Running Me Ragged


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A
reader asked “Why
are there two types of multidimentional arrays? What is the difference between the
(x)(y) and (x,y) notation?”

 

Good
question.  There are two kinds of multidimensional
arrays, called “rectangular” and “ragged“.  A
rectangular array is, well, rectangular.  You
say

 

DIm
MyArrary(3,2)

 

and
you get an array with indices:

 

(0,0)
 (0,1)  (0,2)

(1,0)
 (1,1)  (1,2)

(2,0)
 (2,1)  (2,2)

(3,0)
 (3,1)  (3,2)

 

which
makes a nice rectangle.  A three-dimensional
array makes a rectangular prism, and so on up into the higher dimensions.

 

Now,
as I mentioned earlier, JScript does not have multidimensional arrays.  A
clever trick to simulate multidimensional arrays is to make an array of arrays:

 

var
x = new Array(

  new
Array(1, 2, 3),

  new
Array(4, 5),

  new
Array(6, 7, 8, 9));

 

And
so dereferencing the outer array gives you the inner array, which can then be dereferenced
itself:

 

print(x[2][0]);
// 6

 

But
you notice something about the indices if we write them out as before:

 

[0][0] 
 [0][1]   [0][2]

[1][0] 
 [1][1]

[2][0] 
 [2][1]   [2][2]   [2][3]

 

The
indices make a ragged pattern, not a straight rectangular pattern.

 

You
can have ragged higher dimensional arrays as well, though allocating all the sub-arrays
gets to be a royal pain. 

 

There
are often times when you want ragged arrays even in a language that supports rectangular
multi-dimensional arrays, so VBScript supports both.  If
you say

 

MyArray(2,3)

 

then
you are talking to a rectangular two-dimensional array.  If
you say

 

MyArray(2)(3)

 

then
you are talking to a one dimensional array that contains another one dimensional array. 

Comments (5)

  1. Thanks a lot. Too bad you never wrote a vbscript book.

  2. Eric Lippert says:

    I never did write a VBScript book, but I was the technical editor of "VBScript In A Nutshell, 2nd edition". (And boy, did I earn my fee. The first edition has a LOT of mistakes.)

  3. Shefali says:

    Hi,

    I feel lucky to have found your article "Running Me Ragged " on google today.

    I was desperately looking for information on ragged arrays in vbscript.

    I was trying to achieve a dynamic 3rd dimension for every 1st dimension element in my 3-dimensional rectangular array but it was just not working and now I know why thanks to your article.

    I want to use a ragged 3 dimensional array for the above solution instead but am not sure how.

    Would you be able to suggest a few links or a book where I can find an example of the ragged 3 dimensional arrays and how to use them?

  4. tolga says:

    i need some help..

    i wasted a lot of time while searching on the internet about my problem and i am not sure if i am writing on the correct place.

    My problem is ;

    ** i am a vb 6.0 user

    i have a 3D-D array

    -myArray()

    and i want to redim. it but i dont want to use a prism. So ;

    i assigned NJ(1)=10 ; NJ(2)=14; NJ(3)=5

    NI(1)=10 ; NI(2)=18; NI(3)=5 and i want to redim myarray like ;

    myarray(1,NI(1),NJ(1))

    myarray(2,NI(2),NJ(2))

    myarray(3,NI(3),NJ(3))

    for computing economy i dont need want to dim. my array like ; redim myarray(3,14,18)

    how can i do that ?

    thanks ; Tolga