3 messages · Filippo Monari, Jeff Newmiller, Jiefei Wang
Hi all, I am plying around with fft function from the stats package. Running the example that is listed in the documentation: x <- 1:4 fft(x) #output 1 fft(fft(x), inverse = TRUE)/length(x) #output 2 I was expecting that output 1 and two were the same but I get:
fft(x)
[1] 10+0i -2+2i -2+0i -2-2i
ft(fft(x), inverse = TRUE)/length(x)
[1] 1+0i 2+0i 3+0i 4+0i Am I doing something wrong or is there a bug somewhere? Regards, Filippo
Neither. The discrete Fourier transform is a complex number operation. R-help is per the Posting Guide not an appropriate place to discuss theory in depth, and there is plenty of theory in this question and practically no R, but you can examine your result more closely with the functions described in ?complex.
Hi all, I am plying around with fft function from the stats package. Running the example that is listed in the documentation: x <- 1:4 fft(x) #output 1 fft(fft(x), inverse = TRUE)/length(x) #output 2 I was expecting that output 1 and two were the same but I get:
fft(x)
[1] 10+0i -2+2i -2+0i -2-2i
ft(fft(x), inverse = TRUE)/length(x)
[1] 1+0i 2+0i 3+0i 4+0i Am I doing something wrong or is there a bug somewhere? Regards, Filippo [[alternative HTML version deleted]]
______________________________________________ R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
Sent from my phone. Please excuse my brevity.
Hi Filippo, Why do you expect these two expressions to produce the same output? Especially the second expression contains the first expression. There is no way to have the same result unless the function "fft( * , inverse = TRUE)" does nothing but multiplies the input by the input's length. It does not make any sense. Actually, both results are correct as Jeff said. The first is the result of discrete Fourier transform and the second is the reverse of the transform. That's why the second output is the same as your variable "x". There are some rounding errors in the computation so you might get very small imaginary numbers after doing the reverse transformation. That's why you see "0i" in the second output. Best, Jiefei
Hi all, I am plying around with fft function from the stats package. Running the example that is listed in the documentation: x <- 1:4 fft(x) #output 1 fft(fft(x), inverse = TRUE)/length(x) #output 2 I was expecting that output 1 and two were the same but I get:
fft(x)
[1] 10+0i -2+2i -2+0i -2-2i
ft(fft(x), inverse = TRUE)/length(x)
[1] 1+0i 2+0i 3+0i 4+0i
Am I doing something wrong or is there a bug somewhere?
Regards,
Filippo
[[alternative HTML version deleted]]
______________________________________________ R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.