Odd behaviour of sample when subsetting Seurat objects - r

I am seeing a peculiar behaviour from the R Seurat package, when trying to subset objects to specific sets of cells.
So, say that I generate three sets of random cell names from a Seurat object using sample
library(Seurat)
set.seed(12345)
ten_cells_id <- sample(Cells(pbmc_small), 10)
other_ten_ids <- sample(Cells(pbmc_small), 10)
and_other_ten <- sample(Cells(pbmc_small), 10)
I can now subset the object using [] and print the cell tags
Cells(pbmc_small[, ten_cells_id], pt.size=3)
Cells(pbmc_small[, other_ten_ids], pt.size=3)
Cells(pbmc_small[, and_other_ten], pt.size=3)
No surprises here; it yields three different things as expected.
> Cells(pbmc_small[, ten_cells_id], pt.size=3)
[1] "CATGAGACACGGGA" "CGTAGCCTGTATGC" "ACTCGCACGAAAGT" "CTAGGTGATGGTTG" "TTACGTACGTTCAG" "CATGGCCTGTGCAT"
[7] "ACAGGTACTGGTGT" "AATGTTGACAGTCA" "GATAGAGAAGGGTG" "CATTACACCAACTG"
> Cells(pbmc_small[, other_ten_ids], pt.size=3)
[1] "GGCATATGCTTATC" "ACAGGTACTGGTGT" "CATCAGGATGCACA" "ATGCCAGAACGACT" "GAGTTGTGGTAGCT" "GGCATATGGGGAGT"
[7] "AGAGATGATCTCGC" "GAACCTGATGAACC" "GATATAACACGCAT" "CATGAGACACGGGA"
> Cells(pbmc_small[, and_other_ten], pt.size=3)
[1] "GGGTAACTCTAGTG" "TTTAGCTGTACTCT" "TACATCACGCTAAC" "CTAAACCTGTGCAT" "ATACCACTCTAAGC" "CATGCGCTAGTCAC"
[7] "GATAGAGAAGGGTG" "ATTACCTGCCTTAT" "GCGCATCTTGCTCC" "ACAGGTACTGGTGT"
However, if I do
cells1 <- pbmc_small[, sample(Cells(pbmc_small), 10)]
cells2 <- pbmc_small[, sample(Cells(pbmc_small), 10)]
cells3 <- pbmc_small[, sample(Cells(pbmc_small), 10)]
Cells(cells1)
Cells(cells2)
Cells(cells3)
I get three times the same thing
> Cells(cells1)
[1] "GATAGAGATCACGA" "GGCATATGCTTATC" "ATGCCAGAACGACT" "AGATATACCCGTAA" "TACAATGATGCTAG" "CATGAGACACGGGA"
[7] "GCACTAGACCTTTA" "CGTAGCCTGTATGC" "TTACCATGAATCGC" "ATAAGTTGGTACGT"
> Cells(cells2)
[1] "GATAGAGATCACGA" "GGCATATGCTTATC" "ATGCCAGAACGACT" "AGATATACCCGTAA" "TACAATGATGCTAG" "CATGAGACACGGGA"
[7] "GCACTAGACCTTTA" "CGTAGCCTGTATGC" "TTACCATGAATCGC" "ATAAGTTGGTACGT"
> Cells(cells3)
[1] "GATAGAGATCACGA" "GGCATATGCTTATC" "ATGCCAGAACGACT" "AGATATACCCGTAA" "TACAATGATGCTAG" "CATGAGACACGGGA"
[7] "GCACTAGACCTTTA" "CGTAGCCTGTATGC" "TTACCATGAATCGC" "ATAAGTTGGTACGT"
The values are always the same, independently of the seed I use!
I guess that R is somehow resetting the seed each time. This is not an issue with [] as:
a <- 1:100
a[sample(1:100, 10)]
a[sample(1:100, 10)]
a[sample(1:100, 10)]
Returns three different values.
The only thing I can think of is that something strange is happening because Seurat overloads []. Any ideas?

It looks like this is because [.Seurat() calls subset.Seurat(), which in turn calls WhichCells(). WhichCells() has a seed argument, which defaults to 1. You can override this by setting it to NULL, and thankfully this will also filter through if you pass it to [ like so:
library(Seurat)
#> Attaching SeuratObject
#> Attaching sp
set.seed(12345)
cells1 <- pbmc_small[, sample(Cells(pbmc_small), 10), seed = NULL]
cells2 <- pbmc_small[, sample(Cells(pbmc_small), 10), seed = NULL]
cells3 <- pbmc_small[, sample(Cells(pbmc_small), 10), seed = NULL]
Cells(cells1)
#> [1] "GATAGAGATCACGA" "GGCATATGCTTATC" "ATGCCAGAACGACT" "AGATATACCCGTAA"
#> [5] "TACAATGATGCTAG" "CATGAGACACGGGA" "GCACTAGACCTTTA" "CGTAGCCTGTATGC"
#> [9] "TTACCATGAATCGC" "ATAAGTTGGTACGT"
Cells(cells2)
#> [1] "GTCATACTTCGCCT" "TGGTATCTAAACAG" "ATCATCTGACACCA" "GTTGACGATATCGG"
#> [5] "GACGCTCTCTCTCG" "AGATATACCCGTAA" "CTTCATGACCGAAT" "CTAACGGAACCGAT"
#> [9] "TACTCTGAATCGAC" "GCGTAAACACGGTT"
Cells(cells3)
#> [1] "GTCATACTTCGCCT" "GCTCCATGAGAAGT" "ACAGGTACTGGTGT" "TACATCACGCTAAC"
#> [5] "CCATCCGATTCGCC" "GACGCTCTCTCTCG" "CTTCATGACCGAAT" "GCGTAAACACGGTT"
#> [9] "CATTACACCAACTG" "CTTGATTGATCTTC"
Created on 2022-10-17 with reprex v2.0.2
In my opinion this is quite poorly documented, and the behaviour is confusing enough to possibly justify a new issue at the surat-object GitHub.

Related

Loop including 0 R

I create the following range:
x <- seq(0,22)
Now I want to get some expected poisson estimations:
for (val in x) {
vec[val]<-(dpois(val,6.298387))*124
}
I want also the estimation for val = 0
(dpois(0,6.298387))*124
However, the vector "vec" obtained previously starts at val = 1.
How can I force the loop to take also values = 0?
Since R is 1-indexed, there is no such thing as vec[0]. The first valid index of vec is vec[1], so you probably intended
x <- seq(0,22)
vec <- numeric()
for (val in x) {
vec[val + 1] <- dpois(val, 6.298387) * 124
}
vec
#> [1] 2.280694e-01 1.436469e+00 4.523719e+00 9.497378e+00 1.495454e+01
#> [6] 1.883790e+01 1.977473e+01 1.779270e+01 1.400816e+01 9.803203e+00
#> [11] 6.174437e+00 3.535363e+00 1.855590e+00 8.990174e-01 4.044542e-01
#> [16] 1.698273e-01 6.685238e-02 2.476836e-02 8.666707e-03 2.872962e-03
#> [21] 9.047512e-04 2.713559e-04 7.768656e-05
However, the loop is not necessary, since dpois is vectorized like many R functions. Therefore the above code simplifies to this one-liner:
dpois(0:22, 6.298387) * 124
#> [1] 2.280694e-01 1.436469e+00 4.523719e+00 9.497378e+00 1.495454e+01
#> [6] 1.883790e+01 1.977473e+01 1.779270e+01 1.400816e+01 9.803203e+00
#> [11] 6.174437e+00 3.535363e+00 1.855590e+00 8.990174e-01 4.044542e-01
#> [16] 1.698273e-01 6.685238e-02 2.476836e-02 8.666707e-03 2.872962e-03
#> [21] 9.047512e-04 2.713559e-04 7.768656e-05
Created on 2022-07-22 by the reprex package (v2.0.1)

How to remove elements of a list in R?

I have an igraph object, what I have created with the igraph library. This object is a list. Some of the components of this list have a length of 2. I would like to remove all of these ones.
IGRAPH clustering walktrap, groups: 114, mod: 0.79
+ groups:
$`1`
[1] "OTU0041" "OTU0016" "OTU0062"
[4] "OTU1362" "UniRef90_A0A075FHQ0" "UniRef90_A0A075FSE2"
[7] "UniRef90_A0A075FTT8" "UniRef90_A0A075FYU2" "UniRef90_A0A075G543"
[10] "UniRef90_A0A075G6B2" "UniRef90_A0A075GIL8" "UniRef90_A0A075GR85"
[13] "UniRef90_A0A075H910" "UniRef90_A0A075HTF5" "UniRef90_A0A075IFG0"
[16] "UniRef90_A0A0C1R539" "UniRef90_A0A0C1R6X4" "UniRef90_A0A0C1R985"
[19] "UniRef90_A0A0C1RCN7" "UniRef90_A0A0C1RE67" "UniRef90_A0A0C1RFI5"
[22] "UniRef90_A0A0C1RFN8" "UniRef90_A0A0C1RGE0" "UniRef90_A0A0C1RGX0"
[25] "UniRef90_A0A0C1RHM1" "UniRef90_A0A0C1RHR5" "UniRef90_A0A0C1RHZ4"
+ ... omitted several groups/vertices
For example, this one :
> a[[91]]
[1] "OTU0099" "UniRef90_UPI0005B28A7E"
I tried this but it does not work :
a[lapply(a,length)>2]
Any help?
Since you didn't provide any reproducible data or example, I had to produce some dummy data:
# create dummy data
a <- list(x = 1, y = 1:4, z = 1:2)
# remove elements in list with lengths greater than 2:
a[which(lapply(a, length) > 2)] <- NULL
In case you wanted to remove the items with lengths exactly equal to 2 (question is unclear), then last line should be replaced by:
a[which(lapply(a, length) == 2)] <- NULL

Replace value table with condition in R

I have list of dataset :
> data1
[1] /index.php/search?
[2] /tabel/graphic1_.php?
[3] /mod/Layout/variableView2.php?
[4] /table/tblmon-frameee.php?
and a table:
> tes
[1] http://aladdine/index.php/search?
[2] http://aladdine/mod/params/returnParams.php
[3] http://aladdine/mod/Layout/variableView2.php
[4] http://aladdine/index.php/bos/index?
[5] http://aladdine/index.php/Bos
I want to change the value of the test table with an index on dataset which has a matching string values in the dataset.
I have tried this code:
for(i in 1:length(dataset)){
p = data[i]
for(j in 1:length(tes)){
t = tes [j]
if(grepl(p, t)){
tes[j]=i
}
else tes[j] = "-"
}
}
My expectation result like this,
> tes
[1] 1
[2] -
[3] 3
[4] -
[5] -
But, I always get warning message invalid factor level, NA generated. Why?
Thanks before.
The following code does not do exactly what you need, but effectively it should give you the same information.
data1<-c('/index.php/search?',
'/tabel/graphic1_.php?',
'/mod/Layout/variableView2.php?',
'/table/tblmon-frameee.php?')
tes<-c('http://aladdine/index.php/search?',
'http://aladdine/mod/params/returnParams.php',
'http://aladdine/mod/Layout/variableView2.php',
'http://aladdine/index.php/bos/index?',
'http://aladdine/index.php/Bos')
> lapply(data1,FUN = function(x) which(grepl(x,tes)))
[[1]]
[1] 1
[[2]]
integer(0)
[[3]]
[1] 3
[[4]]
integer(0)
For example, the first output in [[1]] tells which element in "tes" match the first element in "data1" etc...
Probably not the fastest one as i use for loop in this code but hope this provides a solution:
require(data.table)
data1<-c("/index.php/search?","/tabel/graphic1_.php?","/mod/Layout/variableView2.php?","/table/tblmon-frameee.php?")
tes<-c("http://aladdine/index.php/search?","http://aladdine/mod/params/returnParams.php" ,"http://aladdine/mod/Layout/variableView2.php","http://aladdine/index.php/bos/index?","http://aladdine/index.php/Bos")
d<-data.table(d=data1,t=tes)
d$id<-seq(1:nrow(d))
for (i in 1:nrow(d))
{
d$index[i]<-lapply(data1,FUN=function(x) {ifelse(length(grep(x,tes[i]))>0,d$id[i],"-")})[i]
}

knitr vs. interactive R behaviour

I am reposting my problem here, after I noticed that was the approach advised by knitr's author to get more help.
I am a bit puzzle with a .Rmd file that I can proceed line by line in an interactive R session, and also with R CMD BATCH, but that fails when using knit("test.Rmd"). I am not sure where the problem lies, and I tried to narrow the problem down as much as I could. Here is the example (in test.Rmd):
```{r Rinit, include = FALSE, cache = FALSE}
opts_knit$set(stop_on_error = 2L)
library(adehabitatLT)
```
The functions to be used later:
```{r functions}
ld <- function(ltraj) {
if (!inherits(ltraj, "ltraj"))
stop("ltraj should be of class ltraj")
inf <- infolocs(ltraj)
df <- data.frame(
x = unlist(lapply(ltraj, function(x) x$x)),
y = unlist(lapply(ltraj, function(x) x$y)),
date = unlist(lapply(ltraj, function(x) x$date)),
dx = unlist(lapply(ltraj, function(x) x$dx)),
dy = unlist(lapply(ltraj, function(x) x$dy)),
dist = unlist(lapply(ltraj, function(x) x$dist)),
dt = unlist(lapply(ltraj, function(x) x$dt)),
R2n = unlist(lapply(ltraj, function(x) x$R2n)),
abs.angle = unlist(lapply(ltraj, function(x) x$abs.angle)),
rel.angle = unlist(lapply(ltraj, function(x) x$rel.angle)),
id = rep(id(ltraj), sapply(ltraj, nrow)),
burst = rep(burst(ltraj), sapply(ltraj, nrow)))
class(df$date) <- c("POSIXct", "POSIXt")
attr(df$date, "tzone") <- attr(ltraj[[1]]$date, "tzone")
if (!is.null(inf)) {
nc <- ncol(inf[[1]])
infdf <- as.data.frame(matrix(nrow = nrow(df), ncol = nc))
names(infdf) <- names(inf[[1]])
for (i in 1:nc) infdf[[i]] <- unlist(lapply(inf, function(x) x[[i]]))
df <- cbind(df, infdf)
}
return(df)
}
ltraj2sldf <- function(ltr, proj4string = CRS(as.character(NA))) {
if (!inherits(ltr, "ltraj"))
stop("ltr should be of class ltraj")
df <- ld(ltr)
df <- subset(df, !is.na(dist))
coords <- data.frame(df[, c("x", "y", "dx", "dy")], id = as.numeric(row.names(df)))
res <- apply(coords, 1, function(dfi) Lines(Line(matrix(c(dfi["x"],
dfi["y"], dfi["x"] + dfi["dx"], dfi["y"] + dfi["dy"]),
ncol = 2, byrow = TRUE)), ID = format(dfi["id"], scientific = FALSE)))
res <- SpatialLinesDataFrame(SpatialLines(res, proj4string = proj4string),
data = df)
return(res)
}
```
I load the object and apply the `ltraj2sldf` function:
```{r fail}
load("tr.RData")
juvStp <- ltraj2sldf(trajjuv, proj4string = CRS("+init=epsg:32617"))
dim(juvStp)
```
Using knitr("test.Rmd") fails with:
label: fail
Quitting from lines 66-75 (test.Rmd)
Error in SpatialLinesDataFrame(SpatialLines(res, proj4string =
proj4string), (from <text>#32) :
row.names of data and Lines IDs do not match
Using the call directly in the R console after the error occurred works as expected...
The problem is related to the way format produces the ID (in the apply call of ltraj2sldf), just before ID 100,000: using an interactive call, R gives "99994", "99995", "99996", "99997", "99998", "99999", "100000"; using knitr R gives "99994", " 99995", " 99996", " 99997", " 99998", " 99999", "100000", with additional leading spaces.
Is there any reason for this behaviour to occur? Why should knitr behave differently than a direct call in R? I have to admit I'm having hard time with that one, since I cannot debug it (it works in an interactive session)!
Any hint will be much appreciated. I can provide the .RData if it helps (the file is 4.5 Mo), but I'm mostly interested in why such a difference happens. I tried without any success to come up with a self-reproducible example, sorry about that. Thanks in advance for any contribution!
After a comment of baptiste, here are some more details about IDs generation. Basically, the ID is generated at each line of the data frame by an apply call, which in turn uses format like this: format(dfi["id"], scientific = FALSE). Here, the column id is simply a series from 1 to the number of rows (1:nrow(df)). scientific = FALSE is just to ensure that I don't have results such as 1e+05 for 100000.
Based on an exploration of the IDs generation, the problem only occurred for those presented in the first message, i.e. 99995 to 99999, for which a leading space is added. This should not happen with this format call, since I did not ask for a specific number of digit in the output. For instance:
> format(99994:99999, scientific = FALSE)
[1] "99994" "99995" "99996" "99997" "99998" "99999"
However, if the IDs are generated in chunks, it might occur:
> format(99994:100000, scientific = FALSE)
[1] " 99994" " 99995" " 99996" " 99997" " 99998" " 99999" "100000"
Note that the same processed one at a time gives the expected result:
> for (i in 99994:100000) print(format(i, scientific = FALSE))
[1] "99994"
[1] "99995"
[1] "99996"
[1] "99997"
[1] "99998"
[1] "99999"
[1] "100000"
In the end, it's exactly like if the IDs were not prepared one at a time (as I would expect from an apply call by line), but in this case, 6 at a time, and only when close to 1e+05... And of course, only when using knitr, not interactive or batch R.
Here is my session information:
> sessionInfo()
R version 3.0.1 (2013-05-16)
Platform: x86_64-pc-linux-gnu (64-bit)
locale:
[1] LC_CTYPE=fr_FR.UTF-8 LC_NUMERIC=C
[3] LC_TIME=fr_FR.UTF-8 LC_COLLATE=fr_FR.UTF-8
[5] LC_MONETARY=fr_FR.UTF-8 LC_MESSAGES=fr_FR.UTF-8
[7] LC_PAPER=C LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=fr_FR.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] knitr_1.2 adehabitatLT_0.3.12 CircStats_0.2-4
[4] boot_1.3-9 MASS_7.3-27 adehabitatMA_0.3.6
[7] ade4_1.5-2 sp_1.0-11 basr_0.5.3
loaded via a namespace (and not attached):
[1] digest_0.6.3 evaluate_0.4.4 formatR_0.8 fortunes_1.5-0
[5] grid_3.0.1 lattice_0.20-15 stringr_0.6.2 tools_3.0.1
Both Jeff and baptiste were were indeed right! This is an option problem, related to the digits argument. I managed to come up with a working minimal example (e.g. in test.Rmd):
Simple reproducible example : df1 is a data frame of 110,000 rows,
with 2 random normal variables + an `id` variable which is a series
from 1 to the number of row.
```{r example}
df1 <- data.frame(x = rnorm(110000), y = rnorm(110000), id = 1:110000)
```
From this, we create a `id2` variable using `format` and `scientific =
FALSE` to have results with all numbers instead of scientific
notations (e.g. 100,000 instead of 1e+05):
```{r example-continued}
df1$id2 <- apply(df1, 1, function(dfi) format(dfi["id"], scientific = FALSE))
df1$id2[99990:100010]
```
It works as expected using R interactively, resulting in:
[1] "99990" "99991" "99992" "99993" "99994" "99995" "99996"
[8] "99997" "99998" "99999" "100000" "100001" "100002" "100003"
[15] "100004" "100005" "100006" "100007" "100008" "100009" "100010"
However, the results are quite different using knit:
> library(knitr)
> knit("test.Rmd")
[...]
## [1] "99990" "99991" "99992" "99993" "99994" " 99995" " 99996"
## [8] " 99997" " 99998" " 99999" "100000" "100001" "100002" "100003"
## [15] "100004" "100005" "100006" "100007" "100008" "100009" "100010"
Note the additional leading spaces after 99994. The difference actually comes from the digits option, as rightly suggested by Jeff: R uses 7 by default, while knitr uses 4. This difference affects the output of format, although I don't really understand what's going on here. R-style:
> options(digits = 7)
> format(99999, scientific = FALSE)
[1] "99999"
knitr-style:
> options(digits = 4)
> format(99999, scientific = FALSE)
[1] " 99999"
But it should affect all numbers, not just after 99994 (well, to be honest, I don't even understand why it's adding leading spaces at all):
> options(digits = 4)
> format(c(1:10, 99990:100000), scientific = FALSE)
[1] " 1" " 2" " 3" " 4" " 5" " 6" " 7"
[8] " 8" " 9" " 10" " 99990" " 99991" " 99992" " 99993"
[15] " 99994" " 99995" " 99996" " 99997" " 99998" " 99999" "100000"
From this, I have no idea which is at fault: knitr, apply or format? At least, I came up with a workaround, using the argument trim = TRUE in format. It doesn't solve the cause of the problem, but did remove the leading space in the results...
I added a comment to your knitr GitHub issue with this information.
format() adds the extra whitespace when the digits option is not sufficient to display a value but scientific=FALSE is also specified. knitr sets digits to 4 inside code blocks, which causes the behavior you describe:
options(digits=4)
format(99999, scientific=FALSE)
Produces:
[1] " 99999"
While:
options(digits=5)
format(99999, scientific=FALSE)
Produces:
[1] "99999"
Thanks to Aleksey Vorona and Duncan Murdoch, this bug is now fixed in R-devel!
See: https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15411

XCMS Package - Retention time

Is there a simple way to get the list/array of retention times from a xcmsRaw object?
Example Code:
xraw <- xcmsRaw(cdfFile)
So for example getting information from it :
xraw#env$intensity
or
xraw#env$mz
You can see what slots are available in your xcmsRaw instance with
> slotNames(xraw)
[1] "env" "tic" "scantime"
[4] "scanindex" "polarity" "acquisitionNum"
[7] "profmethod" "profparam" "mzrange"
[10] "gradient" "msnScanindex" "msnAcquisitionNum"
[13] "msnPrecursorScan" "msnLevel" "msnRt"
[16] "msnPrecursorMz" "msnPrecursorIntensity" "msnPrecursorCharge"
[19] "msnCollisionEnergy" "filepath"
What you want is xraw#msnRt - it is a vector of numeric.
The env slot is a environment that stores 3 variables:
> ls(xraw#env)
[1] "intensity" "mz" "profile"
More details on the class itself at class?xcmsRaw.
EDIT: The msnRt slot is populated only if you specify includeMSn = TRUE and your input file must be in mzXML or mzML, not in cdf; if you use the faahKO example from ?xcmasRaw, you will see that
xr <- xcmsRaw(cdffiles[1], includeMSn = TRUE)
Warning message:
In xcmsRaw(cdffiles[1], includeMSn = TRUE) :
Reading of MSn spectra for NetCDF not supported
Also, xr#msnRt will only store the retention times for MSn scans, with n > 1. See the xset#rt where xset is an xcmsSet instance for the raw/corrected MS1 retention times as provided by xcms.
EDIT2: Alternatively, have a go with the mzR package
> library(mzR)
> cdffiles[1]
[2] "/home/lgatto/R/x86_64-unknown-linux-gnu-library/2.16/faahKO/cdf/KO/ko15.CDF"
> xx <- openMSfile(cdffiles[1])
> xx
Mass Spectrometry file handle.
Filename: /home/lgatto/R/x86_64-unknown-linux-gnu-library/2.16/faahKO/cdf/KO/ko15.CDF
Number of scans: 1278
> hd <- header(xx)
> names(hd)
[1] "seqNum" "acquisitionNum"
[3] "msLevel" "peaksCount"
[5] "totIonCurrent" "retentionTime"
[7] "basePeakMZ" "basePeakIntensity"
[9] "collisionEnergy" "ionisationEnergy"
[11] "highMZ" "precursorScanNum"
[13] "precursorMZ" "precursorCharge"
[15] "precursorIntensity" "mergedScan"
[17] "mergedResultScanNum" "mergedResultStartScanNum"
[19] "mergedResultEndScanNum"
> class(hd)
[1] "data.frame"
> dim(hd)
[1] 1278 19
but you will be outside of the default xcms pipeline if you take this route (although Steffen Neumann, from xcms, does know mzR very well, oubviously).
Finally, you are better of to use the Bioconductor mailing list of the xcms online forum if you want to maximise your chances to get feedback from the xcms developers.
Hope this helps.
Good answer but i was looking for this :
xraw <- xcmsRaw(cdfFile)
dp_index <- which(duplicated(rawMat(xraw)[,1]))
xraw_rt_dp <- rawMat(xraw)[,1]
xrawData.rt <- xraw_rt_dp[-dp_index]
Now :
xrawData.rt #contains the retention time.
Observation --> Using mzr package:
nc <- mzR:::netCDFOpen(cdfFile)
ncData <- mzR:::netCDFRawData(nc)
mzR:::netCDFClose(nc)
ncData$rt #contains the same retention time for the same input !!!

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