how does this dynamic int[] array work?

Programming Questions

theadlab

how does this dynamic int[] array work?

in Programming Questions • 1 year ago

Hey Guys,

I've noticed that arrays used with split() (from the example given on the reference for split()) appear to be dynamic and require no explicit size initilialisation. That's pretty handy! The nums array below is an example.

int[] nums; // array to store numbers
String numbers = "8 67 5 309";
nums = int(split(numbers, ' '));

I find this interesting because the reference page on arrays doesn't list this sort of array initialisation, this makes it a bit tricky to figure out the syntax for this case, so I ran into problems using it. I'm trying to create a similar dynamic array that stores the line numbers of certain entries of a text file stored in String lines[]. Each line is of the format '1) 5,10,12,5,0' with changing variables. The first number is the line number.

int pen_down_lines[]; // array to store data
int k = 0; // index of the array
for (int i = 0; i<lines.length; i++){ // for each line
String temp[] = split(lines[i], ") "); // split original string into line numbers and data
nums = float(split(temp[1], ", ")); // parse data with comma operator
if (nums[4]==0){ // if a 'pen down' command is located
print(int(temp[0]) + ") "); // print the line number
int h = int(temp[0]); // extract the line number
pen_down_lines[k] = h; // store the line number in an array
k++; // increment the index
}
}

I get a Null Pointer Exception on line 13. Why is this syntax invalid for this case but not for the case on line 7?

I realise there are other ways to use dynamic and undefined arrays in Processing (I've done it before), this just seems like an elegant solution that would be great if I could get it to work.

Thanks,

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Replies(8)

PhiLho

Re: how does this dynamic int[] array work?

1 year ago

The array is dynamic but the final size is known in advance. Ie. you create an array on the fly, but its size is fixed from the start.
If you need an array of variable dimensions, use ArrayList.

theadlab

Re: how does this dynamic int[] array work?

1 year ago

Okay cool, so it stays dynamic until you first use it, then its size is fixed. Is that right?

I've just tried setting up a global variable of int[] = pen_down_lines and then setting it's size in setup() (after determining how big it has to be with some code). This is still useful as I want my array to be global but don't know the size during declaration.

Thanks for the fast response.

trzewiczek

Re: how does this dynamic int[] array work?

1 year ago

So, there is a distiction between the variable declaration and initialization. You can declare a variable of an array type with no actual initialization and initialize it later:

int[] myArray; // declare
myArray = new int[ 25 ]; // init

In the first line the reference object (I'm not trained Java dev, so correct me if I'm wrong) is created. It points to nowhere so if you like to use it, the NullPointerException will visit your home.
In the second line the array object is created and it's address is assigned to the referece, so from this moment on, the variable myArray points the int array object. So - the reference just points the way to the actual objct of 25 ints.
As you probably know, you can create an array object by such an initialization:

int[] myArray = { 1, 2, 3, 4 };

It's all the same way. In one line you declare a reference variable (left side of =), you create a int array object containing four numbers and with a fix size of 4 ints (right side of =) and then you assign an object to the reference variable.

Now a little of magic. Java is a very gentle language. It assumes that if you don't provide an explicit declaration of array content, it's all going to be default values for a certain data type. These two are equivalent:

int[] myArray1 = { 0, 0, 0 };
int[] myArray2 = new int[3];

If you try to println the content of the second array, you'll notice it's all zeros - the default for the int type.

So... The array is not dynamic. It's the difference in declaration and initialization. After initialization the size of an array is always fixed. As phi.lho pointed above - if you would ever need the flexible size of an array, use ArrayList object. It's not so intuitive in the first moment, but gives you some flexibility, that comes handy especially when user input is on stack.

thomas.di..

Re: how does this dynamic int[] array work?

1 year ago

since trzewiczek explained it in detail, there is one more variant: "new int[]{...}" which can be convenient sometimes.

int[] myArray1 = { 0, 0, 0 };
int[] myArray2 = new int[3];
// in case of a new initialisation of myArray1
myArray1 = new int[]{ 1, 2, 3 };
// or as a function parameter
// aFunctionThatWantsAnArray( new int[]{ 1, 2, 3 } );

trzewiczek

Re: Re: how does this dynamic int[] array work?

1 year ago

I forgot about this form! You're so true - sometimes it's very handy, but not so intuitive. Thx!

theadlab

Re: how does this dynamic int[] array work?

1 year ago

Thanks trzewiczek and thomas, that certainly explains things a bit further.

The 'declare then initialise-size-later' method is good to know about for flexible one-shot applications (without getting into ArrayLists). I just finished a flexible text parser with it [oops! I didn't know I wasn't supposed to post links - have removed it after getting a spam complaint!]

Nice to learn and apply something in the same day :-)

PhiLho

Re: how does this dynamic int[] array work?

1 year ago

Lol! I indeed got a spam warning on this thread... because the automaton detected the suspicious words: pen is
Well, it got better, in the past, I had warnings because of words like increase size, which aren't uncommon in the Processing world.

theadlab

Re: Re: how does this dynamic int[] array work?

1 year ago

Ha ha, that is hilarious! I'll never write variables names so innocently again!

And I thought I was in trouble for posting a link to my personal site with the application and source code. Oh well, for fear of another warning here's the link again for anyone who's interested: http://dada.soulwire.co.uk/2012/01/movement-sequencing-from-a-text-file/