How to create array with unequal spacing?
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I want to create an array with unequal spacing. I have h = 1 1/2 1/4 1/8 ..... 2^-60.
Every second entry in this array is 1/2 of the last one. I only know, how to make arrays with equal spacing.
a = 1:5:40
But how do I create array that gets halved with every entry?
5 个评论
h = pow2(0:-1:-60)
Walter Roberson
2018-2-21
timeit() disagrees:
>> timeit(@() pow2(0:-1:-60),0)
ans =
3.45591429508197e-06
>> timeit(@() 2.^-(0:60),0)
Warning: The measured time for F may be inaccurate because it is running too fast. Try measuring something that takes longer.
> In timeit (line 158)
ans =
0
And if you
>> tic;for K = 1 : 1000; h = 2.^-(0:60); end; toc
Elapsed time is 0.000007 seconds.
-- not the first time, at least not from the command line, but after a few times repeating the same line, the JIT fully kicks in.
By way of contrast,
>> tic;for K = 1 : 1000; h = pow2(0:-1:-60); end; toc
Elapsed time is 0.004652 seconds.
was the best I could do.
Ahmad Hasnain
2018-2-22
Stephen23
2018-2-22
@Walter Roberson: Interesting. So what is the usecase for pow2(n) then?
Walter Roberson
2018-2-22
In 2.^-(0:60) the JIT is detecting that the expression is constant and optimizes it, at least in the case of re-use. If you use B = 2; B.^-(0:60) or if you use V = -(0:60); 2.^B then JIT optimization is not as good and pow2 can have better performance than those.
采纳的回答
Walter Roberson
2018-2-21
h = 2.^-(0:60);
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