Re: error: array subscript is not an integer

[Jim Cobban <jcobban at magma dot ca>]

Manal Helal wrote:
> Hi
>
> I am trying to construct a multidimensional array (as linear array in
> memory, but indexed with equations to represent the dimensions) of
> size 30 each, and 6 dimensions, which is 30^6=729000000
>
> using a long type for the array crashes, as it is outside the range
> values. I used double long data type, and I had the above error: array
> subscript is not an integer
>
> Is there a work around this problem that I can implement,  or another
> way of creating similar arrays?
>   
Are you expecting all 729 million entries to be used, or only a small 
fraction? If you only expect a small fraction of the entries to actually 
be used then you should probably use an associative (aka hash) map 
instead.  Bear in mind that even if you actually have the gigabytes of 
memory that would be required to store your linear array that actually 
making the essentially random storage references that such an array 
would require would guarantee cache misses which could severely degrade 
the performance of your application.  Since an associative array only 
uses the number of entries that are actually assigned values it will 
tend to avoid cache misses, which may be enough to compensate for the 
slightly longer path length to locate an entry.  As long as the hash 
vector is significantly larger than the maximum number of entries 
inserted into the "array", and the hash function on the key returns 
sufficiently well distributed values, the time to access a specific 
element is essentially constant.

There are other sparse array techniques that you could try, depending 
upon the exact pattern of access.  For example instead of allocating a 
linear array of 30^6 entries, initially allocate a 30 element array of 
pointers:  Entry ****** dim1[30];  Each time you assign a value to an 
element allocate only that chain of 30 element arrays of pointers 
necessary to get to the specific element you want.  Locating a specific 
element requires using the 6 subscripts to run down the chain of 
pointers until you get the desired element.  This technique is 
guaranteed constant access time, but will in many cases average a longer 
time than the associative "array".

All compilers have an implementation limit on the maximum size of 
arrays.  In the old days that was not much of an issue because the limit 
was typically larger than the maximum amount of memory available.  With 
4GB machines being not at all uncommon these days, this assumption is no 
longer true.

--
Jim Cobban   jcobban@magma.ca
34 Palomino Dr.
Kanata, ON, CANADA
K2M 1M1
+1-613-592-9438