17 Sep
2019
17 Sep
'19
5:31 p.m.
Noel Chiappa wrote:
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
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. 
18 Sep
18 Sep
4:47 a.m.
New subject: block operations in editors, was My EuroBSDcon talk
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(a)cs.dartmouth.edu>
To:<tuhs@tuhs.org>, <jnc(a)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:
C'), pg. 11:
From: Doug
McIlroy
the absence of multidemensional arrays in C.
?? From the 'C Reference Manual' (no date, but circa 'Typesetter
"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
4:52 a.m.
New subject: block operations in editors, was My EuroBSDcon talk
Steve Johnson writes:
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
Remember the words of that great performance pioneer Jimmy Durante: ras-a-ma-cas.
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2 comments
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Doug McIlroy -
Jon Steinhart -
Steve Johnson