What is a magic number, and why is it bad? [closed]

Answer 1

I've always used the term "magic number" differently, as an obscure value stored within a data structure which can be verified as a quick validity check. For example gzip files contain 0x1f8b08 as their first three bytes, Java class files start with 0xcafebabe, etc.

You often see magic numbers embedded in file formats, because files can be sent around rather promiscuously and lose any metadata about how they were created. However magic numbers are also sometimes used for in-memory data structures, like ioctl() calls.

A quick check of the magic number before processing the file or data structure allows one to signal errors early, rather than schlep all the way through potentially lengthy processing in order to announce that the input was complete balderdash.

Answer 2

Have you taken a look at the Wikipedia entry for magic number?

It goes into a bit of detail about all of the ways the magic number reference is made. Here's a quote about magic number as a bad programming practice

The term magic number also refers to the bad programming practice of using numbers directly in source code without explanation. In most cases this makes programs harder to read, understand, and maintain. Although most guides make an exception for the numbers zero and one, it is a good idea to define all other numbers in code as named constants.

Answer 3

A problem that has not been mentioned with using magic numbers...

If you have very many of them, the odds are reasonably good that you have two different purposes that you're using magic numbers for, where the values happen to be the same.

And then, sure enough, you need to change the value... for only one purpose.

Answer 4

It is worth noting that sometimes you do want non-configurable "hard-coded" numbers in your code. There are a number of famous ones including 0x5F3759DF which is used in the optimized inverse square root algorithm.

In the rare cases where I find the need to use such Magic Numbers, I set them as a const in my code, and document why they are used, how they work, and where they came from.

Answer 5

What about return variables?

I specially find it challenging when implementing stored procedures.

Imagine the next stored procedure (wrong syntax, I know, just to show an example):

int procGetIdCompanyByName(string companyName);

It return the Id of the company if it exists in a particular table. Otherwise, it returns -1. Somehow it's a magic number. Some of the recommendations I've read so far says that I'll really have to do design somthing like that:

int procGetIdCompanyByName(string companyName, bool existsCompany);

By the way, what should it return if the company does not exists? Ok: it will set existesCompany as false, but also will return -1.

Antoher option is to make two separate functions:

bool procCompanyExists(string companyName);
int procGetIdCompanyByName(string companyName);

So a pre-condition for the second stored procedure is that company exists.

But i'm afraid of concurrency, because in this system, a company can be created by another user.

The bottom line by the way is: what do you think about using that kind of "magic numbers" that are relatively known and safe to tell that something is unsuccessful or that something does not exists?

Answer 6

In programming, a "magic number" is a value that should be given a symbolic name, but was instead slipped into the code as a literal, usually in more than one place.

It's bad for the same reason SPOT (Single Point of Truth) is good: If you wanted to change this constant later, you would have to hunt through your code to find every instance. It is also bad because it might not be clear to other programmers what this number represents, hence the "magic".

People sometimes take magic number elimination further, by moving these constants into separate files to act as configuration. This is sometimes helpful, but can also create more complexity than it's worth.