Dynamic alternative to 2D array

The_B_Man · 31-05-2012 06:29PM #1

If I have an array like so:

String[][] myArray = [4096][4096];

But may only use it for 10x10 rows, whats the best way to go about it?

Can you do a vector of vectors? ArrayList of ArrayLists? Whats the most efficient option here?

Thanks.

Colonel Panic · 31-05-2012 07:05PM

A dynamic 1D array, a function to index into it and a function to calculate the required size.

unknownlegend · 31-05-2012 07:07PM

Looks like you want key value pairs?

Dictionary datastructure should suffice?

adigilani · 01-06-2012 12:52AM

a hashmap would be good...
read the documentations...

The_B_Man wrote: »

If I have an array like so:

String[][] myArray = [4096][4096];

But may only use it for 10x10 rows, whats the best way to go about it?

Can you do a vector of vectors? ArrayList of ArrayLists? Whats the most efficient option here?

Thanks.

srsly78 · 01-06-2012 09:25AM

What language? Seems to be java? -.-

In c++ you could use vector of vectors.

Anima · 01-06-2012 01:42PM

What are you actually trying to do? Kind of pointless making a suggestion without knowing.

Aswerty · 01-06-2012 02:00PM

Anima wrote: »

What are you actually trying to do? Kind of pointless making a suggestion without knowing.

My thoughts exactly, why are making an String array of size >16M in the first place?

The_B_Man · 01-06-2012 05:29PM

It was actually in a technical test I did the other day. It was the "bad code" section.

They had the array of size [4096][]

then in a loop they were going :

while (true)
{
array[counter][] = new String[7];
//do stuff
counter++;
}

I think it was a String, but not 100%. I think it might have been some object of a class, now that I think of it. Point is though, declaring something that big is ridiculous!! I said they should have used a vector or something more dynamic, but then it struck me that I've never seen a 2D vector before, hence this question.

srsly78 · 01-06-2012 05:35PM

There is no 2d vector because it isn't needed. A vector of vectors achieves the same result.

Aswerty · 01-06-2012 05:45PM

Well the first thing that stands out is you have an infinite loop assuming there is no break in //do stuff. You'd run into a overflow exception if the break doesn't exist. If there is a break a for loop would probably be a better fit.

In this case it looks like a vector of string arrays could be the best choice. Though if the vector is long you want to be sure you don't have to access individual elements a lot since that requires you to iterate through the vector each time.

The_B_Man · 01-06-2012 06:04PM

ye ignore the infinite loop. That was me regurgitating the code from memory, and there was some other stuff inside that got around it.

As I said at the end of my last post, I think it was a class object with 7 member variables. Think of it like a DB table so the array is like "array[row][column]" with an unknown number of rows, and 7 columns.

The tech test is over now so I suppose it doesnt matter. It was just something I encountered and wanted to learn.

So is it possible, in Java, to do a Vector<Vector<String>> myVector etc.... because I don't think a Vector<String> would do the same as a 2D array. Is there a such thing as a Vector[] ?

Aswerty · 01-06-2012 06:28PM

Vector<Vector<String>> myVector can be done.

Not 100% on this (not used Java in a while) but I don't see any reason why a Vector<String>[] would not work since it is just an array of a generic object. While Vectors are extremely useful data structures they are not all that unique and function like any other object.

Opinicus · 02-06-2012 09:55PM

Yeah in a recent college project I had LinkedLists of LinkedLists and an array of LinkedLists also. Learnt a lot about objects and how they worked in java this way.

dregin · 05-06-2012 01:55PM

Glanced through, but couldn't see what language you're talking about.

Diques should be used in python.

Procasinator · 05-06-2012 05:09PM

As Colonel Panic and Aswerty said, you don't need a 2d array/vector.

If you can fix an array that doesn't need to grow, the length would be width * height. Otherwise just use a vector.

Keep hold of your column width, then you could get the index for a (x, y) point (or (column, row) if you prefer) with something like:

function indexOfPoint(x, y) {
    return x + (y * width);
}

Dynamic alternative to 2D array

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