Hi

I have been trying to understand passing an array of strings to a function as a parameter.

In the Processing book it says when you pass an array it just copies the location as a parameter and any manipulation of the array within the function is changing the original contents of the array, and will persist after the function has finished.

This is the behaviour with an array of floats, but if I pass an array of Strings, this doesn't seem to happen and the changes I make to the array are not reflected in the original.

I have to declare the return type of the function as String[], and explicitly return the array from inside the function, and assign it to the array that gets passed in to get them out of the function.

eg thingsToType is a String[], and I am having to use

thingsToType=loadPrefs(thingsToType);

I realise String is a Composite data type rather than a primitive and suspect it is to do with this.

Its working for me, but I am wondering whether this is the best way to do it. Is the function having to copy the contents of the array to return it, is there a better way?

Thanks

Yep your example does exactly what I would expect.

So in the function I a using I am loading text from a file - in this stripped down test a file called fred.txt, that has a few lines of text. I have stripped out all the code that checks whether the file is there and does different things if it isn't (I thought maybe what it did if the file wasn't there was causing the problem as it was initialising the array with  new statement).

But stripping the code right down to just this still produces the behaviour.


Quote:



Produces

[0] "a"
[1] "a"
[2] "a"
inside
[0] "me"
[1] "you"
[2] "banana"
outside
[0] "a"
[1] "a"
[2] "a"

If I send the array in empty I get

null
inside
[0] "me"
[1] "you"
[2] "banana"
outside
null

This is one of those very funny things about parameter passing.  Yes, you are passing in a reference to the array in memory so that you can change the contents of that array.  However, you cannot change the reference itself.  And this is what you are doing in your code -- setting the array in its entirety to a new array (as opposed to the individual elements themselves.)  For example, this will work:

Code:

void loadPrefs( String[] arr){
  String[] arr2= loadStrings("fred.txt");
  for (int i = 0; i < arr.length; i++) {
    arr[i] = arr2[i]; 
  }
  println("inside");
  println (arr);
} 
 

However, what I would suggest doing in this case is simply returning a reference to the new array.

Code:

String[] outsideArr={
  "a", "a", "a"};
 
void setup(){  
  println(outsideArr);  
  outsideArr = loadPrefs();
  println("outside");
  println(outsideArr); 
}
 
String[] loadPrefs(){
  return loadStrings("fred.txt");
}