Best way to push into C array

Question

Good day all  So I know that this is a fairly widely discussed issue  but I can t seem to find a definitive answer  I know that you can do what I am asking for using a List  but that won t solve my issue   I teach a class in beginner C   I have a PHP and Java Script background  and thus tend to think in terms of these languages    I have been looking for the best way to show my students how to add elements to an array on the fly We can t use Lists  as these are not part of the syllabus that I currently teach  although they do come in later in the semester    I am thus looking for the simplest way to perform a Java array push newValue  type scenario that will be understandable to novice coders without teaching them bad practice   Here is how I have been approaching the problem so far   for  int i   0  i  lt  myArray Length  i         if myArray i     null              myArray i    newValue         break           I just need to know if this is an acceptable approach  if I am teaching them any bad practices  or if there is a better   simpler way of achieving my goal   EDIT The crux of the matter is that the element needs to be added into the first empty slot in an array  lie a Java push function would do   Any advice would be greatly appreciated

User · Answer

As per comment  That is not pushing to an array  It is merely assigning to it   If you looking for the best practice to assign value to array then its only way that you can assign value   Array index   value    there is only way to assign value when you do not want to use List

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I surggest using the CopyTo method  so here is my two cent on a solution that is easy to explain and simple  The CopyTo method copies the array to another array at a given index  In this case myArray is copied to newArr starting at index 1 so index 0 in newArr can hold the new value   quot Push quot  to array online demo var newValue   1  var myArray   new int 5    2  3  4  5  6    var newArr   new int myArray Length   1    myArray CopyTo newArr  1   newArr 0    newValue         debug for var i   0  i  lt  newArr Length  i         Console WriteLine newArr i  ToString          output 1 2 3 4 5 6

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As said before  List provides functionality to add elements in a clean way  to do the same with arrays  you have to resize them to accomodate extra elements  see code below   int   arr   new int 2   arr 0    1  arr 1    2    without this line we d get a exception Array Resize ref arr  3   arr 2    3    Regarding your idea with loop   elements of an array are set to their default values when array is initialized  So your approach would be good  if you want to fill  blanks  in array holding reference types  which have default value null    But it wouldn t work with value types  as they are initialized with 0

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Here is my solution for this public void ArrayPush lt T gt  ref T   table  object value        Array Resize ref table  table Length   1      Resizing the array for the cloned length   -    1      table SetValue value  table Length - 1      Setting the value for the new element    How to use it  So simple  Array Push Example string   table      quot apple quot    quot orange quot     ArrayPush ref table   quot banana quot     Array ForEach table   element    gt  Console WriteLine element        quot apple quot      quot orange quot      quot banana quot   Really simple  amp  useful  It is some tricky but it works

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C  is a little different in this respect with JavaScript  Because of the strict checks  you define the size of the array and you are supposed to know everything about the array such as its bounds  where the last item was put and what items are unused  You are supposed to copy all the elements of the array into a new  bigger one  if you want to resize it   So  if you use a raw array  there s no other way other than to maintain the last empty  available index assign items to the array at this index  like you re already doing   However  if you d like to have the runtime maintain this information and completely abstract away the array  but still use an array underneath  then C  provides a class named ArrayList that provides this abstraction   The ArrayList abstracts a loosely typed array of Objects  See the source here   It takes care of all the issues like resizing the array  maintaining the last index that is available  etc  However  it abstracts   encapsulates this information from you  All you use is the ArrayList   To push an item of any type into the underlying array at the end of the underlying array  call the Add method on the ArrayList like so      you may or may not define a size using a constructor overload    var arrayList   new ArrayList      arrayList Add  Foo      EDIT  A note about type restriction  Like every programming language and runtime does  C  categorizes heap objects from those that will not go on the heap and will only stay on the function s argument stack  C  notes this distinction by the name Value types vs  Reference types  All things whose value goes on the stack are called Value types  and those that will go on the heap are called Reference types  This is loosely similar to JavaScript s distinction between objects and literals   You can put anything into an ArrayList in C   whether the thing is a value type or a reference type  This makes it closest to the JavaScript array in terms of typelessness  although neither of the three -- the JavaScript array  the JavaScript language and the C  ArrayList -- are actually type-less   So  you could put a number literal  a string literal  an object of a class you made up  a boolean  a float  a double  a struct  just about anything you wanted into an ArrayList   That is because the ArrayList internally maintains and stores all that you put into it  into an array of Objects  as you will have noted in my original answer and the linked source code   And when you put something that isn t an object  C  creates a new object of type Object  stores the value of the thing you put into the ArrayList into this new Object type object  This process is called boxing and isn t very much unlike the JavaScript boxing mechanism   For e g  in JavaScript  while you could use a numeric literal to invoke a function on the Number object  you couldn t add something to the number literal s prototype      Valid javascript var s   4 toString        Invalid JavaScript code 4 prototype square        gt  4   4  var square   4 square      Just like JavaScript boxes the numeric literal 4 in the call to the toString method  C  boxes all things that are not objects into an Object type when putting them into an ArrayList   var arrayList   new ArrayList     arrayList Add 4      The value 4 is boxed into a  new Object    first and then that new object is inserted as the last element in the  ArrayList     This involves a certain penalty  as it does in the case of JavaScript as well   In C   you can avoid this penalty as C  provides a strongly typed version of the ArrayList  known as the List lt T gt   So it follows that you cannot put anything into a List lt T gt   just T types   However  I assume from your question s text that you already knew that C  had generic structures for strongly typed items  And your question was to have a JavaScript like data structure exhibiting the semantics of typelessness and elasticity  like the JavaScript Array object  In this case  the ArrayList comes closest   It is also clear from your question that your interest was academic and not to use the structure in a production application   So  I assume that for a production application  you would already know that a generic   strongly typed data structure  List lt T gt  for example  is better performing than its non-typed one  ArrayList for example

User · Answer

There are couple of ways this can be done    First  is converting to list and then to array again   List lt int gt  tmpList   intArry ToList    tmpList Add anyInt   intArry   tmpList ToArray      Now this is not recommended as you convert to list and back again to array  If you do not want to use a list  you can use the second way which is assigning values directly into the array   int   terms   new int 400   for  int runs   0  runs  lt  400  runs          terms runs    value      This is the direct approach and if you do not want to tangle with lists and conversions   this is recommended for you

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array push is like List lt T gt  Add    NET arrays are fixed-size so you can t actually add a new element   All you can do is create a new array that is one element larger than the original and then set that last element  e g   Array Resize ref myArray  myArray Length   1   myArray myArray GetUpperBound 0     newValue    EDIT   I m not sure that this answer actually applies given this edit to the question      The crux of the matter is that the element needs to be added into the   first empty slot in an array  lie a Java push function would do    The code I provided effectively appends an element   If the aim is to set the first empty element then you could do this   int index   Array IndexOf myArray  null    if  index    -1        myArray index    newValue      EDIT   Here s an extension method that encapsulates that logic and returns the index at which the value was placed  or -1 if there was no empty element   Note that this method will work for value types too  treating an element with the default value for that type as empty   public static class ArrayExtensions       public static int Push lt T gt  this T   source  T value                var index   Array IndexOf source  default T             if  index    -1                        source index    value                     return index

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I don t think there is another way other than assigning value to that particular index of that array

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I don t understand what you are doing with the for loop  You are merely iterating over every element and assigning to the first element you encounter  If you re trying to push to a list go with the above answer that states there is no such thing as pushing to a list  That really is getting the data structures mixed up  Javascript might not be setting the best example  because a javascript list is really also a queue and a stack at the same time

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Check out this documentation page  https   msdn microsoft com en-us library ms132397 v vs 110  aspx  The Add function is the first one under Methods

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Your question is a little off the mark   In particular  you say  that the element needs to be added into the first empty slot in an array  lie  sic  a Java push function would do     Java s Array does not have a push operation - JavaScript does   Java and JavaScript are two very different languages JavaScript s push function does not behave as you describe   When you  push  a value into a JavaScript array  the array is extended by one element  and that new element is assigned the pushed value  see  Mozilla s Array prototype push function docs   The verb  Push  is not something that is used with an Array in any language that I know of except JavaScript   I suspect that it s only in JavaScript because it could be there  since JavaScript is a completely dynamic language    I m pretty sure it wasn t designed in intentionally   A JavaScript-style Push operation in C  could be written in this somewhat inefficient manner   int    myArray   new int     1  2  3  4   var tempList   myArray ToList    tempList Add 5   myArray   tempList ToArray        equiv  myArray Push 5      Push  is used in some types of containers  particularly Stacks  Queues and Deques  which get two pushes - one from the front  one from the back    I urge you not to include Push as a verb in your explanation of arrays   It adds nothing to a CS student s vocabulary   In C   as in most traditional procedural languages  an array is a collection of elements of a single type  contained in a fixed length contiguous block of memory   When you allocate an array  the space for every array element is allocated  and  in C  those elements are initialized to the default value of the type  null for reference types    In C   arrays of reference types are filled with object references  arrays of value types are filled with instances of that value type   As a result  an array of 4 strings uses the same memory as an array of 4 instance of your application class  since they are both reference types    But  an array of 4 DateTime instances is significantly longer that of an array of 4 short integers   In C   an instance of an array is an instance of System Array  a reference type   Arrays have a few properties and methods  like the Length property    Otherwise  there isn t much you can do with an array  you can read  or write  from  or to  individual elements using an array index   Arrays of type T also implement IEnumerable lt T gt   so you can iterate through the elements of an array   Arrays are mutable  the values in an array can be written to   but they have a fixed length - they can t be extended or shortened   They are ordered  and they can t be re-arranged  except by swizzling the values manually    C  arrays are covariant   If you were to ask the C  language designers  this would be the feature they regret the most   It s one of the few ways you can break C  type safety   Consider this code  assuming that Cat and Dog classes inherit from Animal    Cat   myCats   new Cat   myCat  yourCat  theirCat   Animal   animals    Animal    myCats        legal but dangerous animals 1    new Dog                        heading off the cliff myCats 1  Speak                             Woof    That  feature  is the result of the lack of generic types and explicit covariance contravariance in the initial version of the  NET Framework and the urge to copy a Java  feature    Arrays do show up in many core  NET APIs  for example  System Reflection    They are there  again  because the initial release did not support generic collections   In general  an experienced C  programmer will not use many arrays in his applications  preferring to use more capable collections such as List lt T gt   Dictionary lt TKey  TValue gt   HashSet lt T gt  and friends   In particular  that programmer will tend to pass collections around using IEnumerable lt T gt  an interface that all collections implement   The big advantage of using IEnumerable lt T gt  as parameters and return types  where possible and logical  is that collections accessed via IEnumerable lt T gt  references are immutable   It s kinda-sorta like using const correctly in C     One thing you might consider adding in to your lectures on arrays - after everyone has mastered the basics - is the new Span lt T gt  type  Spans may make C  arrays useful   Finally  LINQ  Language Integrated Query  introduced a lot of functionality to collections  by adding Extension Methods to IEnumerable lt T gt     Make sure your student do not have a using System Linq  statement up at the top of their code - mixing LINQ in to a beginning student s class on arrays would bewilder him or her   BTW  what kind of class is it you teach  At what level

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The is no array push newValue  in C   You don t push to an Array in C   What we use for this is a List lt T gt   What you may want to consider  for teaching purpose only  is the ArrayList  no generic and it is a IList  so       static void Main            Create an ArrayList and add 3 elements      ArrayList list   new ArrayList        list Add  quot One quot       Add is your push     list Add  quot Two quot        list Add  quot Three quot

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This is acceptable as assigning to an array  But if you are asking for pushing  I am pretty sure its not possible in array  Rather it can be achieved by using Stack  Queue or any other data structure  Real arrays doesn t have such functions  But derived classes such as ArrayList have it

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