Functional Programming – How to ‘Remember’ Values in Functional Programming

functional programmingpseudocode

I've decided to take upon myself the task of learning functional programming. So far it's been a blast, and I've 'seen the light' as it were. Unfortunately, I don't actually know any functional programmer that I can bounce questions off of. Introducing Stack Exchange.

I'm taking a web/software development course, but my instructor isn't familiar with functional programming. He's fine with me using it, and he just asked me to help him understand how it works so he can read my code better.

I decided the best way to do this would be by illustrating a simple mathematical function, like raising a value to a power. In theory I could easily do that with a prebuilt function, but that would defeat the purpose of an example.

Anyway, I'm having some difficulty figuring out how to hold a value. Since this is functional programming I can't change variable. If I were to code this imperatively, it would look something like this:

(The following is all pseudocode)

f(x,y) {
  int z = x;
  for(int i = 0, i < y; i++){
    x = x * z;
  }
  return x;
}

In functional programming, I wasn't sure. This is what I came up with:

f(x,y,z){
  if z == 'null',
    f(x,y,x);
  else if y > 1,
    f(x*z,y-1,z);
  else
    return x;
}

Is this right? I need to hold a value, z in both cases, but I wasn't sure how to do this in function programming. In theory, the way I did it works, but I wasn't sure if it was 'right'. Is there a better way to do it?

Best Answer

First of all, congratulations on "seeing the light". You've made the software world a better place by expanding your horizons.

Second, there is honestly no way a professor who doesn't understand functional programming is going to be able to say anything useful about your code, other than trite comments such as "the indentation looks off". This isn't that surprising in a web development course, as most web development is done using HTML/CSS/JavaScript. Depending on how much you actually care about learning web development, you might want to put in the effort to learn the tools your professor is teaching (painful though it may be - I know from experience).

To address the stated question: if your imperative code uses a loop, then chances are your functional code is going to be recursive.

(* raises x to the power of y *)
fun pow (x: real) (y: int) : real = 
    if y = 1 then x else x * (pow x (y-1))

Note that this algorithm is actually more or less identical to the imperative code. In fact, one could consider the loop above to be syntactic sugar for iterative recursive processes.

As a side note, there's no need for a value of z in either your imperative or functional code, in fact. You should have written your imperative function like so:

def pow(x, y):
    var ret = 1
    for (i = 0; i < y; i++)
         ret = ret * x
    return ret

rather than changing the meaning of the variable x.

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