title: How to update only missing values in R based on parameters
body: <p>I have data set as below mentioned script </p>
  
  <pre><code>library(ggmap)
  countries &lt;- c('Ghana', 'Guinea', 'Mali', 'Niger')
  withLocation&lt;- data.frame(countries, geocode(countries))
  </code></pre>
  
  <p>once I run the command then I get data like this </p>
  
  <pre><code>       countries        lon        lat
  1          Ghana  -1.023194   7.946527
  2         Guinea  -9.696645   9.945587
  3           Mali  -3.996166  17.570692
  4          Niger         NA         NA
  </code></pre>
  
  <p>Now I have missing values for 'Niger' and want to update that row only as running the google API with complete list will miss different country, please help me to achieve this </p>