| Re: strange behaviour of bytscl by large arrays [message #80025 is a reply to message #79963] |
Tue, 24 April 2012 10:30   |
Lajos Foldy
Messages: 176
Registered: December 2011
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Senior Member |
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On Monday, April 23, 2012 10:22:08 PM UTC+2, Chris Torrence wrote:
> Well, wrong is perhaps too strong of a word. The real word is "fast". I just did a test where I changed the internal implementation of FINDGEN to use an integer counter. The "float" counter is 4 times faster than using an integer counter and converting it to floats.
>
> However, perhaps we could look at the size of the input array, and switch to using the slower integer counter if it was absolutely necessary. I'll give it a thought.
>
> Thanks for reporting this!
>
> Cheers,
> Chris
> Exelis VIS
I could not reproduce this 4x slowdown. The integer counter + conversion method is only 30% slower in the following C test program (Intel Core i5-2500, 64 bit Linux):
#include <time.h>
#include <stdio.h>
#include <stdlib.h>
double timediff(struct timeval* tv1, struct timeval* tv2)
{
return tv2->tv_sec-tv1->tv_sec+(tv2->tv_usec-tv1->tv_usec)*1e-6;
}
int main()
{
int n=1000000000, j;
float* x=malloc(n*sizeof(float));
float f;
struct timeval tv1, tv2;
gettimeofday(&tv1, NULL);
for (j=0; j<n; j++) x[j]=j;
gettimeofday(&tv2, NULL);
printf("integer counter: %lf %f\n", timediff(&tv1, &tv2), x[n-1]);
gettimeofday(&tv1, NULL);
f=0.0;
for (j=0; j<n; j++) x[j]=f++;
gettimeofday(&tv2, NULL);
printf("float counter: %lf %f\n", timediff(&tv1, &tv2), x[n-1]);
}
Also, IDL help says:
The FINDGEN function creates a floating-point array of the specified dimensions. Each element of the array is set to the value of its one-dimensional subscript.
So it should be equivalent to float(lindgen()), as one-dimensional subscript is an integer.
But I don't want to convince you, I can accept that it is a feature :-)
regards,
Lajos
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