What exactly is an algorithm?

I know that this may sound a bit out of the box, in fact i used to always think inside the box, but recently i've been thinking, possibly because computer science provides an high degree of freedom, about ways to devise programs other than the ones taught in university.

Consider the factorial function. Typically we define this function like

int fact(int n) 
 { 
 int r = 1; 
 for(int i=2;i<=n;i++) 
 r = r*i; 
 return r; 
 }

I'd call this an algorithm and have no doubt that this is the right way to do it. Then, i wondered "can i do this in constant time?", which let to the following idea: what if i had an array of integers where array[n] houses the factorial of n? Once this array is filled i could simply define fact as:

int fact(int n) 
 { 
 return array[n]; 
 }

Still i cant seem to cal this an algorithm, even though it provides the correct result and operates in constant time O(1). Can this be called an algorithm? Otherwise, why not? I could argue that filling the array would require an algorithm to have operated at some time, even if it was in our brain in order for us to fill the array, but could this be the criteria? How are these aspects handled formally?

Note that this concept could be extended to any function operating over integers independly of its number of arguments, i would just have to use a matrix if the function had 2 arguments, or 3 if the function had 3 arguments, and so forth. Also, aren't these solutions used simply because of memory consumption?

Also, not that functions may also encompass any program with output, since i could find a way to index every single possible output that a program could provide.

As another exemple, consider the common use of an array: i allocate an array initially of size N, then i add elements to array by storing the value at index n and increasing n by one unit. Then, if i want to look for an elemento, i cant help but to perform a linear search over the array. If instead i created an array of

The informal definition of an algorithm in a popular textbook goes something like:

An algorithm is (1) a well defined computational procedure (2) that takes some input and (3) produces some output (4) for a well defined computational problem.

In your first case you have coded an algorithm where:
The problem is to find the factorial (part 4 of definition),
given int n as input (part 2 of definition),
the code describes the computation to be performed (part 1 of definition),
the output is the factorial (part 3 of definition).

In your second case:
The problem is to find the array element at position n (part 4 of definition),
given n as input (part 3 of definition),
the code describes the computation to be performed (part 2 of definition),
the output is the element at position n (part 1 of definition).

You have stored factorials there so it gives you factorials. If you had stored squares or cubes there you'd get squares or cubes, so it cannot be said that the second snippet by itself is an algorithm to compute factorials.

And if you say that an array look up along with an array having f(n) at position n is an algorithm to compute f(n) then you have gone so deep that there is no more computation below. A well defined computational procedure should be a finite piece of information. If an infinite array of factorials is a part of the computational procedure this does not hold. So that wouldn't be an algorithm to compute factorials.

StackExchange Computer Science Q#31932, answer score: 9