array slices

array slices

[gshibona @ b e l l s o u t h . n e t]

I wonder if someone could point to the error in this code.  I'm looking
to fill array a with a "row" slice
of array c.   There doesn't appear to be much info on this particular
facet of Ada, yet what I have seen
suggests that the syntax below is correct.

procedure ada_2daryslice is

a : array(1..4) of integer;
c : array( 1..2, 1..4 ) of integer;

begin
    c :=  (
         1 => (0, 1, 2, 4 ),
         2 => (8, 16, 32, 64)
         );
    -- this seems to yield the expected results
    a(1..4) := ( 1,2,3,4);

    -- however, this does not...
    a(1..4) := c(1)(1..4);

end ada_2daryslice;

The compiler complains of too few subscripts in the array reference.
To wit...

gnatmake -g -gnatv ada2daryslice.adb
gnatgcc -c -g -gnatv ada2daryslice.adb

GNAT 3.13p  (20000509) Copyright 1992-2000 Free Software Foundation,
Inc.

Compiling: ada2daryslice.adb (source file time stamp: 2003-05-13
01:20:09)

    17.     a(1..4) := c(1)(1..4);
                                   |
        >>> too few subscripts in array reference

 17 lines: 1 error
gnatmake: "ada2daryslice.adb" compilation error

It's quite possible that I've been looking at this for much too long to
be objective.   Thanks in advance
for any help.

Ger

Re: array slices

[Jeffrey Carter]

gshibona @ b e l l s o u t h . n e t wrote:
> I wonder if someone could point to the error in this code.  I'm looking
> to fill array a with a "row" slice
> of array c.   There doesn't appear to be much info on this particular
> facet of Ada, yet what I have seen
> suggests that the syntax below is correct.
> 
> procedure ada_2daryslice is
> 
> a : array(1..4) of integer;
> c : array( 1..2, 1..4 ) of integer;
> 
> begin
>     c :=  (
>          1 => (0, 1, 2, 4 ),
>          2 => (8, 16, 32, 64)
>          );
>     -- this seems to yield the expected results
>     a(1..4) := ( 1,2,3,4);

This is perhaps better written as

A := (1, 2, 3, 4);

> 
>     -- however, this does not...
>     a(1..4) := c(1)(1..4);
> 
> end ada_2daryslice;

You can only slice a one-dimensional array. C is a two-dimensional 
array, and must have 2 indices. C (1) is meaningless here.

You have 2 choices. Well, you probably have more, but 2 main ones:

1. Use a loop

for I in A'range loop
    A (I) := C (C'First, I);
end loop;

2. Make C a one-dimensional array of one-dimensional arrays (this is not 
the same thing as a two-dimensional array):

type One_D is array (1 .. 4) of Integer;

A : One_D;
C : array (1 .. 2) of One_D;
...
A := C (1);

-- 
Jeff Carter
"Monsieur Arthur King, who has the brain of a duck, you know."
Monty Python & the Holy Grail

Re: array slices

[tmoran]

> a : array(1..4) of integer;
> c : array( 1..2, 1..4 ) of integer;
>   a(1..4) := c(1)(1..4);
There are two problems here.  1) you can only take slices of one
dimensional arrays (yes, that is annoying).  2) A and C are of different
(anonymous) types so the compiler will point that out if you try to assign
a chunk of one to the other.
  If you do
type Rows is array(1 .. 4) of integer;
a : Rows;
c : array(1 .. 2) of Rows;
then any of
 a(1 .. 4) := c(1)(1 .. 4);
 a := c(1);
 a(3 .. 4) := c(2)(1 .. 2);
are ok.

or
type Rows is array(integer range <>) of integer;
a : Rows(1 .. 4);
c : array(1 .. 2) of Rows(1 .. 4);
d : array(5 .. 6) of Rows(11 .. 25);
a(1 .. 2) := d(6)(11 .. 12);
will also work.

Re: array slices

[James S. Rogers]

"gshibona @ b e l l s o u t h . n e t" <gshibona@bellsouth.net> wrote in
message news:3EC0494F.A7D67AA4@bellsouth.net...
> I wonder if someone could point to the error in this code.  I'm looking
> to fill array a with a "row" slice
> of array c.   There doesn't appear to be much info on this particular
> facet of Ada, yet what I have seen
> suggests that the syntax below is correct.
>
> procedure ada_2daryslice is
>
> a : array(1..4) of integer;
> c : array( 1..2, 1..4 ) of integer;
>
> begin
>     c :=  (
>          1 => (0, 1, 2, 4 ),
>          2 => (8, 16, 32, 64)
>          );
>     -- this seems to yield the expected results
>     a(1..4) := ( 1,2,3,4);
>
>     -- however, this does not...
>     a(1..4) := c(1)(1..4);
>
> end ada_2daryslice;
>
> The compiler complains of too few subscripts in the array reference.

Others have noted that you can only slice a one dimensional array.
I will focus on another issue in your code.

This error message is correct. You defined array c as a two dimensional
array. You are accessing it as an array of arrays. Those are two different
concepts in Ada.

To access the elemets of array c you want to use the following syntax:

   c(1,1)

Note that this is different from

  c(1)(1)

Your problem can be better resolved using an array of arrays.

type Row is array (1..4) of integer;
type Matrix is array (1..2) of Row;

a : Row := (1,2,3,4);
c : Matrix := (1 =>(0,1,2,4),
                     2 => (8,16,32,64));

a := c(1);

This works because each element of C is a Row as defined above.
A is also a Row. You can also do the following:

a(1..2) := c(2)(3..4);

Jim Rogers