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digitalmars.D - Built-in arrays as output ranges

reply Steve Teale <steve.teale britseyeview.com> writes:
Not a bug, but would it be reasonable to expand a little on the documentation
for an output range, perhaps along the lines:

r.put(e) puts e in the range (in a range-dependent manner) and advances to the
popFront position in the range. Successive calls to r.put add elements to the
range. put may throw to signal failure.

When using a built-in array as an output range, to do anything useful it is
necessary to take a reference to the original array before using put. The
original array will be nibbled away by put operations (see std.array). For
example:

    int[] a = [ 1,2,3 ];
    int[] b = a;
    a.put(-a[0]);
    a.put(-a[0]);
    writefln("([%s] [%s]", b, a);
   // [ -1, -2, 3] [ 3 ]
Feb 07 2010
parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Steve Teale wrote:

 Not a bug, but would it be reasonable to expand a little on the documentation
for an output range, perhaps along the lines:
 
 r.put(e) puts e in the range (in a range-dependent manner) and advances to the
popFront position in the range. Successive calls to r.put add elements to the
range. put may throw to signal failure.
 
 When using a built-in array as an output range, to do anything useful it is
necessary to take a reference to the original array before using put. The
original array will be nibbled away by put operations (see std.array). For
example:
 
     int[] a = [ 1,2,3 ];
     int[] b = a;
     a.put(-a[0]);
     a.put(-a[0]);
     writefln("([%s] [%s]", b, a);
    // [ -1, -2, 3] [ 3 ]


Right. Speaking of which, I think it's sensible to always leave the test
for empty range in, even in release unsafe builds.

void put(T, E)(ref T[] range, E element) if (!isSomeString!(T[]))
{
     enforce(!range.empty, "Attempting to put in an empty array");
     *range.ptr = element;
     range.popFront();
}

enforce() will never be disabled.

As an aside, I just realized I haven't implemented put for strings yet,
and also that I'd promised a check in this weekend.


Andrei
Feb 07 2010
parent reply Steve Teale <steve.teale britseyeview.com> writes:
 
 enforce() will never be disabled.
 
 As an aside, I just realized I haven't implemented put for strings yet,
 and also that I'd promised a check in this weekend.
 
 
 Andrei


Actually, thinking about this overnight, I'm a bit unhappy about giving 
the impression that a built-in array can serve as an output range. It 
really isn't true unless you never want to see the output again. If you 
do, some data structure is required, either a loose combination of an 
array and an unprotected reference to its original state (arrays a and 
b), or something more explicit like:

struct arrayOutputRange(T)
{
    T[] array;
    uint pos;

    this(uint sx) { ... }
    void put(T val) { ... }
}

Steve
Feb 07 2010
parent Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Steve Teale wrote:

 enforce() will never be disabled.

 As an aside, I just realized I haven't implemented put for strings yet,
 and also that I'd promised a check in this weekend.


 Andrei


 
 Actually, thinking about this overnight, I'm a bit unhappy about giving 
 the impression that a built-in array can serve as an output range. It 
 really isn't true unless you never want to see the output again. If you 
 do, some data structure is required, either a loose combination of an 
 array and an unprotected reference to its original state (arrays a and 
 b), or something more explicit like:
 
 struct arrayOutputRange(T)
 {
     T[] array;
     uint pos;
 
     this(uint sx) { ... }
     void put(T val) { ... }
 }
 
 Steve
 


copy(source, target) does make sense for arrays as output ranges. Since 
target is passed by value, your copy will see what's been copied.

Andrei
Feb 08 2010