Chapter 7: Recursion

Example 7.1, Page number: 219

In [1]:
# Variable declaration, calculation and result
str = raw_input('Enter a string: ')

print str

Enter a string: HELLO
HELLO

Example 7.2, Page number: 221

In [2]:
# Variable declaration, calculation and result
str = raw_input('Enter a string: ')

print str[::-1]

Enter a string: HELLO
OLLEH

Example 7.3, Page number: 224

In [3]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def fact (n) :
   if n>0 :
      return n * fact (n-1)
   else :
      return 1

print ('Factorial of %d is %d' % (n, fact (n)))

Enter any number 5
Factorial of 5 is 120

Example 7.4, Page number: 227

In [4]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def printnum (n) :
   if n>0 :
      printnum (n-1)
      print n,

print ('Counting up to %d' % n)
printnum (n)

Enter any number 10
Counting up to 10
1 2 3 4 5 6 7 8 9 10

Example 7.5, Page number: 229

In [5]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def printeven (n) :
	if not hasattr(printeven, "x") : 
		printeven.x = 2
      
	if printeven.x <= n :
		print printeven.x,
		printeven.x = printeven.x + 2
		printeven (n)

print ('Even numbers up to %d are' % n)
printeven (n)

Enter any number 10
Even numbers up to 10 are
2 4 6 8 10

Example 7.6, Page number: 231

In [6]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def printodd (x, n) :
	if x <= n : 
		print x,
		printodd (x+2, n)

print ('Odd numbers up to %d are' % n)
printodd (1, n)

Enter any number 10
Odd numbers up to 10 are
1 3 5 7 9

Example 7.7, Page number: 232

In [7]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def printnum (fro, to) :
	if fro <= to : 
		print fro,
		printnum (fro+1, to)

print ('Number line for %d is' % n)
printnum (-n, n)

Enter any number 5
Number line for 5 is
-5 -4 -3 -2 -1 0 1 2 3 4 5

Example 7.8, Page number: 234

In [8]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def summate (n) :
	if n > 0 : 
		return n + summate (n-1)
	else :
		return 0

print ('Sum of first %d natural numbers is %d' % (n, summate (n)))

Enter any number 10
Sum of first 10 natural numbers is 55

Example 7.9, Page number: 237

In [9]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def factors (n) :
	if not hasattr(factors, "i") : 
		factors.i = 1
		
	if factors.i <= n : 
		if n % factors.i == 0 :
			print factors.i,
		factors.i = factors.i + 1
		factors (n)

print ('The factors of %d are' % n)
factors (n)

Enter any number 28
The factors of 28 are
1 2 4 7 14 28

Example 7.10, Page number: 239

In [10]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def multiple (n) :
	if not hasattr(multiple, "i") : 
		multiple.i = 1
		
	if multiple.i <= 10 : 
		print ('%d X %d = %d' % (n, multiple.i, n*multiple.i))
		multiple.i = multiple.i + 1
		multiple (n)

print ('The first 10 multiples of %d are' % n)
multiple (n)

Enter any number 6
The first 10 multiples of 6 are
6 X 1 = 6
6 X 2 = 12
6 X 3 = 18
6 X 4 = 24
6 X 5 = 30
6 X 6 = 36
6 X 7 = 42
6 X 8 = 48
6 X 9 = 54
6 X 10 = 60

Example 7.11, Page number: 241

In [11]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def checkprime (n) :
   flag = 1
   i = 2
   while i<n and flag :
      if n%i == 0 :
         flag = 0
      i = i+1
   return flag

result = checkprime (n)

if result :
   print ('The number %d is PRIME ' % n)
else :
   print ('The number %d is COMPOSITE ' % n)

Enter any number 19
The number 19 is PRIME

Example 7.12, Page number: 243

In [12]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def Fibonacci (n) :
	if n < 2 :
		return n
	else :
		return Fibonacci (n-1) + Fibonacci (n-2)
	

print ('The %d terms of Fibonacci series are' % n)
for i in range (0, n) :
	print ('%d' % Fibonacci (i)),

Enter any number 10
The 10 terms of Fibonacci series are
0 1 1 2 3 5 8 13 21 34

Example 7.13, Page number: 244

In [13]:
# Variable declaration
n = int(raw_input('Enter any number '))
sum = n

# Function declaration, calculation and result
def sumofdigit (n, sum) :
   while sum>=10 :
      sum = 0
      while n>0 :
         sum = sum + n%10
         n = n/10
      n = sum
   return sum

result = sumofdigit (n, sum)

print ('Sum of digits of %d is %d' % (n, result))

Enter any number 8626
Sum of digits of 8626 is 4

Example 7.14, Page number: 245

In [15]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def prime (n) :
   for num in range (3, n+1) :
      flag = 1
      for i in range (2, num) :
         if num % i == 0 : 
            flag = 0
      if flag :
         print ('%d ' % num),

print ('Prime numbers upto %d are' % n)
prime (n)

Enter any number 30
Prime numbers upto 30 are
3  5  7  11  13  17  19  23  29

Example 7.15, Page number: 246

In [16]:
# Variable declaration
n = int (raw_input('Enter any number '))

# Function declaration, calculation and result
def pattern (n) :
	if n > 0 :
		pattern (n-1)
		for count in range (1, n+1) :
			print ('%d' % count),
   	print
   	
pattern (n)

Enter any number 4

1
1 2
1 2 3
1 2 3 4

Example 7.16, Page number: 249

In [17]:
# Variable declaration
n = int(raw_input('Enter any number '))

# Function declaration, calculation and result
def printline (x, total) :
	if x > 0 :
		printline (x-1, total)
		for blk in range (1, total-x+1) :
			print ' ',
		for up in range (1, x+1) :
			print ('%d' % up),
		down = x-1
		while down>0 :
			print ('%d' % down),
			down = down-1
		print

printline (n, n)

Enter any number 4
      1
    1 2 1
  1 2 3 2 1
1 2 3 4 3 2 1

Example 7.17, Page number: 253

In [18]:
# Function declaration, calculation and result
def listsum () :
	n = int(raw_input('Enter any number '))
	if n > 0 :
		return n + listsum ()
	else :
		return 0
		
sum = listsum ()
print ('Sum of list of numbers is %d' % sum)

Enter any number 1
Enter any number 2
Enter any number 3
Enter any number -4
Sum of list of numbers is 6