Thread: Mod limits.h? Function to check for prime #s? If not, could one create a .h to do so?

After you modify limits.h, feel free to speed the next few months of your life re-writing the rest of the C library so it works with your modified limits.h.

Hint: Get BigNum or related library and use it.
Arbitrary-precision arithmetic - Wikipedia, the free encyclopedia

Tim S.

First modifying a standard header is not recommended. Plus just "modifying" the header will not allow you to use a type with a larger value. These limits are the limits of the compiler. You would be better off investigating something like: GNU Multiple Precision Library .

Jim

Generally, you can create your own .h file and add your own #defines. Never modify system header files unless you've got a really good reason.

For your case, you simply cannot calculate numbers that high, as (assuming you have a newer computer) 2^64 is the highest value you can store (uint64_t). This only adds up to about 20 digits, only a fifth of what you want. If you really need anything higher, you're gonna need a library.

edit: ninja'd by jim and stahta

Originally Posted by andrew89

I can take only being able to calculate prime numbers up to 18,446,744,073,709,551,616 with a grain of salt and deal with it :P.

Jim, I'm going to look into that, as the one that Tim recommended seems to only be available in C# and .NET

If you only want to go up until that number, you don't need a library, just use the data type uint64_t whenever making calculations.

Originally Posted by andrew89

I have 6 gb of RAM and an Intel quad core 2.5ghz processor... that change anything

Not anymore, the limits are pretty standardized now. The problem mainly affects really old computers.

I have 6 gb of RAM and an Intel quad core 2.5ghz processor... that change anything

Nope. The limits defined in limits.h are the limits that the compiler can handle.

Jim

Originally Posted by andrew89

So after installing GMP I'd use:

Code:

uint64_t primeNum;

in order to declare something larger than what my compiler would normally be able to handle?

No, you don't need to install GMP if you're only using uint64_t.

GMP is for numbers longer than ~20 digits, anything shorter is natively supported by C.

EDIT: make sure you include <stdint.h> or it won't necessarily work.

Originally Posted by andrew89

A while back I created a program to check if a number was prime. Basically it checked to see if a number was divisible by any number between 2 and it's square root, if it returned true, the number wasn't prime, if it returned false, it would print it out to a file with a comma and a space ', '

I don't mind redoing it in C (the original was done in C++) but I haven't made a .h file and I figured it would be a learning experience.

I also heard about limits.h and I'd like to change the limit--temporarily--to one centillion (a number with 100 '0s' after it) to see how high I can compute prime numbers. As I think I'd need to do this to calculate any prime number above 2,147,483,647 (or below -2,147,483,647 for that matter).

The algorithm I don't really need help with, mostly just the .h files and any info on creating a function and modifying 'limits.h' to suit my purposes.

The values in limits.h are determined by
A) The design of your compiler
B) The size of your CPU's registers

Most compilers will support 64 bit integers (long long int, int64_t) ONLY if the underlying hardware supports it.

the limits.h file is only a set of constants that *reflect* the limits of the compiler... they do not *set* them, that's pretty much hard wired into the code.

The flatout last thing you want to do is take a stable compiler system (code, libs, headers) and start messing with it... Well, unless you want *every* program you write to contain unfixable errors, that is.