Array of an unknown length in C

Question

I ve just started learning C  and in the introduction to arrays they showed how to establish a variable as an array but is seems that one must specify the length of the array at assignment  so what if I don t know the length of the array

User · Answer

In a nutshell  please use Collections and Generics   It s a must for any C  developer  it s worth spending time to learn

User · Answer

Use List lt  gt  to build up an  array  of unknown length   Use List lt  gt  ToArray   to return a real array  and not a List   var list   new List lt int gt     list Add 1   list Add 2   list Add 3   var array   list ToArray

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A little background information   As said  if you want to have a dynamic collection of things  use a List lt T gt   Internally  a List uses an array for storage too  That array has a fixed size just like any other array  Once an array is declared as having a size  it doesn t change  When you add an item to a List  it s added to the array  Initially  the List starts out with an array that I believe has a length of 16  When you try to add the 17th item to the List  what happens is that a new array is allocated  that s  I think  twice the size of the old one  so 32 items  Then the content of the old array is copied into the new array  So while a List may appear dynamic to the outside observer  internally it has to comply to the rules as well    And as you might have guessed  the copying and allocation of the arrays isn t free so one should aim to have as few of those as possible and to do that you can specify  in the constructor of List  an initial size of the array  which in a perfect scenario is just big enough to hold everything you want  However  this is micro-optimization and it s unlikely it will ever matter to you  but it s always nice to know what you re actually doing

User · Answer

As detailed above  the generic List lt   is the best way of doing it   If you re stuck in  NET 1    then you will have to use the ArrayList class instead  This does not have compile-time type checking and you also have to add casting - messy   Successive versions have also implemented various variations - including thread safe variants

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var yummy   new List lt string gt     while person FeelsHappy          yummy Add person GetNewFavoriteFood       Console WriteLine  Sweet  I have a list of size  0     list Count   Console WriteLine  I didn t even need to know how big to make it          until I finished making it

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If you  really need to use an array instead of a list  then you can create an array whose size is calculated at run time like so     e g i want a two dimensional array of size n by n  n will be gotten at run time from the user  int n   0  bool isInteger   int TryPase Console ReadLine    out n   var x   new int n n

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Arrays must be assigned a length  To allow for any number of elements  use the List class   For example   List lt int gt  myInts   new List lt int gt     myInts Add 5   myInts Add 10   myInts Add 11   myInts Count      3

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Use an ArrayList if in  NET 1 x  or a List lt yourtype gt  if in  NET 2 0 or 3 x   Search for them in System Collections and System Collections Generics

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You can create an array with the size set to a variable  i e   int size   50  string   words   new string size      contains 50 strings   However  that size can t change later on  if you decide you need 100 words   If you need the size to be really dynamic  you ll need to use a different sort of data structure   Try List

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try a generic list instead of array

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You might also want to look into Dictionarys if your data is unique  This will give you two columns to work with   User name   Total bill  it gives you a lot of built in tools to search and update just the value

[c#] Array of an unknown length in C#

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