Object-oriented Software in Ada 95

66 Procedures and functions
5.5.3 Summary of access to formal parameters
Formal parameter specified by:
(using as an example an
Integer formal parameter)
Write to
formal
parameter
allowed
Read from
formal
parameter
Can be used as a
parameter to
item: Integer r √ procedure or function
item: in Integer r √ procedure or function
item: in out Integer √ √ procedure only
item: out Integer √ √ procedure only
5.6 Recursion
Recursion is the ability of a procedure or function to make a call on itself from within its own code body. Whilst
this initially may seem a strange idea, it can lead to very elegant code sequences that otherwise would require
many more lines of code. In certain exceptional cases recursion is the only way to implement a problem.
An example of a recursive procedure to write a natural number using only character based output is sketched
in outline below:
Write a natural number: (write_natural)
• Split the natural number into two components
(a) The first digit (remainder when number divided by 10)
(b) The other digits (number divided by 10).
For example:
123 would be split into:
3 (first digit)
12 (other digits).
• If the other digits are greater than or equal to 10 then write the other digits by recursively
calling the code to write a decimal number.
• Output the first digit as a character.
The sequence of calls made is
Call
Implemented as
write_natural( 123 ) write_natural(12); output first digit 3
write_natural( 12 ) write_natural(1); output first digit 2
write_natural( 1 ) output first digit 1
This process is diagrammatically expressed in Figure 5.4.
Initial call
Split
recursive call on 12
Split
recursive call on 1
123
Split
but no recursive call required
12 3
1
2
1
Unwind
Figure 5.4 Illustration of recursive calls to print the natural number 123.
© M A Smith - May not be reproduced without permission