Recursion in Perl

Recursion is a mechanism when a function calls itself again and again till the required condition is met. When the function call statement is written inside the same function then such a function is referred to as a recursive function.

The argument passed to a function is retrieved from the default array @_ whereas each value can be accessed by $_[0], $_[1] and so on.

Example 1: The example below finds factorial of a number.



Factorial of any number n is (n)*(n-1)*(n-2)*....*1.
e.g.:
4! = 4*3*2*1 = 24
3! = 3*2*1 = 6
2! = 2*1 = 2
1! = 1
0! = 0
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#!/usr/bin/perl
  
# Perl Program to calculate Factorial 
sub fact
{
      
# Retriving the first argument 
# passed with function calling
my $x = $_[0];
  
# checking if that value is 0 or 1
if ($x == 0 || $x == 1)
{
    return 1;
}
  
# Recursively calling function with the next value
# which is one less than current one
else
{
    return $x * fact($x - 1);
}
}
  
# Driver Code
$a = 5;
  
# Function call and printing result after return
print "Factorial of a number $a is ", fact($a);

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Here is how the program works :
Step 1- When the value of scalar a is 0 or 1, the function will return 1 because the value of both 0! and 1! is 1.
Step 2- When the value of scalar a is 2 then fac(x-1) makes a call to fac(1) and this function returns 1.
So, it is 2*factorial(1) = 2*1 = 2.So, it will return 2.
Step 3- Similarly when higher values are passed to function at every call argument value decreases by 1 and computes till the value reaches 1.

Example 2: Example below computes the Fibonacci series till a given number.

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#!/usr/bin/perl
  
# Perl Program to print Fibonacci series
sub fib 
{
    # Retriving values from the parameter
    my $x = shift;
    my $y = shift;
      
    # Number till which the series is to be printed
    my $n = shift;
      
    # Check for the end value
    if ($y > $n
    {
        return 1;
    }
      
    # Printing the number 
    print " $y";
  
    # Recursive Function Call
    fib($y, $x + $y, $n);     
}
  
# Driver Code
  
# Number till which series is to be printed
$a = 5;
  
# First two elements of the series
$c = 0;
$d = 1;
  
print "$c";
  
# Function call with required parameters
fib($c, $d, $a);

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Here is how the program works :
Step 1- Function fib() is called with 3 parameters starting values which will be 0 and 1 while $n is a number till which a series is to be printed
Step 2- These values are transferred in the form of an array whose values are retrieved with the use of shift.
Step 3- At each call first two values are retrieved using shift and these values are stored in scalars x and y. Now these two values are added to get the next value in the series. This step continues till the value reaches the ending value provided by the user



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