This is a long shot, but here's a plausible scenario:

as part of its pipeline, ravetools::mvfftw computes the mean of the input vector **and then centers it to a mean of zero** (intentionally or accidentally?)

because variables are passed to compiled code by reference (someone can feel free to correct my terminology), this means that the original vector in R now has a mean of zero

the first element of fft() is mean(x)*length(x), so if mean(x) has been forced to zero, that would explain your issue.

  I don't know about the non-reproducibility part.

On 10/19/21 7:06 PM, Dipterix Wang wrote:
Dear R-devel Team,

I'm developing a neuroscience signal pipeline package in R 
(https://github.com/dipterix/ravetools) and I noticed a weird issue that failed 
my unit test.

Basically I was trying to use `fftw3` library to implement fast multivariate 
fft function in C++. When I tried to compare my results with stats::fft, the 
test result showed the first element of **expected** (which was produced by 
stats::fft) was zero, which, I am pretty sure, is wrong, and I can confirm that 
my function produces correct results.

However, somehow I couldn’t reproduce this issue on my personal computer (osx, 
M1, R4.1.1), the error simply went away.

The catch is my function produced consistent and correct results but stats::fft 
was not. This does not mean `stats::fft` has bugs. Instead, I suspect there 
could be some weird interactions between my code and stats::fft at C/C++ level, 
but I couldn’t figure it out why.

+++ Details:

Here’s the code I used for the test:

https://github.com/dipterix/ravetools/blob/4dc35d64763304aff869d92dddad38a7f2b30637/tests/testthat/test-fftw.R#L33-L41

————————Test code————————
set.seed(1)
x <- rnorm(1000)
dim(x) <- c(100,10)
a <- ravetools:::mvfftw_r2c(x, 0)
c <- apply(x, 2, stats::fft)[1:51,]
expect_equal(a, c)
————————————————————————

Here are the tests that gave me the errors:

The test logs on win-builder
https://win-builder.r-project.org/07586ios8AbL/00check.log

Test logs on GitHub
https://github.com/dipterix/ravetools/runs/3944874310?check_suite_focus=true


—————————————— Failed tests ——————————————
  -- Failure (test-fftw.R:41:3): mvfftw_r2c 
--------------------------------------
  `a` (`actual`) not equal to `c` (`expected`).

  actual vs expected
                                      [,1]                    [,2]              
    [,3]                  [,4]                    ...
  - actual[1, ]     10.8887367+ 0.0000000i  -3.7808077+ 0.0000000i   2.967354+ 
0.000000i   5.160186+ 0.000000i ...
  + expected[1, ]    0.0000000+ 0.0000000i  -3.7808077+ 0.0000000i   2.967354+ 
0.000000i   5.160186+ 0.000000i...

————————————————————————

The first columns are different, `actual` is the results I produced via 
`ravetools:::mvfftw_r2c`, and `expected` was produced by `stats::fft`


Any help or attention is very much appreciated.
Thanks,
- Zhengjia
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Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
Director, School of Computational Science and Engineering
Graduate chair, Mathematics & Statistics

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