C - How to implement Set data structure?

Question

Is there any tricky way to implement a set data structure (a collection of unique values) in C? All elements in a set will be of the same type and there is a huge RAM memory

Answer 1

The way to get genericity in C is by void *, so you're going to be using pointers anyway, and pointers to different objects are unique. This means you need a hash map or binary tree containing pointers, and this will work for all data objects.

The downside of this is that you can't enter rvalues independently. You can't have a set containing the value 5; you have to assign 5 to a variable, which means it won't match a random 5. You could enter it as (void *) 5, and for practical purposes this is likely to work with small integers, but if your integers can get into large enough sizes to compete with pointers this has a very small probability of failing.

Nor does this work with string values. Given char a[] = "Hello, World!"; char b[] = "Hello, World!";, a set of pointers would find a and b to be different. You would probably want to hash the values, but if you're concerned about hash collisions you should save the string in the set and do a strncmp() to compare the stored string with the probing string.

(There's similar problems with floating-point numbers, but trying to represent floating-point numbers in sets is a bad idea in the first place.)

Therefore, you'd probably want a tagged value, one tag for any sort of object, one for integer value, and one for string value, and possibly more for different sorts of values. It's complicated, but doable.