls/dwishlist/

* Although there hasn't been shown a positive correlation between the D wishlist
and the D language. It functions more like a salve. ;)

Cheers,
--AJG.
Aug 08 2005
prev sibling next sibling parent reply Burton Radons <burton-radons smocky.com> writes:
Regan Heath wrote:

 Simple question.
 
 Does anyone else find themselves with this problem?
 
 //contrived example, actual values/data not important
 char[] string = "abcdefgh";
 foreach(char c; string)
 {
   if (c == 'e') break;
 }
 //at this point I'd like to do something if the item wasn't found.. how  
 can I tell?


I've found that this problem is symptomatic of over-complex control flow 
and should be delegated into a function (inner or outer) which tells you 
whether it found any entries and possibly acts on found entries if it 
can handle more than one.  A method which goes along this path will 
become harder and harder to manipulate easily.  Inner functions aren't 
just for doing a task more than once; they also act as 
self-documentation and control flow, reducing the core function to a 
series of modular parts to manipulate at will.

This reminds me that I need to barge in on Parcel code and do some 
refactoring, I even use goto a couple times.


 I can tell when using a for loop:
 
 int i;
 for(i = 0; i < string.length; i++)
 {
   if (string[i] == 'e') break;
 }
 if (i == string.length) {} //not found
 
 But I thought the idea was to use foreach so that I can change the  
 container type at a later date with no other changes required.


It's no substitute for interfaces or templating.
Aug 04 2005
parent "Regan Heath" <regan netwin.co.nz> writes:
On Thu, 04 Aug 2005 08:23:29 -0700, Burton Radons  
<burton-radons smocky.com> wrote:

 Regan Heath wrote:

 Simple question.
  Does anyone else find themselves with this problem?
  //contrived example, actual values/data not important
 char[] string = "abcdefgh";
 foreach(char c; string)
 {
   if (c == 'e') break;
 }
 //at this point I'd like to do something if the item wasn't found..  
 how  can I tell?


 I've found that this problem is symptomatic of over-complex control flow  
 and should be delegated into a function (inner or outer) which tells you  
 whether it found any entries and possibly acts on found entries if it  
 can handle more than one.  A method which goes along this path will  
 become harder and harder to manipulate easily.  Inner functions aren't  
 just for doing a task more than once; they also act as  
 self-documentation and control flow, reducing the core function to a  
 series of modular parts to manipulate at will.

 This reminds me that I need to barge in on Parcel code and do some  
 refactoring, I even use goto a couple times.


Ok, can you give me a quick example?

We're you thinking...

void func()
{
   char[] string = "abcdefg";

   bool findItem(char item)
   {
     foreach(char c; string)
     {
       if (c == item) return true;
     }
     return false;
   }

   if (!findItem('e')) {
   }

   ..etc..
}


 I can tell when using a for loop:
  int i;
 for(i = 0; i < string.length; i++)
 {
   if (string[i] == 'e') break;
 }
 if (i == string.length) {} //not found
  But I thought the idea was to use foreach so that I can change the   
 container type at a later date with no other changes required.


 It's no substitute for interfaces or templating.


I thought the idea was that foreach was a template for iterating over a  
container.

Regan
Aug 04 2005
prev sibling parent reply John Demme <me teqdruid.com> writes:
import std.stdio;
void main() {
  char[] string = "abcdfgh";
  foreach(char c; string)
    {
      if (c == 'e') goto found;
    }
  writefln("not found");
 found:
  writefln("done");
}


Hehe..  Might just be an acceptable use of "goto".  Doesn't require the
use of an extra variable, or an extra operator within the loop.


On Fri, 2005-08-05 at 00:27 +1200, Regan Heath wrote:

 char[] string = "abcdefgh";
 foreach(char c; string)
 {
    if (c == 'e') break;
 }
Aug 04 2005
parent reply AJG <AJG_member pathlink.com> writes:
Hi,


import std.stdio;
void main() {
  char[] string = "abcdfgh";
  foreach(char c; string)
    {
      if (c == 'e') goto found;
    }
  writefln("not found");
 found:
  writefln("done");
}


I'm guessing you wanna throw a return in there. Otherwise the "not found" case
also reaches the "found" case. No?

--AJG.
Aug 04 2005
parent John Demme <me teqdruid.com> writes:
I dunno what the poster's intent was.

On Fri, 2005-08-05 at 00:05 +0000, AJG wrote:

 Hi,
 

import std.stdio;
void main() {
  char[] string = "abcdfgh";
  foreach(char c; string)
    {
      if (c == 'e') goto found;
    }
  writefln("not found");
 found:
  writefln("done");
}


 
 I'm guessing you wanna throw a return in there. Otherwise the "not found" case
 also reaches the "found" case. No?
 
 --AJG.
Aug 04 2005