R 以地址取得經緯座標值

假如data.frame data中的lat值為0時，則利用google maps api geocode 查詢地址的經緯座標，並存回 data$lat 與 data$lng 中：

library(ggmap)

for (i in 1:nrow(data)) {
     if (data$lat[i] == 0) {
         addr <- data$service_addr[i]
         url = paste('http://maps.google.com/maps/api/geocode/xml?address=', addr,'&sensor=false',sep='')
         doc = xmlTreeParse(url) 
         root = xmlRoot(doc) 
         data$lat[i] = xmlValue(root[['result']][['geometry']][['location']][['lat']]) 
         data$lng[i] = xmlValue(root[['result']][['geometry']][['location']][['lng']])
     }
}

# 將 data 內容寫入檔案 mymap.csv
write.table(data, file = "mymap.csv", sep = "," )

#