Re: [TUHS] block operations in editors, was My EuroBSDcon talk

["Steve Johnson" <scj@yaccman.com>]

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This is one of my pet peeves.  "Random Access" memory is far from
random when you look at the time it takes to do the accesses.  With
modern memories, accessing a column can be 20 to 40x slower than
accessing a row.  This is particularly irritating when doing AI
training, where training reuses 4-d tensors transposed, a very painful
operation.

In FORTRAN days, I once used a vector package in which you described a
vector by giving the first element, the second element, and a count. 
So you could describe rows, columns, a matrix diagonal, and even rows
and columns from back to front.  Fortran passed arguments by address,
which made the whole thing easy and fast.

Steve

----- Original Message -----
From: "Doug McIlroy" <doug@cs.dartmouth.edu>
To:<tuhs@tuhs.org>, <jnc@mercury.lcs.mit.edu>
Cc:
Sent:Tue, 17 Sep 2019 13:31:52 -0400
Subject:Re: [TUHS] block operations in editors, was My EuroBSDcon talk

 Noel Chiappa wrote:

 > > From: Doug McIlroy
 >
 > > the absence of multidemensional arrays in C.
 >
 >?? From the 'C Reference Manual' (no date, but circa 'Typesetter
C'), pg. 11:
 >
 > "If the unadorned declarator D would specify an n-dimensional array
.. then
 > the declarator D[in+1] yields an (n+1)-dimensional array"
 >
 >
 >I'm not sure if I've _ever_ used one, but they are there.

 Yes, C allows arrays of arrays, and I've used them aplenty.
 However an "n-dimensional array" has one favored dimension,
 out of which you can slice an array of lower dimension. For
 example, you can pass a row of a 2D array to a function of a
 1D variable, but you can't pass a column. That asymmetry
 underlies my assertion. In Python, by contrast, n-dimensional
 arrays can be sliced on any dimension.

 Doug



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