Translating a `for loop` into purrr using `map` function in r

Question

I need to download weather data from NASA’s POWER (Prediction Of Worldwide Energy Resource). The package nasapower is a package developed for data retrieval using R. I need to download many locations (lat, long coordinates). To do this I tried a simple loop with three locations as a reproducible example.

library(nasapower) data1 <- read.csv(text = " location,long,lat loc1, -56.547, -14.2427 loc2, -57.547, -15.2427 loc3, -58.547, -16.2427") i=1 all.weather <- data.frame() for (i in seq_along(1:nrow(data1))) { weather.data <- get_power(community = "AG", lonlat = c(data1$long[i],data1$lat[i]), dates = c("2015-01-01", "2015-01-10"), temporal_average = "DAILY", pars = c("T2M_MAX")) all.weather <-rbind(all.weather, weather.data) }

This works perfect. The problem is that I am trying to mimic this using purrr::map since I want to have an alternative within tidyverse. This is what I did but it does not work:

library(dplyr) library(purrr) all.weather <- data1 %>% group_by(location) %>% map(get_power(community = "AG", lonlat = c(long, lat), dates = c("2015-01-01", "2015-01-10"), temporal_average = "DAILY", site_elevation = NULL, pars = c("T2M_MAX")))

I got the following error:

Error in isFALSE(length(lonlat != 2)) : object 'long' not found

Any hint on how to run this using purrr?

is get_power compliant with tidy eval ? if not, you may have to specify data1$long, data1$lat... — GuedesBF
– GuedesBF, Commented May 28, 2021 at 20:18

stefan · Accepted Answer · 2021-05-30 08:21:33Z

To make your code work make use of purrr::pmap instead of map like so:

map is for one argument functions, map2 for two argument funs and pmap is the most general one allowing for funs with more than two arguments.
pmap will loop over the rows of your df. As your df has 3 columns 3 arguments are passed to the function, even if the first argument location is not used. To make this work and to make use of the column names you have to specify the function and the argument names via function(location, long, lat)

library(nasapower) data1 <- read.csv(text = " location,long,lat loc1, -56.547, -14.2427 loc2, -57.547, -15.2427 loc3, -58.547, -16.2427") library(dplyr) library(purrr) all.weather <- data1 %>% pmap(function(location, long, lat) get_power(community = "AG", lonlat = c(long, lat), dates = c("2015-01-01", "2015-01-10"), temporal_average = "DAILY", site_elevation = NULL, pars = c("T2M_MAX"))) %>% # Name list with locations setNames(data1$location) %>% # Add location names as identifiers bind_rows(.id = "location") head(all.weather) #> NASA/POWER SRB/FLASHFlux/MERRA2/GEOS 5.12.4 (FP-IT) 0.5 x 0.5 Degree Daily Averaged Data #> Dates (month/day/year): 01/01/2015 through 01/10/2015 #> Location: Latitude -14.2427 Longitude -56.547 #> Elevation from MERRA-2: Average for 1/2x1/2 degree lat/lon region = 379.25 meters Site = na #> Climate zone: na (reference Briggs et al: http://www.energycodes.gov) #> Value for missing model data cannot be computed or out of model availability range: NA #> #> Parameters: #> T2M_MAX MERRA2 1/2x1/2 Maximum Temperature at 2 Meters (C) #> #> # A tibble: 6 x 9 #> location LON LAT YEAR MM DD DOY YYYYMMDD T2M_MAX #> <chr> <dbl> <dbl> <dbl> <int> <int> <int> <date> <dbl> #> 1 loc1 -56.5 -14.2 2015 1 1 1 2015-01-01 29.9 #> 2 loc1 -56.5 -14.2 2015 1 2 2 2015-01-02 30.1 #> 3 loc1 -56.5 -14.2 2015 1 3 3 2015-01-03 27.3 #> 4 loc1 -56.5 -14.2 2015 1 4 4 2015-01-04 28.7 #> 5 loc1 -56.5 -14.2 2015 1 5 5 2015-01-05 30 #> 6 loc1 -56.5 -14.2 2015 1 6 6 2015-01-06 28.7

one question is why the location names from my df are not pulled in the output. Is there any way to conserve the location names in the output?
Hi @GiuseppePetri. I just made a small edit to my code to add a column with the location names to the final output

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