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*To*: srfi-25@xxxxxxxxxxxxxxxxx*Subject*: ideas in blitz++*From*: Sebastien.deMentendeHorne@xxxxxxxxxxxxxx*Date*: Fri, 16 Nov 2001 16:18:44 +0100*Delivered-to*: srfi-25@xxxxxxxxxxxxxxxxx

Great SRFI!! However, if I can suggest to have a look at the library blitz++ : this is a template library in C++ (a bag of clever macros) which deals with arrays. They have defined some basic operations and others not so basic to deal with multi-dimensional arrays. Some concepts from this library : - represent the data as a linear vector - store shape information in some "vectors": - base vector : store the lower index in each dimension - length vector : store number of indices in each dimension - step vector : store the step size for each dimension - stride vector : store the product of the length of the dimension g - permutation vector : store if any permutation - a different object to reprensent shapes Hence, in order to acces the element (array-ref a 3 5 6), we compute : i = (3-base[permutation[0]])/step[permutation[0]]*stride[permutation[0]] + (5-base[permutation[1]])/step[permutation[1]]*stride[permutation[1]] + (6-base[permutation[2]])/step[permutation[2]]*stride[permutation[2]] and give (vector-ref v i). In the SRFI-25 way of thinking, the analogies are: (shape a) = (base[0] (+ base[0] length[0]) ... base[n] (+ base[n] length[n])) permutation = (0 1 ... n) (no permutation of indices) step = (1 1 ... 1) (indices increase only by 1) stride = (lentgh[0]*...*length[n-1]*1 lentgh[1]*...*length[n-1]*1 ... length[n-1]*1 1) Some advantages of the representation are: - the possibility to transpose arrays with no cost by doing: permutation = (1 0 2) instead of (0 1 2) - having steps (may be negative or positive) - representing slice (with affine maps) by playing only on "base","step" and "permutation" - shapes are not arrays, indices are not arrays because they both need more economic and efficient representations Well, I think it is worth the look. Maybe those ideas are only ideas for implementations but anyway, this is a srfI ! Moreover, this can be efficiently done (no vector nestings) and can be a great tool for numeric calculus (speed AND high level language!!!) Sebastien de Menten

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