R Subset by row numbers -


i have large matrix “a” (14 columns 250 000 rows). has no id column.

i have matrix “b” (14x1) 14 numbers. 

1   1   3   7   15  31  63  127 255 511 1023    2047    4095    8191

i subset “a” taking rows have number in b. eg. row 1, 3, 7, 15, 31, etc.

i have tried following: 

newa <- a[nrow(a) %in% c(b),]

i didn’t work. have tried add id column large matrix (e.g. a$id <- 1:nrow(a) didn’t work either.

try converting b numeric vector , use index - works matrices regardless of how many rows , columns have. 

b<-as.numeric(as.character(b))  newa<-a[b,]

edit - refined clarity , information on applicability matrices of dimensions.


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