Array traversal from a given index to the first

Question

I've taken an Algorithm course in which the instructor has decided to use Ada as programming language tool to test our understanding. The argument was that ADA language is quite close to the Pseudo code that appears in CLRS book.

Below code is a snip for Selection sort in ADA:

   procedure Selection_Sort(Data : in out List) is
   begin
      for Destination_Index in Data'Range loop
         for Source_Index in Destination_Index + 1 .. Data'Last loop
            if Data(Source_Index) < Data(Destination_Index) then
               Swap(Data, Source_Index, Destination_Index);
            end if;
         end loop;
      end loop;
   end Selection_Sort;

The following:

for Source_Index in Destination_Index + 1 .. Data'Last loop

will cause traversing the array from given element (Data1) to last element, however if I wish to traverse from the last element to the first, the following don't seem to work.

for Source_Index in Data'Last .. Data'First loop

It would be great help if someone can help me with this trivial problem, so that I can get on with the main problem at hand, which is learning Algorithms.

Ondrej Tucny · Accepted Answer · 2016-02-15T22:28:34.077

5

A range A .. B having A > B results in an empty range. In respect to the for cycle, there's the reverse keyword to define a descending loop (end of ARM 5.5 (9)). Hence reverse A .. B, where A < B, will result in looping from B down to A.

The rest is left up to you for the sake of learning. In order to learn something, you cannot avoid learning at the first place.

edited Feb 15 '16 at 22:28

answered Feb 15 '16 at 15:13

Ondrej Tucny

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It’s more that a descending range (`for J in 10 .. 0`) is empty, so that the loop is traversed zero times. – Simon Wright Feb 15 '16 at 16:24
@SimonWright This is what I meant by “That would result in an empty range.” Feel free to edit and clarify my answer if it's not clear. – Ondrej Tucny Feb 15 '16 at 18:29
@SimonWright I think according to note 7 (§ 12) the reverse range requires `A < B`, because the discrete subtype definition is evaluated regardless of the `reverse` keyword. – Ondrej Tucny Feb 15 '16 at 19:46
@SimonWright Your edit indicated `A` should be *greater* than `B`. However, in its current state I believe the answer is correct. – Ondrej Tucny Feb 16 '16 at 16:48
Ondrej, sorry! I hadn’t realised you’d had to correct my error. – Simon Wright Feb 16 '16 at 16:54
@SimonWright No problem and thanks for your input :-) – Ondrej Tucny Feb 16 '16 at 16:56

Array traversal from a given index to the first

1 Answers1