What is a magic number and why is it bad

Question

What is a magic number   Why should it be avoided   Are there cases where it s appropriate

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Another advantage of extracting a magic number as a constant gives the possibility to clearly document the business information   public class Foo                   Max age in year to get child rate for airline tickets                The value of the constant is   value              public static final int MAX AGE FOR CHILD RATE   2       public void computeRate              if  person getAge    lt  MAX AGE FOR CHILD RATE                   applyChildRate

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A magic number can also be a number with special  hardcoded semantics  For example  I once saw a system where record IDs   0 were treated normally  0 itself was  new record   -1 was  this is the root  and -99 was  this was created in the root   0 and -99 would cause the WebService to supply a new ID   What s bad about this is that you re reusing a space  that of signed integers for record IDs  for special abilities  Maybe you ll never want to create a record with ID 0  or with a negative ID  but even if not  every person who looks either at the code or at the database might stumble on this and be confused at first  It goes without saying those special values weren t well-documented   Arguably  22  7  -12 and 620 count as magic numbers  too   -

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A Magic Number is a hard-coded value that may change at a later stage  but that can be therefore hard to update   For example  let s say you have a Page that displays the last 50 Orders in a  Your Orders  Overview Page  50 is the Magic Number here  because it s not set through standard or convention  it s a number that you made up for reasons outlined in the spec   Now  what you do is you have the 50 in different places - your SQL script  SELECT TOP 50   FROM orders   your Website  Your Last 50 Orders   your order login  for  i   0  i  lt  50  i     and possibly many other places   Now  what happens when someone decides to change 50 to 25  or 75  or 153  You now have to replace the 50 in all the places  and you are very likely to miss it  Find Replace may not work  because 50 may be used for other things  and blindly replacing 50 with 25 can have some other bad side effects  i e  your Session Timeout   50 call  which is also set to 25 and users start reporting too frequent timeouts    Also  the code can be hard to understand  i e   if a  lt  50 then bla  - if you encounter that in the middle of a complicated function  other developers who are not familiar with the code may ask themselves  WTF is 50      That s why it s best to have such ambiguous and arbitrary numbers in exactly 1 place -  const int NumOrdersToDisplay   50   because that makes the code more readable   if a  lt  NumOrdersToDisplay   it also means you only need to change it in 1 well defined place   Places where Magic Numbers are appropriate is everything that is defined through a standard  i e  SmtpClient DefaultPort   25 or TCPPacketSize   whatever  not sure if that is standardized   Also  everything only defined within 1 function might be acceptable  but that depends on Context

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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

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eed3si9n  I d even suggest that  1  is a magic number   -   A principle that s related to magic numbers is that every fact your code deals with should be declared exactly once  If you use magic numbers in your code  such as the password length example that  marcio gave  you can easily end up duplicating that fact  and when your understand of that fact changes you ve got a maintenance problem

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A magic number is a sequence of characters at the start of a file format  or protocol exchange   This number serves as a sanity check   Example  Open up any GIF file  you will see at the very start  GIF89    GIF89  being the magic number   Other programs can read the first few characters of a file and properly identify GIFs    The danger is that random binary data can contain these same characters   But it is very unlikely    As for protocol exchange  you can use it to quickly identify that the current  message  that is being passed to you is corrupted or not valid      Magic numbers are still useful

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I assume this is a response to my answer to your earlier question  In programming  a magic number is an embedded numerical constant that appears without explanation  If it appears in two distinct locations  it can lead to circumstances where one instance is changed and not another  For both these reasons  it s important to isolate and define the numerical constants outside the places where they re used

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Magic Number Vs  Symbolic Constant  When to replace   Magic  Unknown semantic  Symbolic Constant -  Provides both correct semantic and correct context for use  Semantic  The meaning or purpose of a thing    Create a constant  name it after the meaning  and replace the number with it   -- Martin Fowler  First  magic numbers are not just numbers  Any basic value can be  magic   Basic values are manifest entities such as integers  reals  doubles  floats  dates  strings  booleans  characters  and so on  The issue is not the data type  but the  magic  aspect of the value as it appears in our code text   What do we mean by  magic   To be precise  By  magic   we intend to point to the semantics  meaning or purpose  of the value in the context of our code  that it is unknown  unknowable  unclear  or confusing  This is the notion of  magic   A basic value is not magic when its semantic meaning or purpose-of-being-there is quickly and easily known  clear  and understood  not confusing  from the surround context without special helper words  e g  symbolic constant    Therefore  we identify magic numbers by measuring the ability of a code reader to know  be clear  and understand the meaning and purpose of a basic value from its surrounding context  The less known  less clear  and more confused the reader is  the more  magic  the basic value is   Helpful Definitions   confuse  cause  someone  to become bewildered or perplexed  bewildered  cause  someone  to become perplexed and confused  perplexed  completely baffled  very puzzled  baffled  totally bewilder or perplex  puzzled  unable to understand  perplexed  understand  perceive the intended meaning of  words  a language  or speaker    meaning  what is meant by a word  text  concept  or action  meant  intend to convey  indicate  or refer to  a particular thing or notion   signify  signify  be an indication of   indication  a sign or piece of information that indicates something   indicate  point out  show  sign  an object  quality  or event whose presence or occurrence indicates the probable presence or occurrence of something else    Basics  We have two scenarios for our magic basic values  Only the second is of primary importance for programmers and code    A lone basic value  e g  number  from which its meaning is unknown  unknowable  unclear or confusing  A basic value  e g  number  in context  but its meaning remains unknown  unknowable  unclear or confusing    An overarching dependency of  magic  is how the lone basic value  e g  number  has no commonly known semantic  like Pi   but has a locally known semantic  e g  your program   which is not entirely clear from context or could be abused in good or bad context s    The semantics of most programming languages will not allow us to use lone basic values  except  perhaps  as data  i e  tables of data   When we encounter  magic numbers   we generally do so in a context  Therefore  the answer to       Do I replace this magic number with a symbolic constant     is        How quickly can you assess and understand the semantic meaning of the   number  its purpose for being there  in its context     Kind of Magic  but not quite  With this thought in mind  we can quickly see how a number like Pi  3 14159  is not a  magic number  when placed in proper context  e g  2 x 3 14159 x radius or 2 Pi r   Here  the number 3 14159 is mentally recognized Pi without the symbolic constant identifier    Still  we generally replace 3 14159 with a symbolic constant identifier like Pi because of the length and complexity of the number  The aspects of length and complexity of Pi  coupled with a need for accuracy  usually means the symbolic identifier or constant is less prone to error  Recognition of  Pi  as a name is a simply a convenient bonus  but is not the primary reason for having the constant   Meanwhile  Back at the Ranch  Laying aside common constants like Pi  let s focus primarily on numbers with special meanings  but which those meanings are constrained to the universe of our software system  Such a number might be  2   as a basic integer value    If I use the number 2 by itself  my first question might be  What does  2  mean  The meaning of  2  by itself is unknown and unknowable without context  leaving its use unclear and confusing  Even though having just  2  in our software will not happen because of language semantics  we do want to see that  2  by itself carries no special semantics or obvious purpose being alone   Let s put our lone  2  in a context of  padding    2  where the context is a  GUI Container   In this context the meaning of 2  as pixels or other graphical unit  offers us a quick guess of its semantics  meaning and purpose   We might stop here and say that 2 is okay in this context and there is nothing else we need to know  However  perhaps in our software universe this is not the whole story  There is more to it  but  padding   2  as a context cannot reveal it   Let s further pretend that 2 as pixel padding in our program is of the  default padding  variety throughout our system  Therefore  writing the instruction padding   2 is not good enough  The notion of  default  is not revealed  Only when I write  padding   default padding as a context and then elsewhere  default padding   2 do I fully realize a better and fuller meaning  semantic and purpose  of 2 in our system   The example above is pretty good because  2  by itself could be anything  Only when we limit the range and domain of understanding to  my program  where 2 is the default padding in the GUI UX parts of  my program   do we finally make sense of  2  in its proper context  Here  2  is a  magic  number  which is factored out to a symbolic constant default padding within the context of the GUI UX of  my program  in order to make it use as default padding quickly understood in the greater context of the enclosing code   Thus  any basic value  whose meaning  semantic and purpose  cannot be sufficiently and quickly understood is a good candidate for a symbolic constant in the place of the basic value  e g  magic number    Going Further  Numbers on a scale might have semantics as well  For example  pretend we are making a D amp D game  where we have the notion of a monster  Our monster object has a feature called life force  which is an integer  The numbers have meanings that are not knowable or clear without words to supply meaning  Thus  we begin by arbitrarily saying    full life force  INTEGER   10 -- Very alive  and unhurt  minimum life force  INTEGER   1 -- Barely alive  very hurt  dead  INTEGER   0 -- Dead undead  INTEGER   -1 -- Min undead  almost dead  zombie  INTEGER   -10 -- Max undead  very undead    From the symbolic constants above  we start to get a mental picture of the aliveness  deadness  and  undeadness   and possible ramifications or consequences  for our monsters in our D amp D game  Without these words  symbolic constants   we are left with just the numbers ranging from -10    10  Just the range without the words leaves us in a place of possibly great confusion and potentially with errors in our game if different parts of the game have dependencies on what that range of numbers means to various operations like attack elves or seek magic healing potion   Therefore  when searching for and considering replacement of  magic numbers  we want to ask very purpose-filled questions about the numbers within the context of our software and even how the numbers interact semantically with each other   Conclusion  Let s review what questions we ought to ask    You might have a magic number if       Can the basic value have a special meaning or purpose in your softwares universe  Can the special meaning or purpose likely be unknown  unknowable  unclear  or confusing  even in its proper context  Can a proper basic value be improperly used with bad consequences in the wrong context  Can an improper basic value be properly used with bad consequences in the right context  Does the basic value have a semantic or purpose relationships with other basic values in specific contexts  Can a basic value exist in more than one place in our code with different semantics in each  thereby causing our reader a confusion    Examine stand-alone manifest constant basic values in your code text  Ask each question slowly and thoughtfully about each instance of such a value  Consider the strength of your answer  Many times  the answer is not black and white  but has shades of misunderstood meaning and purpose  speed of learning  and speed of comprehension  There is also a need to see how it connects to the software machine around it    In the end  the answer to replacement is answer the measure  in your mind  of the strength or weakness of the reader to make the connection  e g   get it    The more quickly they understand meaning and purpose  the less  magic  you have   CONCLUSION  Replace basic values with symbolic constants only when the magic is large enough to cause difficult to detect bugs arising from confusions

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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

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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

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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

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A magic number is a direct usage of a number in the code   For example  if you have  in Java    public class Foo       public void setPassword String password                don t do this          if  password length    gt  7                  throw new InvalidArgumentException  password                         This should be refactored to   public class Foo       public static final int MAX PASSWORD SIZE   7       public void setPassword String password             if  password length    gt  MAX PASSWORD SIZE                  throw new InvalidArgumentException  password                         It improves readability of the code and it s easier to maintain  Imagine the case where I set the size of the password field in the GUI  If I use a magic number  whenever the max size changes  I have to change in two code locations  If I forget one  this will lead to inconsistencies   The JDK is full of examples like in Integer  Character and Math classes   PS  Static analysis tools like FindBugs and PMD detects the use of magic numbers in your code and suggests the refactoring

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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

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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

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What about initializing a variable at the top of the class with a default value  For example   public class SomeClass       private int maxRows   15000                 Inside another method     for  int i   0  i  lt  maxRows  i                 Do something            public void setMaxRows int maxRows            this maxRows   maxRows             public int getMaxRows             return this maxRows          In this case  15000 is a magic number  according to CheckStyles   To me  setting a default value is okay  I don t want to have to do   private static final int DEFAULT MAX ROWS   15000  private int maxRows   DEFAULT MAX ROWS    Does that make it more difficult to read  I never considered this until I installed CheckStyles

[language-agnostic] What is a magic number, and why is it bad?

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