What are magic numbers?

Question

What is meant by magic numbers from a programming point of view.

Answer 1

Magic numbers are any number in your code that isn't immediately obvious to someone with very little knowledge.

For example, the following piece of code:

sz = sz + 729;

has a magic number in it and would be far better written as:

sz = sz + CAPACITY_INCREMENT;

Some extreme views state that you should never have any numbers in your code except -1, 0 and 1 but I prefer a somewhat less dogmatic view since I would instantly recognise 24, 1440, 86400, 3.1415, 2.71828 and 1.414 - it all depends on your knowledge.

However, even though I know there are 1440 minutes in a day, I would probably still use a MINS_PER_DAY identifier since it makes searching for them that much easier. Whose to say that the capacity increment mentioned above wouldn't also be 1440 and you end up changing the wrong value? This is especially true for the low numbers: the chance of dual use of 37197 is relatively low, the chance of using 5 for multiple things is pretty high.

Use of an identifier means that you wouldn't have to go through all your 700 source files and change 729 to 730 when the capacity increment changed. You could just change the one line:

#define CAPACITY_INCREMENT 729

to:

#define CAPACITY_INCREMENT 730

and recompile the lot.

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Contrast this with magic constants which are the result of naive people thinking that just because they remove the actual numbers from their code, they can change:

x = x + 4;

to:

#define FOUR 4
x = x + FOUR;

That adds absolutely zero extra information to your code and is a total waste of time.

Answer 2

"magic numbers" are numbers that appear in statements like

if days == 365

Assuming you didn't know there were 365 days in a year, you'd find this statement meaningless. Thus, it's good practice to assign all "magic" numbers (numbers that have some kind of significance in your program) to a constant,

DAYS_IN_A_YEAR = 365

And from then on, compare to that instead. It's easier to read, and if the earth ever gets knocked out of alignment, and we gain an extra day... you can easily change it (other numbers might be more likely to change).

Answer 3

The term magic number is usually used to describe some numeric constant in code. The number appears without any further description and thus its meaning is esoteric.

The use of magic numbers can be avoided by using named constants.

Answer 4

Anything that doesn't have a readily apparent meaning to anyone but the application itself.

if (foo == 3) {
    // do something
} else if (foo == 4) {
    // delete all users
}

Answer 5

Using numbers in calculations other than 0 or 1 that aren't defined by some identifier or variable (which not only makes the number easy to change in several places by changing it in one place, but also makes it clear to the reader what the number is for).

Answer 6

Magic numbers are special value of certain variables which causes the program to behave in an special manner.

For example, a communication library might take a Timeout parameter and it can define the magic number "-1" for indicating infinite timeout.

Answer 7

Wikipedia is your friend (Magic Number article)

Answer 8

Most of the answers so far have described a magic number as a constant that isn't self describing. Being a little bit of an "old-school" programmer myself, back in the day we described magic numbers as being any constant that is being assigned some special purpose that influences the behaviour of the code. For example, the number 999999 or MAX_INT or something else completely arbitrary.

The big problem with magic numbers is that their purpose can easily be forgotten, or the value used in another perfectly reasonable context.

As a crude and terribly contrived example:

while (int i != 99999)
{
  DoSomeCleverCalculationBasedOnTheValueOf(i);      

  if (escapeConditionReached)
  {
    i = 99999;
  }
}

The fact that a constant is used or not named isn't really the issue. In the case of my awful example, the value influences behaviour, but what if we need to change the value of "i" while looping?

Clearly in the example above, you don't NEED a magic number to exit the loop. You could replace it with a break statement, and that is the real issue with magic numbers, that they are a lazy approach to coding, and without fail can always be replaced by something less prone to either failure, or to losing meaning over time.

Answer 9

There's more than one meaning. The one given by most answers already (an arbitrary unnamed number) is a very common one, and the only thing I'll say about that is that some people go to the extreme of defining...

#define ZERO 0
#define ONE 1

If you do this, I will hunt you down and show no mercy.

Another kind of magic number, though, is used in file formats. It's just a value included as typically the first thing in the file which helps identify the file format, the version of the file format and/or the endian-ness of the particular file.

For example, you might have a magic number of 0x12345678. If you see that magic number, it's a fair guess you're seeing a file of the correct format. If you see, on the other hand, 0x78563412, it's a fair guess that you're seeing an endian-swapped version of the same file format.

The term "magic number" gets abused a bit, though, referring to almost anything that identifies a file format - including quite long ASCII strings in the header.

http://en.wikipedia.org/wiki/File_format#Magic_number