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Hi,
I created an harmonic with the following command:
a1=sin(pi*time)
where time is a trend series. I expected a zero vector,
but the program returns something which is very close to
zero but not exactly, as shown in the tabulation
Obs b2
1981:1 1.22461e-016
1981:2 -2.44921e-016
1981:3 3.67382e-016
...
and in the graph, which is not a straight line at y=0.
I understand this is due to the numerical accuracy at which the program
works,
but I wonder if it's possible to set an option to show 0 whenever
values are lower than a fixed level (say 10^-5).
Thanks,
Marco Marini
Istat
Attachments:
- attachment.html (text/html — 1006 bytes)
On Fri, April 20, 2007 17:13, Marco Marini wrote:
Hi,
I created an harmonic with the following command:
a1=sin(pi*time)
where time is a trend series. I expected a zero vector,
but the program returns something which is very close to
zero but not exactly, as shown in the tabulation
Obs b2
1981:1 1.22461e-016
1981:2 -2.44921e-016
1981:3 3.67382e-016
...
and in the graph, which is not a straight line at y=0.
I understand this is due to the numerical accuracy at which the program
works,
but I wonder if it's possible to set an option to show 0 whenever
values are lower than a fixed level (say 10^-5).
I don't think you really want to do this.
suppose you have
nulldata 200
setobs 1 1 --special
genr time
x = sin(pi*time)
and suppose you set the threshold at 5.0e-14 (arbitrary). You'd end up
seeing stuff like this:
Obs x
1 0
2 0
3 0
...
161 0
162 0
163 5.19355e-14
164 -6.27161e-14
165 7.34967e-14
166 0
167 0
168 0
169 0
170 0
171 0
172 0
173 0
174 -5.68353e-14
175 6.76159e-14
176 -7.83965e-14
177 0
178 0
179 0
which is not very nice, is it?
IMHO, anyone who uses gretl should know what the exponential notation
means. I mean, I'm all for user-friendliness, but this is not a
kindergarten.
Riccardo (Jack) Lucchetti
Dipartimento di Economia
Facoltà di Economia "G. Fuà"
Ancona
On Fri, 20 Apr 2007, Marco Marini wrote:
I created an harmonic with the following command:
a1=sin(pi*time)
where time is a trend series. I expected a zero vector,
but the program returns something which is very close to
zero but not exactly, as shown in the tabulation
Obs b2
1981:1 1.22461e-016
1981:2 -2.44921e-016
1981:3 3.67382e-016
...
and in the graph, which is not a straight line at y=0.
I understand this is due to the numerical accuracy at which the
program works, but I wonder if it's possible to set an option to
show 0 whenever values are lower than a fixed level (say 10^-5).
You can construct a series that's screened for computer-numerical
noise if you wish, for example:
tmp = sin(pi*time)
NOISE = 1.99e-14
a1 = tmp * (abs(tmp) > NOISE)
Allin Cottrell
6213
days inactive
6213
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Allin Cottrell -
Marco Marini -
Riccardo (Jack) Lucchetti