Object-oriented Software in Ada 95

186 Child libraries
13.4 The cat program revisited
The program to concatenate the contents of files previously seen in Section 3.11 can now be re-written to give a
sensible error message to the user when an attempt is made to list a file that does not exist. In this program the
following exception occur:
Exception
Name_Error
Status_Error
Explanation
File does not exist.
File is already open.
This new program is:
with Ada.Text_Io, Ada.Command_Line;
use Ada.Text_Io, Ada.Command_Line;
procedure Cat is
Fd : Ada.Text_Io.File_Type; --File descriptor
Ch : Character;
--Current character
begin
if Argument_Count >= 1 then
for I in 1 .. Argument_Count loop --Repeat for each file
begin
Open( File=>Fd, Mode=>In_File, --Open file
Name=>Argument(I) );
while not End_Of_File(Fd) loop --For each Line
while not End_Of_Line(Fd) loop--For each character
Get(Fd,Ch); Put(Ch); --Read / Write character
end loop;
Skip_Line(Fd); New_Line; --Next line / new line
end loop;
Close(Fd);
--Close file
exception
when Name_Error =>
Put("cat: " & Argument(I) & " no such file" );
New_Line;
when Status_Error =>
Put("cat: " & Argument(I) & " all ready open" );
New_Line;
end;
end loop;
else
Put("Usage: cat file1 ... "); New_Line;
end if;
end Cat;
13.5 A stack
A stack is a structure used to store and retrieve data items. Data items are pushed onto the structure and retrieved
in reverse order. This is commonly referred to as ‘first in last out’. This process is illustrated in Figure 13.1
Stack
2
1 1 1
After sending
message
Push(1) Push(2) Pop() Pop()
2 1
Figure 13.1 Example of operations on a stack.
© M A Smith - May not be reproduced without permission