Reading in different raster files with different bands using a loop

Question

I've downloaded 6 multi-band raster files (each file has 4 bands) from the National Weather Service containing precipitation data for different time periods (30, 60, 90, 120, and 365 day periods). Here's where I downloaded the data from: https://water.weather.gov/precip/download.php

I want to write a loop to pull each of those files into R, with each raster band assigned to a different variable. Here's an example for just one raster file for the 30 day time period:

library(rgdal)
library(raster)

y <- format(Sys.Date()-1,"%Y")
m <- format(Sys.Date()-1,"%m")
d <- format(Sys.Date()-1,"%d")

NWS1_prev30 <- raster(paste0('/path/nws_precip_last30days_', y, m, d, '_conus.tif'), band = 1)
NWS2_prev30 <- raster(paste0('/path/nws_precip_last30days_', y, m, d, '_conus.tif'), band = 2)
NWS3_prev30 <- raster(paste0('/path/nws_precip_last30days_', y, m, d, '_conus.tif'), band = 3)
NWS4_prev30 <- raster(paste0('/path/nws_precip_last30days_', y, m, d, '_conus.tif'), band = 4)

I don't really want to copy and paste that 5 other times, so was hoping to write a loop. However, I'm receiving an error with my code as it is so far. Here's what I have:

pathname <- c(paste0("/path/nws_precip_last30days_", y, m, d, "_conus.tif"),paste0("/path/nws_precip_last60days_", y, m, d,"_conus.tif"),paste0("/path/nws_precip_last9days_", y, m, d,"_conus.tif"),paste0("path//nws_precip_last120days_", y, m, d,"_conus.tif"), 
              paste0("/path/nws_precip_last180days_", y, m, d,"_conus.tif"), paste0("/path/nws_precip_last120days_", y, m, d,"_conus.tif"), paste0("/path/nws_precip_last365days_", y, m, d,"_conus.tif"))

# Bulk
for (j in 1:length(pathname)) {
  for (k in 1:4) {
    assign(paste0("NWS", k, j), raster(j, band = k))
  }
}

# Error in (function (classes, fdef, mtable)  : 
#  unable to find an inherited method for function ‘raster’ for signature ‘"integer"’

Answer 1

Right now you're trying to load a raster using j - which is just an integer. Instead, you want to load it from the selection of path names. So, if you use indexing:

for (j in 1:length(pathname)) {
  for (k in 1:4) {
    assign(paste0("NWS", k, j), raster(pathname[j], band = k))
  }
}

you may have better luck.