A recursive procedure or function is one that calls itself. Why would you need it? Sometimes you need the result of the previous calculation in order to be able to calculate the current one. Take for example a very classical example: N!. Here all numbers from 1 to N are multiplied. This could be done using a for-loop and multiplying the current index with the previous multiplicant or this could be done recursively, where 1! is calculated then using this 2! is calculated.. N! is calculated using (N-1)! In orfer for this to be possible, we have to define recursive functions calling themselves.

Let's implement the factorial function:
 

function Factor(N:integer):integer;

begin

if N=0 then

Factor:=1

else

Factor:=Factor(N-1)*N;

end;

Important notes:

• Do not forget to give a terminating condition, one that does not call itself anymore, so that the function can retrun a value, otherwise it continues to execute until infinity.
• Try to avoid using recursive functions wherever you can find an iterative way to solve the problem, unless you really think you can master it, recursive functions are no child's play!