Dienstag, 5. März 2013

About Closures And Loops

About Closures And Loops

A frequently asked questions on StackOverflow is why the value of a variable that a function closes over isn't the value the programmer expects but another one. Usually this kind of question comes in combination with loop. Lets have a look on an example

The expected outcome of this would certainly be 1,4,9 but its 16,16,16.

What went wrong?

Less experienced programmers might think that creating a closures somehow copies the values of the accessed context during the creation of that closure. However that is not true.

What really happens!

When a function closes over a surrounding context it saves a reference to that enclosing context - that includes all variables within that context.
Now when accessing the context and its variables one gets the state of that context as it is at the time of access and not as it was during the creation of the closure.
In this specific case the value of the variable >>i<< after the finishing the first loop is 4. When executing the functions within the second loop its still 4.

What is the solution?

What you need is a way to create different context for each of the functions that you create.
The best way is to define a function that returns a function as a result. The returned function again closes over the defining function and its variables and parameters.

This will yield the expected outcome of 1,4,9.

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