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From: b.mcguinness747@gmail.com (Brian9000)
Newsgroups: comp.lang.ada
Subject: How can I make Reduce run from right to =?UTF-8?B?bGVmdD8=?=
Date: Tue, 10 Dec 2024 17:20:53 +0000
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I am trying to find a way to make Reduce operate from right to left like
APL reduction, but this doesn't seem to work.

I wrote a test program:

pragma Ada_2022;
pragma Extensions_Allowed (On);

with Ada.Text_IO;

procedure Reverse_Reduction is
  type Real   is digits 15;
  type Vector is array(Natural range <>) of Real;

  data : constant Vector := [for i in 0 .. 11 => Real (i + 1)];
begin
  for i in data'Range loop
    Ada.Text_IO.Put_Line (i'Image & ": " & data(i)'Image);
  end loop;

  Ada.Text_IO.New_Line;
  Ada.Text_IO.Put_Line (Real'Image ([ for i in 1 .. data'Last => data(i)
]'Reduce("-", data(0))));
  Ada.Text_IO.Put_Line (Real'Image ([ for i in reverse 0 .. data'Last -
1 => data(i) ]'Reduce("-", data(data'Last))));
end Reverse_Reduction;

but the compiler won't accept "reverse" in the next-to-last line, even
though you can use it in a normal loop.  The other lines compile and run
without any problems.

$ gnatmake reverse_reduction.adb
x86_64-linux-gnu-gcc-12 -c reverse_reduction.adb
reverse_reduction.adb:18:47: error: missing operand
x86_64-linux-gnu-gnatmake-12: "reverse_reduction.adb" compilation error

Of course I can just write a normal loop to do this, but I wanted to see
if I could do it with Reduce.

--- Brian