# Program to apply Russian Peasant Multiplication in Python

PythonServer Side ProgrammingProgramming

#### Beyond Basic Programming - Intermediate Python

Most Popular

36 Lectures 3 hours

#### Practical Machine Learning using Python

Best Seller

91 Lectures 23.5 hours

#### Practical Data Science using Python

22 Lectures 6 hours

Suppose we are given four integer numbers p, q, r, and k. We will use a method called the Russian Peasant Multiplication method and determine the value of (p + q.i)^r = r + s.i. We have to return the value of r mod k and s mod k.

So, if the input is like p = 3, q = 0, r = 8, k = 10000, then the output will be (6561, 0) 3^8 = 6561, as q = 0 value of r mod k = 6561.

To solve this, we will follow these steps −

• if r is same as 0, then
• return 1
• therwise when r is same as 1, then
• return a pair containing (p mod k, q mod k)
• therwise when r mod 2 is same as 0, then
• return solve((p*p - q*q) mod k, 2*p*q mod k, r/2, k)
• therwise,
• a pair (pr, qr) = solve(p, q, r-1, k)
• return a pair containing ((p * pr - q * qr) mod k, (p * qr + q * pr) mod k)

## Example

Let us see the following implementation to get better understanding −

def solve(p, q, r, k):
if r == 0:
return 1
elif r == 1:
return (p % k, q % k)
elif r % 2 == 0:
return solve((p*p - q*q) % k, 2*p*q % k, r/2, k)
else:
(pr, qr) = solve(p, q, r-1, k)
return ((p * pr - q * qr) % k, (p * qr + q * pr) % k)

print(solve(3, 0, 8, 10000))

## Input

3, 0, 8, 10000


## Output

(6561, 0)
Updated on 23-Oct-2021 07:52:50