What in layman s terms is a Recursive Function using PHP

Question

Can anyone please explain a recursive function to me in PHP  without using Fibonacci  in layman language and using examples  i was looking at an example but the Fibonacci totally lost me   Thank you in advance  -  Also how often do you use them in web development

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Its a function that calls itself  Its useful for walking certain data structures that repeat themselves  such as trees  An HTML DOM is a classic example   An example of a tree structure in javascript and a recursive function to  walk  the tree       1          2   3              4   5   --  var tree       id  1    left        id  2      left  null      right  null        right        id  3      left          id  4        left  null        right  null            right          id  5        left  null        right  null                To walk the tree  we call the same function repeatedly  passing the child nodes of the current node to the same function  We then call the function again  first on the left node  and then on the right   In this example  we ll get the maximum depth of the tree  var depth   0   function walkTree node  i         Increment our depth counter and check   i      if  i  gt  depth  depth   i       call this function again for each of the branch nodes  recursion     if  node left    null  walkTree node left  i     if  node right    null  walkTree node right  i        Decrement our depth counter before going back up the call stack   i--      Finally we call the function  alert  Tree depth     walkTree tree  0      A great way of understanding recursion is to step through the code at runtime

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Here is a practical example  there are several good ones already   I just wanted to add one that is useful to almost any developer   At some point  developers will need to parse an object as with a response from an API or some type of object or array   This function is initially called to parse an object which may just contain parameters  but what if the object also contains other objects or arrays  This will need to be addressed  and for the most part the basic function already does this  so the function just calls itself again  after confirming that the key or value is either an object or an array  and parses this new object or array  Ultimately what is returned is a string that creates each parameter on a line by itself for readability  but you could just as easily log the values to a log file or insert into a DB or whatever   I added the  prefix parameter to use the parent element to help describe the end variable so that we can see what the value pertains to  It doesn t include things like null values  but this can be amended from this example   If you have the object    apiReturn   new stdClass     apiReturn- gt shippingInfo   new stdClass     apiReturn- gt shippingInfo- gt fName    Bill    apiReturn- gt shippingInfo- gt lName    Test    apiReturn- gt shippingInfo- gt address1    22 S  Deleware St     apiReturn- gt shippingInfo- gt city    Chandler    apiReturn- gt shippingInfo- gt state    AZ    apiReturn- gt shippingInfo- gt zip   85225   apiReturn- gt phone    602-312-4455    apiReturn- gt transactionDetails   array       totalAmt    gt   100 00         currency    gt   USD         tax    gt   5 00         shipping    gt   5 00      apiReturn- gt item   new stdClass     apiReturn- gt item- gt name    T-shirt    apiReturn- gt item- gt color    blue    apiReturn- gt item- gt itemQty   1    and use   var dump  apiReturn     it will return      object stdClass  1  4      shippingInfo     object stdClass  2  6      fName     string 4   Bill    lName     string 4   Test    address1     string 18   22 S  Deleware St     city     string 8   Chandler    state     string 2   AZ    zip     int 85225      phone     string 12   602-312-4455    transactionDetails     array 4      totalAmt     string 6   100 00    currency     string 3   USD    tax     string 4   5 00    shipping     string 4   5 00      item     object stdClass  3  3      name     string 7   T-shirt    color     string 4   blue    itemQty     int 1        and here is the code to parse it into a string with a line break for each parameter   function parseObj  obj   prefix             stringRtrn           foreach  obj as  key  gt  value           if  prefix               switch   key                    case is array  key                       foreach  key as  k  gt  v                            stringRtrn    parseObj  key   obj                                             break                  case is object  key                        stringRtrn    parseObj  key   obj                       break                  default                      switch   value                            case is array  value                                stringRtrn    parseObj  value   key                               break                          case is object  value                                stringRtrn    parseObj  value   key                               break                          default                               stringRtrn     prefix        key          value    lt br gt                                break                                            break                          else      end if  prefix              switch  key                   case is array  key                        stringRtrn    parseObj  key   obj                       break                  case is object  key                         stringRtrn    parseObj  key   obj                       break                  default                      switch   value                            case is array  value                                stringRtrn    parseObj  value   key                               break                          case is object  value                                stringRtrn    parseObj  value   key                               break                                                default                               stringRtrn     key          value    lt br gt                                break                           end inner switch                   end outer switch              end else          end foreach  obj as  key  gt  value      return  stringRtrn       END parseObj     This will return the object as follows   shippingInfo fName   Bill shippingInfo lName   Test shippingInfo address1   22 S  Deleware St  shippingInfo city   Chandler shippingInfo state   AZ shippingInfo zip   85225 phone   602-312-4455 transactionDetails totalAmt   100 00 transactionDetails currency   USD transactionDetails tax   5 00 transactionDetails shipping   5 00 item name   T-shirt item color   blue item itemQty   1   I did the nested switch statements to avoid confusion with if       ifelse       else  but it was almost as long  If it helps  just ask for the if conditionals and I can paste them for those who need it

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Recursion is something that repeats itself   Like a function that calls itself from within itself   Let me demonstrate in a somewhat pseudo example   Imagine you re out with your buddies drinking beer  but your wife is going to give you hell if you don t come home before midnight  For this purpose  let s create the orderAndDrinkBeer  time  function where  time is midnight minus the time it takes you to finish your current drink and get home   So  arriving at the bar  you order your first beer and commence the drinking    timeToGoHome    23       Let s give ourselves an hour for last call and getting home  function orderAndDrinkBeer  timeToGoHome        Let s create the function that s going to call itself       beer   New Beer        Let s grab ourselves a new beer      currentTime   date  G       Current hour in 24-hour format      while   beer- gt status     empty         Time to commence the drinking loop          beer- gt drink        Take a sip or two of the beer or chug if that s your preference                Now we re out of the drinking loop and ready for a new beer      if   currentTime  lt   timeToGoHome       BUT only if we got the time         orderAndDrinkBeer  timeToGoHome       So we make the function call itself again        else       Aw  snap   It is time  S         break     Let s go home              Now let s just hope you weren t able to drink enough beer to become so intoxicated that your wife is going to make you sleep on the couch regardless of being home on time - -  But yeah  that s pretty much how recursion goes

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It is very simple  when a function calls itself for accomplishing a task for undefined and finite number of time  An example from my own code  function for populating a  with  multilevel category tree  function category tree  parent 0  sep            q  select id name from categorye where parent id    parent       rs mysql query  q       while  rd mysql fetch object  rs                 echo  id       sep  rd- name              category tree  rd- id  sep  --

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Recursion is an alternative to loops  it s quite seldom that they bring more clearness or elegance to your code  A good example was given by Progman s answer  if he wouldn t use recursion he would be forced to keep track in which directory he is currently  this is called state  recursions allows him to do the bookkeeping using the stack  the area where variables and return adress of a method are stored   The standard examples factorial and Fibonacci are not useful for understanding the concept because they re easy to replace by a loop

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Simply put  a recursive function is a function that calls itself

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An example would be to print every file in any subdirectories of a given directory  if you have no symlinks inside these directories which may break the function somehow   A pseudo-code of printing all files looks like this   function printAllFiles  dir        foreach  getAllDirectories  dir  as  f            printAllFiles  f      here is the recursive call           foreach  getAllFiles  dir  as  f            echo  f            The idea is to print all sub directories first and then the files of the current directory  This idea get applied to all sub directories  and thats the reason for calling this function recursively for all sub directories   If you want to try this example you have to check for the special directories   and     otherwise you get stuck in calling printAllFiles      all the time  Additionally you must check what to print and what your current working directory is  see opendir    getcwd

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Laymens terms  A recursive function is a function that calls itself A bit more in depth  If the function keeps calling itself  how does it know when to stop  You set up a condition  known as a base case  Base cases tell our recursive call when to stop  otherwise it will loop infinitely  What was a good learning example for me  since I have a strong background in math  was factorial  By the comments below  it seems the factorial function may be a bit too much  I ll leave it here just in case you wanted it  function fact  n      if   n     0       our base case      return 1        else        return  n   fact  n-1       lt --calling itself         In regards to using recursive functions in web development  I do not personally resort to using recursive calls  Not that I would consider it bad practice to rely on recursion  but they shouldn t be your first option  They can be deadly if not used properly  Although I cannot compete with the directory example  I hope this helps somewhat   4 20 10  Update  It would also be helpful to check out this question  where the accepted answer demonstrates in laymen terms how a recursive function works  Even though the OP s question dealt with Java  the concept is the same   Understanding basic recursion

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It work a simple example recursive  Y     lt  php    function factorial  y  x          if   y  lt   x             echo  y         else            echo  x           factorial  y  x 1              y 10    x 0  factorial  y  x       gt

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Basically this  It keeps calling itself until its done  void print folder string root        Console WriteLine root       foreach var folder in Directory GetDirectories root                 print folder folder             Also works with loops   void pretend loop int c        if c  0  return      print  hi       pretend loop c-       You can also trying googling it  Note the  Did you mean   click on it      http   www google com search q recursion amp spell 1

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Best explanation I have found when I was learning that myself is here http   www elated com articles php-recursive-functions   Its because one thing   The function when its called is created in memory  new instance is created   So the recursive function IS NOT CALLLING ITSELF  but its calling other instance - so its not one function in memory doing some magic  Its couple of instances in memory which are returning themselves some values - and this behavior is the same when for example function a is calling function b  You have two instances as well as if recursive function called new instance of itself   Try draw memory with instances on paper - it will make sense

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Walking through a directory tree is a good example  You can do something similar to process an array  Here is a really simple recursive function that simply processes a string  a simple array of strings  or a nested array of strings of any depth  replacing instances of  hello  with  goodbye  in the string or the values of the array or any sub-array    function replaceHello  a        if    is array  a              a   str replace  hello    goodbye    a         else           foreach  a as  key   gt   value                 a  key    replaceHello  value                       return  a     It knows when to quit because at some point  the  thing  it is processing is not an array  For example  if you call replaceHello  hello    it will return  goodbye   If you send it an array of strings  though it will call itself once for every member of the array  then return the processed array

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This is a very simple example of factorial with Recursion   Factorials are a very easy maths concept  They are written like 5  and this means 5   4   3   2   1  So 6  is 720 and 4  is 24   function factorial  number          if   number  lt  2             return 1         else            return   number   factorial  number-1                hope this is usefull for you

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Recursion is a fancy way of saying  Do this thing again until its done    Two important things to have     A base case - You ve got a goal to get to  A test - How to know if you ve got to where you re going    Imagine a simple task  Sort a stack of books alphabetically  A simple process would be take the first two books  sort them  Now  here comes the recursive part  Are there more books  If so  do it again  The  do it again  is the recursion  The  are there any more books  is the test  And  no  no more books  is the base case

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Recursion used for Kaprekar s constant   function KaprekarsConstant  num   count   1         input   str split  num       sort  input         ascendingInput    implode  input        descendingInput   implode array reverse  input          result    ascendingInput  gt   descendingInput             ascendingInput -  descendingInput             descendingInput -  ascendingInput       if   result    6174            return KaprekarsConstant sprintf   04d    result    count   1              return  count       The function keeps calling itself with the result of the calculation until it reaches Kaprekars constant  at which it will return the amount of times the calculations was made    edit For anyone that doesn t know Kaprekars Constant  it needs an input of 4 digits with at least two distinct digits

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If you add a certain value  say   1   to Anthony Forloney s example  everything would be clear   function fact 1      if  1     0       our base case   return 1        else     return 1   fact 1-1       lt --calling itself          original   function fact  n      if   n     0       our base case     return 1        else     return  n   fact  n-1       lt --calling itself

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Recursion occurs when something contains  or uses  a similar version of itself When talking specifically about computer programming  recursion occurs when a function calls itself   The following link helps me to understand recursion better  even I consider this is the best so far what I ve learned  It comes with an example to understand how the inner recursive  functions called while executing  Please go through the article and I have pasted the test program in case if the article got trashed  You may please run the program in local server to see it in action  https   www elated com php-recursive-functions   lt  php  function factorial   n           Base case   if    n    0         echo  quot Base case   n   0  Returning 1    lt br gt  quot       return 1            Recursion   echo  quot  n    n  Computing  n   factorial   quot      n-1     quot       lt br gt  quot      result      n   factorial   n-1        echo  quot Result of  n   factorial   quot      n-1     quot       result  Returning  result    lt br gt  quot     return  result     echo  quot The factorial of 5 is   quot    factorial  5       gt

[php] What in layman's terms is a Recursive Function using PHP

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