[Numpy-discussion] Need help for implementing a fast clip in numpy (was slow clip)
David Cournapeau
david at ar.media.kyoto-u.ac.jp
Fri Jan 12 02:32:01 CST 2007
Charles R Harris wrote:
>
>
> Well, the common machines are 32 bit and 64 bit, so for instance
> extended precision (usually 80 bits) ends up as 96 bits (3*32) on the
> first and 128 (2*64) bits on the second, with the extra bits ignored.
> The items in c structures will often have empty spaces filling in
> between them unless specific compiler directives are invoked and the
> whole will be aligned on the appropriate boundary. For instance, on my
> machine
>
> struct bar {
> char a;
> int b;
> };
So the data buffer in numpy is not really a C array, but more like a
structure ? (I am talking about the actual binary data)
In C, I think you always have sizeof(*a) bytes between a[0] and a[i].
That is sizeof(item) and addresses shift always match: we always have
a[i] = (char*)a + sizeof(*a) * i. For example, in your example for
extended precision, the computation may be done in 80 bits, but
sizeof(long double) is 12 on my machine, not 10. There is a match
between sizeof and spacing between items; otherwise, I don't see how
pointer arithmetic would be possible with arrays in C otherwise ?
But this goes quite further from my usual C knowledge, so I may be wrong
here too.
cheers,
David
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