Ackermann Function In C

Last Updated on June 13, 2023 by Mayank Dham

The Ackermann function is a two-parameter function that takes non-negative integers as inputs and produces a non-negative integer as its result. While it may seem deceptively simple, this function exhibits astonishing growth rates, rapidly surpassing the capabilities of traditional computational methods.

In this article, we embark on a journey to explore the Ackermann function and implement it in the versatile programming language, C. We will delve into the recursive nature of the function and discuss the challenges and considerations involved in its implementation.

Our exploration will begin with an in-depth understanding of the Ackermann function’s recursive definition and its mathematical properties. We will examine how the function evolves with each iteration, uncovering the remarkable exponential growth it exhibits and the consequent implications for computational complexity.

What is the Ackermann Function in C?

The Ackermann function, named after the German mathematician Wilhelm Ackermann, is a recursive mathematical function that takes two non-negative integers as inputs and produces a non-negative integer as its output. In C, the Ackermann function can be implemented using recursion.

The function is defined as follows:

int ackermann(int m, int n) {
    if (m == 0) {
        return n + 1;
    } else if (n == 0) {
        return ackermann(m - 1, 1);
    } else {
        return ackermann(m - 1, ackermann(m, n - 1));
    }
}

Let’s break down the implementation:

The base cases:

• If m is 0, the function returns n + 1.
• If n is 0, the function makes a recursive call with m decremented by 1 and n set to 1.

The recursive case:

• If neither m nor n is 0, the function makes a recursive call with m decremented by 1 and n set to the result of another recursive call with m and n decremented by 1.

Ackermann function is defined as:

The Ackermann function is known for its rapid growth rate, even for small inputs. As the values of m and n increase, the number of recursive calls and the complexity of the computation increase exponentially. This growth makes it a challenging function to compute for larger values of m and n.

Ackermann algorithm:

Ackermann(m, n)
{next and goal are arrays indexed from 0 to m, initialized so that next[O] through next[m] are 0, goal[O] through goal[m - l] are 1, and goal[m] is -1} 
repeat
value ← next[O] + 1 
transferring ← true 
current ← O 
while transferring do begin
if next[current] = goal[current] then goal[current] ← value
else transferring ← false
next[current] ← next[current]+l
current ← current + 1 
end while
until next[m] = n + 1 
return value {the value of A(m, n)}
end Ackermann 

Here’s the explanation of the given Algorithm:

Let me explain the algorithm by taking the example A(1, 2) where m = 1 and n = 2
So according to the algorithm initially the value of next, goal, value and current are:

Though next[current] != goal[current], so else statements will execute and transfer become false.
So now, the value of next, goal, value and current are:

Similarly by tracing the algorithm until next[m] = 3 the value of next, goal, value and current are changing accordingly. Here’s the explanation how the values are changing,

Finally returning the value e.g 4

Analysis of this algorithm:

• The time complexity of this algorithm is: O(mA(m, n)) to compute A(m, n)
• The space complexity of this algorithm is: O(m) to compute A(m, n)

Let’s Take an example to solve the ackermann function in C for values 1 and 2.
Solve A(1, 2)?
Answer:
Given problem is A(1, 2)
Here m = 1, n = 2 e.g m > 0 and n > 0
Hence applying third condition of Ackermann function
A(1, 2) = A(0, A(1, 1)) ———- (1)
Now, Let’s find A(1, 1) by applying third condition of Ackermann function
A(1, 1) = A(0, A(1, 0)) ———- (2)
Now, Let’s find A(1, 0) by applying second condition of Ackermann function
A(1, 0) = A(0, 1) ———- (3)
Now, Let’s find A(0, 1) by applying first condition of Ackermann function
A(0, 1) = 1 + 1 = 2
Now put this value in equation 3
Hence A(1, 0) = 2
Now put this value in equation 2
A(1, 1) = A(0, 2) ———- (4)
Now, Let’s find A(0, 2) by applying first condition of Ackermann function
A(0, 2) = 2 + 1 = 3
Now put this value in equation 4
Hence A(1, 1) = 3
Now put this value in equation 1
A(1, 2) = A(0, 3) ———- (5)
Now, Let’s find A(0, 3) by applying first condition of Ackermann function
A(0, 3) = 3 + 1 = 4
Now put this value in equation 5
Hence A(1, 2) = 4
So, A (1, 2) = 4

Code Implementation

#include <stdio.h>
int ack(int m, int n)
{
    if (m == 0){
        return n+1;
    }
    else if((m > 0) && (n == 0)){
        return ack(m-1, 1);
    }
    else if((m > 0) && (n > 0)){
        return ack(m-1, ack(m, n-1));
    }
}

int main(){
    int A;
    A = ack(1, 2);
    printf("%d", A);
    return 0;
}

Output

4

Time Complexity
In terms of time complexity, the Ackermann function in C programming language can be approximated as a tower of exponents, represented as O(2^n).

Space Complexity:
The space complexity of the Ackermann function in C programming language is also significant, as it relies on recursive function calls. Each recursive call adds a new frame to the call stack, consuming additional memory. The space complexity is directly related to the depth of the recursive calls, which grows with the values of m and n. Therefore, the space complexity can be approximated as O(m) or O(n)

Conclusion
In conclusion, the Ackermann function stands as a fascinating mathematical construct that pushes the boundaries of recursive computation. Through our exploration of implementing the Ackermann function in C, we have witnessed the exponential growth and complexity that arise from seemingly simple inputs.
Furthermore, we examined the time and space complexity of the Ackermann function. As the values of m and n increase, the computation becomes increasingly resource-intensive, reaching the limits of practicality for larger inputs. The time complexity can be approximated as O(2^n), and the space complexity as O(m) or O(n).

FAQ Related to Ackermann Function In C

Q1: What are some practical applications of the Ackermann function in C?
A: The Ackermann function is primarily used as a benchmark to test the efficiency and performance of recursive algorithms and programming languages. It helps identify the limitations of recursive computation and serves as a theoretical tool for exploring the boundaries of computational complexity.

Q2: Can the Ackermann function be optimized in C to handle larger inputs?
A: Due to the exponential growth of the Ackermann function, it becomes computationally expensive for larger inputs. While certain optimizations can be applied, such as memoization or tail recursion, these techniques can only provide marginal improvements. Ultimately, the Ackermann function reaches the limits of practical computation for larger inputs.

Q3: Are there alternative approaches to compute the Ackermann function in C?
A: Apart from the traditional recursive implementation, alternative approaches like using loops or dynamic programming can be explored to compute the Ackermann function in C. However, these approaches may not overcome the exponential growth or significantly improve the performance of the function for larger inputs.

Q4: Are there alternative functions similar to the Ackermann function?
A: Yes, several other functions, such as the Tower of Hanoi function or the Fibonacci function, exhibit similar recursive characteristics and computational complexity. These functions are often used in the realm of theoretical computer science to analyze the behavior and limits of recursive algorithms.