问题
I have uploaded a raster file and polyline shapefile into R and use the extract function to to extract the data from every pixel along the polyline. How do I turn the list output by extract into a CSV file?
回答1:
If I understand what it is you're asking, I think you could resolve your situation by using unlist().
d <- c(1:10) # creates a sample data frame to use
d <- as.list(d) # converts the data frame into a list
d <- unlist(d) # converts the list into a vector
回答2:
Always include a simple self-contained reproducible example (this one is taken from ?raster::extract
library(raster)
r <- raster(ncol=36, nrow=18, vals=1:(18*36))
cds1 <- rbind(c(-50,0), c(0,60), c(40,5), c(15,-45), c(-10,-25))
cds2 <- rbind(c(80,20), c(140,60), c(160,0), c(140,-55))
lines <- spLines(cds1, cds2)
e <- extract(r, lines)
e is a list
> e
[[1]]
[1] 126 127 161 162 163 164 196 197 200 201 231 232 237 266 267 273 274 302 310 311 338 346 381 382 414 417 450 451 452 453 487 488
[[2]]
[1] 139 140 141 174 175 177 208 209 210 213 243 244 249 250 279 286 322 358 359 394 429 430 465 501 537
and you cannot directly write this to a csv because the list elements (vectors) have different lengths.
So first make them all the same length
x <- max(sapply(e, length))
ee <- sapply(e, `length<-`, x)
Let's see
head(ee)
# [,1] [,2]
#[1,] 126 139
#[2,] 127 140
#[3,] 161 141
#[4,] 162 174
#[5,] 163 175
#[6,] 164 177
tail(ee)
# [,1] [,2]
#[27,] 450 NA
#[28,] 451 NA
#[29,] 452 NA
#[30,] 453 NA
#[31,] 487 NA
#[32,] 488 NA
And now you can write to a csv file
write.csv(ee, "test.csv", row.names=FALSE)
来源：https://stackoverflow.com/questions/56403276/turn-extraction-list-to-csv-file