# Python Program to Generate Gray Codes using Recursion

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Recursive function:

In its definition, recursion is just calling the same function many times (this means, we are using that same function within its body).

Binary Code:

As previously stated, Binary Code is a Base-2 representation of a number. In Binary, all numbers are represented by simply two symbols: 0 and 1. Binary (also known as base-2) is a numerical system with only two digits: 0 and 1.

Gray Code:

A Gray Code is a method of encoding integers in which consecutive numbers differ by by one bit. Gray Code is created by arranging the binary numeral system in such a way that two consecutive values differ only by one bit.

The number of bits n has been specified. The task at hand is to generate n-bit Gray code. The Gray code is a binary number ordering in which two consecutive codewords differ by only one bit.

Examples:

Example1:

Input:

given number of bits = 5

Output:

Printing all 5 bits Gray Codes :
['00000', '00001', '00011', '00010', '00110', '00111', '00101', '00100', '01100', '01101', '01111', '01110', '01010',
'01011', '01001', '01000', '11000', '11001', '11011', '11010', '11110', '11111', '11101', '11100', '10100', '10101',
'10111', '10110', '10010', '10011', '10001', '10000']

Example2:

Input:

given number of bits = 7

Output:

Printing all 7 bits Gray Codes :
['0000000', '0000001', '0000011', '0000010', '0000110', '0000111', '0000101', '0000100', '0001100', '0001101',
'0001111', '0001110', '0001010', '0001011', '0001001', '0001000', '0011000', '0011001', '0011011', '0011010',
'0011110', '0011111', '0011101', '0011100', '0010100', '0010101', '0010111', '0010110', '0010010', '0010011',
'0010001', '0010000', '0110000', '0110001', '0110011', '0110010', '0110110', '0110111', '0110101', '0110100',
'0111100', '0111101', '0111111', '0111110', '0111010', '0111011', '0111001', '0111000', '0101000', '0101001',
'0101011', '0101010', '0101110', '0101111', '0101101', '0101100', '0100100', '0100101', '0100111', '0100110',
'0100010', '0100011', '0100001', '0100000', '1100000', '1100001', '1100011', '1100010', '1100110', '1100111',
'1100101', '1100100', '1101100', '1101101', '1101111', '1101110', '1101010', '1101011', '1101001', '1101000',
'1111000', '1111001', '1111011', '1111010', '1111110', '1111111', '1111101', '1111100', '1110100', '1110101',
'1110111', '1110110', '1110010', '1110011', '1110001', '1110000', '1010000', '1010001', '1010011', '1010010',
'1010110', '1010111', '1010101', '1010100', '1011100', '1011101', '1011111', '1011110', '1011010', '1011011',
'1011001', '1011000', '1001000', '1001001', '1001011', '1001010', '1001110', '1001111', '1001101', '1001100',
'1000100', '1000101', '1000111', '1000110', '1000010', '1000011', '1000001', '1000000']

## Program to Generate Gray Codes using Recursion in Python

There are several ways to generate gray codes  using recursion some of them are:

### Method #1:Using Recursion (Static Input)

Approach:

• Give the value of numb as static input.
• There is a function called printGrayCodes that is defined.
• It accepts as an argument the number of bits n.
• It returns a list of n-bit Gray codes.
• To begin, the function obtains the (n – 1)-bit Gray code.
• The first half of the n-bit Gray codewords are simply the (n – 1)-bit Gray codewords prefixed with a 0.
• The second half of the n-bit Gray codewords is made up of (n – 1)-bit Gray codewords listed in reverse and prefixed with a 1.
• The 0-bit Gray code is essentially the empty string.

Below is the implementation:

def printGrayCodes(numb):
# Return a list of n-bit Gray codes.
if numb == 0:
return ['']
# The first half of the n-bit Gray codewords are simply the (n – 1)-bit Gray
# codewords prefixed with a 0.
firstHalfBits = printGrayCodes(numb - 1)
secondHalfBits = firstHalfBits.copy()
# The second half of the n-bit Gray codewords is made up of (n – 1)-bit Gray codewords
# listed in reverse and prefixed with a 1.
firstHalfBits = ['0' + code for code in firstHalfBits]
secondHalfBits = ['1' + code for code in reversed(secondHalfBits)]
# printing the result
return firstHalfBits + secondHalfBits

# Give the value of number of bits as static input
numb = 5
# passing the given number of bits to print gray codes
getOutput = printGrayCodes(numb)
print('Printing all', numb, 'bits', 'Gray Codes : ')
print(getOutput)


Output:

Printing all 5 bits Gray Codes :
['00000', '00001', '00011', '00010', '00110', '00111', '00101', '00100', '01100', '01101', '01111', '01110', '01010',
'01011', '01001', '01000', '11000', '11001', '11011', '11010', '11110', '11111', '11101', '11100', '10100', '10101',
'10111', '10110', '10010', '10011', '10001', '10000']

### Method #2:Using Recursion (User Input)

Approach:

• Give some random number of bits as user input using int(input()) function.
• There is a function called printGrayCodes that is defined.
• It accepts as an argument the number of bits n.
• It returns a list of n-bit Gray codes.
• To begin, the function obtains the (n – 1)-bit Gray code.
• The first half of the n-bit Gray codewords are simply the (n – 1)-bit Gray codewords prefixed with a 0.
• The second half of the n-bit Gray codewords is made up of (n – 1)-bit Gray codewords listed in reverse and prefixed with a 1.
• The 0-bit Gray code is essentially the empty string.

Below is the implementation:

def printGrayCodes(numb):
# Return a list of n-bit Gray codes.
if numb == 0:
return ['']
# The first half of the n-bit Gray codewords are simply the (n – 1)-bit Gray
# codewords prefixed with a 0.
firstHalfBits = printGrayCodes(numb - 1)
secondHalfBits = firstHalfBits.copy()
# The second half of the n-bit Gray codewords is made up of (n – 1)-bit Gray codewords
# listed in reverse and prefixed with a 1.
firstHalfBits = ['0' + code for code in firstHalfBits]
secondHalfBits = ['1' + code for code in reversed(secondHalfBits)]
# printing the result
return firstHalfBits + secondHalfBits

# Give the value of number of bits as static input
numb = int(input('Enter some random number of bits = '))
# passing the given number of bits to print gray codes
getOutput = printGrayCodes(numb)
print('Printing all', numb, 'bits', 'Gray Codes : ')
print(getOutput)


Output:

Enter some random number of bits = 7
Printing all 7 bits Gray Codes :
['0000000', '0000001', '0000011', '0000010', '0000110', '0000111', '0000101', '0000100', '0001100', '0001101',
'0001111', '0001110', '0001010', '0001011', '0001001', '0001000', '0011000', '0011001', '0011011', '0011010',
'0011110', '0011111', '0011101', '0011100', '0010100', '0010101', '0010111', '0010110', '0010010', '0010011',
'0010001', '0010000', '0110000', '0110001', '0110011', '0110010', '0110110', '0110111', '0110101', '0110100',
'0111100', '0111101', '0111111', '0111110', '0111010', '0111011', '0111001', '0111000', '0101000', '0101001',
'0101011', '0101010', '0101110', '0101111', '0101101', '0101100', '0100100', '0100101', '0100111', '0100110',
'0100010', '0100011', '0100001', '0100000', '1100000', '1100001', '1100011', '1100010', '1100110', '1100111',
'1100101', '1100100', '1101100', '1101101', '1101111', '1101110', '1101010', '1101011', '1101001', '1101000',
'1111000', '1111001', '1111011', '1111010', '1111110', '1111111', '1111101', '1111100', '1110100', '1110101',
'1110111', '1110110', '1110010', '1110011', '1110001', '1110000', '1010000', '1010001', '1010011', '1010010',
'1010110', '1010111', '1010101', '1010100', '1011100', '1011101', '1011111', '1011110', '1011010', '1011011',
'1011001', '1011000', '1001000', '1001001', '1001011', '1001010', '1001110', '1001111', '1001101', '1001100',
'1000100', '1000101', '1000111', '1000110', '1000010', '1000011', '1000001', '1000000']

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