title: Using is.na with Sapply function in R
body: <p>Can anyone tell me what the line of code written below do?</p>
  
  <p><code>sapply(X, function(x) sum(is.na(x))) / nrow(airports) * 100</code></p>
  
  <p>What is understood is that it will drop <code>NA</code>s when it applies the sum function but keeps them in the matrix.</p>
  
  <p>Any help is appreciated.</p>
  
  <p>Thank you</p>