A.18.16 Array Sorting
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The language-defined generic procedures Containers.Generic_Array_Sort
and Containers.Generic_Constrained_Array_Sort provide sorting on arbitrary
array types. Static Semantics
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The generic library procedure Containers.Generic_Array_Sort
has the following declaration: 3/2
generic
type Index_Type is (<>);
type Element_Type is private;
type Array_Type is array (Index_Type range <>) of Element_Type;
with function "<" (Left, Right : Element_Type)
return Boolean is <>;
procedure Ada.Containers.Generic_Array_Sort (Container : in out Array_Type);
pragma Pure(Ada.Containers.Generic_Array_Sort);
4/2
Reorders the elements
of Container such that the elements are sorted smallest first as determined
by the generic formal "<" operator provided. Any exception
raised during evaluation of "<" is propagated.
5/2
The actual function for
the generic formal function "<" of Generic_Array_Sort is
expected to return the same value each time it is called with a particular
pair of element values. It should define a strict ordering relationship,
that is, be irreflexive, asymmetric, and transitive; it should not modify
Container. If the actual for "<" behaves in some other manner,
the behavior of the instance of Generic_Array_Sort is unspecified. How
many times Generic_Array_Sort calls "<" is unspecified.{unspecified
[partial]}
5.a/2
Ramification: This
implies swapping the elements, usually including an intermediate copy.
This of course means that the elements will be copied. Since the elements
are nonlimited, this usually will not be a problem. Note that there is
Implementation Advice below that the implementation should use a sort
that minimizes copying of elements.
5.b/2
The sort is not required
to be stable (and the fast algorithm required will not be stable). If
a stable sort is needed, the user can include the original location of
the element as an extra "sort key". We considered requiring
the implementation to do that, but it is mostly extra overhead -- usually
there is something already in the element that provides the needed stability.
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The generic library procedure Containers.Generic_Constrained_Array_Sort
has the following declaration: 7/2
generic
type Index_Type is (<>);
type Element_Type is private;
type Array_Type is array (Index_Type) of Element_Type;
with function "<" (Left, Right : Element_Type)
return Boolean is <>;
procedure Ada.Containers.Generic_Constrained_Array_Sort
(Container : in out Array_Type);
pragma Pure(Ada.Containers.Generic_Constrained_Array_Sort);
8/2
Reorders the elements
of Container such that the elements are sorted smallest first as determined
by the generic formal "<" operator provided. Any exception
raised during evaluation of "<" is propagated.
9/2
The actual function for
the generic formal function "<" of Generic_Constrained_Array_Sort
is expected to return the same value each time it is called with a particular
pair of element values. It should define a strict ordering relationship,
that is, be irreflexive, asymmetric, and transitive; it should not modify
Container. If the actual for "<" behaves in some other manner,
the behavior of the instance of Generic_Constrained_Array_Sort is unspecified.
How many times Generic_Constrained_Array_Sort calls "<"
is unspecified.{unspecified [partial]}
Implementation Advice
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The worst-case time complexity of a call on an
instance of Containers.Generic_Array_Sort or Containers.Generic_Constrained_Array_Sort
should be O(N**2) or better, and the average time complexity
should be better than O(N**2), where N is the length
of the Container parameter. 10.a/2
Implementation Advice:
Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort
should have an average time complexity better than O(N**2)
and worst case no worse than O(N**2).
10.b/2
Discussion: In
other words, we're requiring the use of a sorting algorithm better than
O(N**2), such as Quicksort. No bubble sorts allowed!
11/2
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Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort
should minimize copying of elements. 11.a/2
Implementation Advice:
Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort
should minimize copying of elements.
11.b/2
To be honest: We
do not mean “absolutely minimize” here; we're not intending
to require a single copy for each element. Rather, we want to suggest
that the sorting algorithm chosen is one that does not copy items unnecessarily.
Bubble sort would not meet this advice, for instance.
Extensions to Ada 95
11.c/2
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{extensions to Ada 95} The
generic packages Containers.Generic_Array_Sort and Containers.Generic_Constrained_Array_Sort
are new.
Sponsored by Ada-Europe