What does yield mean in PHP

Question

I ve recently stumbled over this code   function xrange  min   max         for   i    min   i  lt    max   i              yield  i            I ve never seen this yield keyword before  Trying to run the code I get      Parse error  syntax error  unexpected T VARIABLE on line x   So what is this yield keyword  Is it even valid PHP  And if it is  how do I use it

User · Answer

This function is using yield  function a  items        foreach   items as  item            yield  item   1           It is almost the same as this one without  function b  items         result           foreach   items as  item             result      item   1            return  result     The only one difference is that a   returns a generator and b   just a simple array  You can iterate on both  Also  the first one does not allocate a full array and is therefore less memory-demanding

User · Answer

yield keyword serves for definition of  quot generators quot  in PHP 5 5  Ok  then what is a generator  From php net   Generators provide an easy way to implement simple iterators without the overhead or complexity of implementing a class that implements the Iterator interface  A generator allows you to write code that uses foreach to iterate over a set of data without needing to build an array in memory  which may cause you to exceed a memory limit  or require a considerable amount of processing time to generate  Instead  you can write a generator function  which is the same as a normal function  except that instead of returning once  a generator can yield as many times as it needs to in order to provide the values to be iterated over   From this place  generators   generators  other functions  just a simple functions    functions  So  they are useful when   you need to do things simple  or simple things   generator is really much simplier then implementing the Iterator interface  other hand is  ofcource  that generators are less functional  compare them   you need to generate BIG amounts of data - saving memory  actually to save memory we can just generate needed data via functions for every loop iteration  and after iteration utilize garbage  so here main points is - clear code and probably performance  see what is better for your needs   you need to generate sequence  which depends on intermediate values  this is extending of the previous thought  generators can make things easier in comparison with functions  check Fibonacci example  and try to make sequence without generator  Also generators can work faster is this case  at least because of storing intermediate values in local variables   you need to improve performance  they can work faster then functions in some cases  see previous benefit

User · Answer

None of the answers above show a concrete example using massive arrays populated by non-numeric members  Here is an example using an array generated by explode   on a large  txt file  262MB in my use case    lt  php  ini set  memory limit   1000M     echo  quot Starting memory usage   quot    memory get usage      quot  lt br gt  quot     path      file txt    content   file get contents  path    foreach explode  quot  n quot    content  as  ex         ex   trim  ex      echo  quot Final memory usage   quot    memory get usage     The output was  Starting memory usage  415160 Final memory usage  270948256  Now compare that to a similar script  using the yield keyword   lt  php  ini set  memory limit   1000M     echo  quot Starting memory usage   quot    memory get usage      quot  lt br gt  quot    function x          path      file txt        content   file get contents  path       foreach explode  quot  n quot    content  as  x            yield  x           foreach x   as  ex         ex   trim  ex      echo  quot Final memory usage   quot    memory get usage     The output for this script was  Starting memory usage  415152 Final memory usage  415616  Clearly memory usage savings were considerable   MemoryUsage ----- gt   270 5 MB in first example   450B in second example

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An interesting aspect  which worth to be discussed here  is yielding by reference  Every time we need to change a parameter such that it is reflected outside of the function  we have to pass this parameter by reference  To apply this to generators  we  simply prepend an ampersand  amp  to the name of the generator and to the variable used in the iteration     lt  php          Yields by reference      param int  from     function  amp counter  from        while   from  gt  0            yield  from           foreach  counter 100  as  amp  value         value--      echo  value                Output  99   98   97   96   95      The above example shows how changing the iterated values within the foreach loop changes the  from variable within the generator  This is because  from is yielded by reference due to the ampersand before the generator name  Because of that  the  value variable within the foreach loop is a reference to the  from variable within the generator function

User · Answer

simple example   lt  php echo   start main     function a        echo   start        for  i 1   i lt  9   i            yield  i      echo   end       foreach a   as  v      echo  v      echo   end main      gt    output   start main   start 1 2 3 4 5 6 7 8 9  end   end main    advanced example   lt  php echo   start main     function a        echo   start        for  i 1   i lt  9   i            yield  i      echo   end       foreach a   as  k   gt   v       if  k     5          break      echo  k    gt    v        echo   end main      gt    output   start main   start 0  gt 1 1  gt 2 2  gt 3 3  gt 4 4  gt 5  end main

User · Answer

With yield you can easily describe the breakpoints between multiple tasks in a single function  That s all  there is nothing special about it    closure   function   injected1   injected2             returned   array          task1 on  injected1      returned      returned1    I need a breakpoint here                                task2 on  injected2      returned      returned2                return  returned      returned    closure  injected1   injected2          If task1 and task2 are highly related  but you need a breakpoint between them to do something else    free memory between processing database rows run other tasks which provide dependency to the next task  but which are unrelated by understanding the current code doing async calls and wait for the results and so on       then generators are the best solution  because you don t have to split up your code into many closures or mix it with other code  or use callbacks  etc    You just use yield to add a breakpoint  and you can continue from that breakpoint if you are ready   Add breakpoint without generators    closure1   function   injected1         task1 on  injected1     return  returned1      closure2   function   injected2         task2 on  injected2     return  returned1            returned1    closure1  injected1     breakpoint between task1 and task2  returned2    closure2  injected2           Add breakpoint with generators   closure   function          injected1   yield        task1 on  injected1      injected2    yield  returned1          task2 on  injected2      injected3    yield  returned2                  yield  returnedN       generator    closure     returned1    generator- gt send  injected1     breakpoint between task1 and task2  returned2    generator- gt send  injected2          returnedN    generator- gt send  injectedN     note  It is easy to make mistake with generators  so always write unit tests before you implement them  note2  Using generators in an infinite loop is like writing a closure which has infinite length

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The below code illustrates how using a generator returns a result before completion  unlike the traditional non generator approach that returns a complete array after full iteration  With the generator below  the values are returned when ready  no need to wait for an array to be completely filled    lt  php   function sleepiterate  length        for   i 0   i  lt   length   i              sleep 2           yield  i           foreach  sleepiterate 5  as  i        echo  i  PHP EOL

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What is yield  The yield keyword returns data from a generator function   The heart of a generator function is the yield keyword  In its simplest form  a yield statement looks much like a return statement  except that instead of stopping execution of the function and returning  yield instead provides a value to the code looping over the generator and pauses execution of the generator function   What is a generator function  A generator function is effectively a more compact and efficient way to write an Iterator  It allows you to define a function  your xrange  that will calculate and return values while you are looping over it  function xrange  min   max        for   i    min   i  lt    max   i              yield  i                  foreach  xrange 1  10  as  key   gt   value        echo  quot  key   gt   value quot   PHP EOL     This would create the following output  0   gt  1 1   gt  2     9   gt  10  You can also control the  key in the foreach by using yield  someKey   gt   someValue   In the generator function   someKey is whatever you want appear for  key and  someValue being the value in  val  In the question s example that s  i  What s the difference to normal functions  Now you might wonder why we are not simply using PHP s native range function to achieve that output  And right you are  The output would be the same  The difference is how we got there  When we use range PHP  will execute it  create the entire array of numbers in memory and return that entire array to the foreach loop which will then go over it and output the values  In other words  the foreach will operate on the array itself  The range function and the foreach only  quot talk quot  once  Think of it like getting a package in the mail  The delivery guy will hand you the package and leave  And then you unwrap the entire package  taking out whatever is in there  When we use the generator function  PHP will step into the function and execute it until it either meets the end or a yield keyword  When it meets a yield  it will then return whatever is the value at that time to the outer loop  Then it goes back into the generator function and continues from where it yielded  Since your xrange holds a for loop  it will execute and yield until  max was reached  Think of it like the foreach and the generator playing ping pong  Why do I need that  Obviously  generators can be used to work around memory limits  Depending on your environment  doing a range 1  1000000  will fatal your script whereas the same with a generator will just work fine  Or as Wikipedia puts it   Because generators compute their yielded values only on demand  they are useful for representing sequences that would be expensive or impossible to compute at once  These include e g  infinite sequences and live data streams   Generators are also supposed to be pretty fast  But keep in mind that when we are talking about fast  we are usually talking in very small numbers  So before you now run off and change all your code to use generators  do a benchmark to see where it makes sense  Another Use Case for Generators is asynchronous coroutines  The yield keyword does not only return values but it also accepts them  For details on this  see the two excellent blog posts linked below  Since when can I use yield  Generators have been introduced in PHP 5 5  Trying to use yield before that version will result in various parse errors  depending on the code that follows the keyword  So if you get a parse error from that code  update your PHP  Sources and further reading   Official docs The original RFC kelunik s blog  An introduction to generators ircmaxell s blog  What generators can do for you NikiC s blog  Cooperative multitasking using coroutines in PHP Co-operative PHP Multitasking What is the difference between a generator and an array  Wikipedia on Generators in general

[php] What does yield mean in PHP?

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