您如何检查字符串是否以 Ada 中的另一个字符串结尾？

Question

There are canonical answers to this question for every popular language, even though that answer usually boils down to: "Use string.endsWith() from the standard library". For Ada, as far as I can find in the docs for the Fixed String package , there is no string.endswith function.

So, given two fixed strings A and B, how do you check if A ends with B?

declare
   A : constant String := "John Johnson";
   B : constant String := "son";
begin
   if A.Ends_With(B) then -- this doesn't compile
      Put_Line ("Yay!");
   end if;
end

My intent is to establish a standard answer for Ada.

Answer 1

A slight simplification of Simon's answer:

function Ends_With (Source, Pattern : String) return Boolean is
begin
   return Pattern'Length <= Source'Length 
     and then Source (Source'Last - Pattern'Length + 1 .. Source'Last) = Pattern;
end Ends_With;

Answer 2

Well, here's a possible solution:

main.adb

with Ada.Text_IO;       use Ada.Text_IO;
with Ada.Strings.Fixed; use Ada.Strings.Fixed;

procedure Main is

   A : constant String := "John Johnson";
   B : constant String := "son";

begin

   if Tail (A, B'Length) = B then
      Put_Line ("Yay!");
   end if;

end Main;

output

$ ./main
Yay!

---

UPDATE (2)

Another update (thanks @Brian Drummond for the comment; comment disappeared though), again using Tail . This is now almost identical to @Zerte's answer, except for the dependency on Ada.Strings.Fixed :

main.adb

with Ada.Text_IO;       use Ada.Text_IO;
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
with Ada.Assertions;    use Ada.Assertions;

procedure Main is

   function Ends_With (Source, Pattern : String) return Boolean is
   begin
      return Source'Length >= Pattern'Length and then
        Tail (Source, Pattern'Length) = Pattern;
   end Ends_With;   

begin

   Assert (Ends_With ("John Johnson", "son")  = True);
   Assert (Ends_With ("hi", "longer than hi") = False);

   Assert (Ends_With (""  , ""  ) = True);
   Assert (Ends_With (" " , ""  ) = True);
   Assert (Ends_With (""  , " " ) = False);
   Assert (Ends_With (" " , " " ) = True);

   Assert (Ends_With ("n ", "n ") = True);
   Assert (Ends_With (" n", "n" ) = True);
   Assert (Ends_With ("n" , " n") = False);
   Assert (Ends_With (" n", " n") = True);

   Put_Line ("All OK.");

end Main;

output

$ ./main
All OK.

Answer 3

Here is an example without any explicit loops.

with Ada.Assertions; use Ada.Assertions;
with Ada.Text_IO; use Ada.Text_IO;

procedure Main is
   function Ends_With(Source : String; Pattern : String) return Boolean is
      result : Boolean := False;
   begin
      if Pattern'Length <= Source'Length then
         if Pattern'Length > 0 then
            result := Source((Source'Last - Pattern'Length + 1)..Source'Last) = Pattern;
         else 
            result := True;
         end if;
      end if;
      return result;
   end Ends_With;

begin

   Assert (Ends_With ("John Johnson", "son") = True);


   Assert (Ends_With (""  , ""  ) = True);
   Assert (Ends_With (" " , ""  ) = True);
   Assert (Ends_With (""  , " " ) = False);
   Assert (Ends_With (" " , " " ) = True);   

   Assert (Ends_With (""  , "n" ) = False);
   Assert (Ends_With ("n"  , "" ) = True);

   Assert (Ends_With ("n ", "n ") = True);
   Assert (Ends_With (" n", "n" ) = True);
   Assert (Ends_With ("n" , " n") = False);
   Assert (Ends_With (" n", " n") = True);

   Put_Line("All OK");
end Main;

Answer 4

As a slight simplification of Jim's answer , this works too:

   function Ends_With (Source, Pattern : String) return Boolean is
   begin
      if Pattern'Length > Source'Length then
         return False;
      else
         return Source (Source'Last - Pattern'Length + 1 .. Source'Last)
           = Pattern;
      end if;
   end Ends_With;

but, even better (thanks, Zerte),

function Ends_With (Source, Pattern : String) return Boolean is
  (Pattern'Length <= Source'Length and then
     Source (Source'Last - Pattern'Length + 1 .. Source'Last) = Pattern);