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Get distance and value for origin-destination pairs from Google Maps API
google_maps <- function(datafile, gapikey, save_file = FALSE){
# This function takes origin-destination pairs and returns the distance and
# trip duration between these pairs as a data frame
# _datafile_ is a string that is the name of a csv file in which each line
# contains an origin and a destination separated by a comma.
# _gapikey_ is the Google Maps API key, also a string.
# Note: distances are in meters, durations are in seconds.
gdist <- as.numeric()
gdur <- as.numeric()
data <- read.csv(file = paste0(datafile), header = FALSE)
names(data) <- c("from", "to")
for (i in 1:nrow(data)){
jsonData <- fromJSON(paste0("", data[i,1] ,
"&destination=", data[i,2], "&key=",gapikey))
gdist <- c(gdist, unlist(jsonData$routes$legs[1])["distance.value"])
gdur <- c(gdur, unlist(jsonData$routes$legs[1])["duration.value"])
} else {
gdist <- c(gdist, NA)
gdur <- c(gdur, NA)
data <- cbind(data, gdist, gdur)
if (save_file == TRUE){
write.csv(data, file = paste0("dist_matrix_", datafile))
message(paste0('Note: results saved as "dist_matrix_', datafile, '"'))
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