title: R dplyr with multiple columns with same stem name
body: <p>I have some data columns that I need to set to NA whenever a corresponding column is >0.</p>
  
  <p>I can do this with mutate and the names of the two columns, but I want a scoped version where I create the name of the corresponding column from the name of the 1st column</p>
  
  <pre><code>(x&lt;-data.frame(x1=(1:4),map.x1=c(0,0,7,0),x2=c(2,2,2,2),map.x2=c(0,7,0,0)))
  
  mutate(x, x1=ifelse(map.x1&gt;0, NA, x1))
  
  mutate_at(x, vars(starts_with("x")), function(v) { 
    m.name &lt;- paste0("map.", deparse(substitute(v)))
    ifelse(get(m.name)&gt;0, NA, v)
    )
  }
  </code></pre>
  
  <p>I can see ifelse() is unhappy because it wants the 1st argument to be an object, and I have given an expression.</p>
  
  <p>I could not find a way. I even wondered if there was some way to avoid the function(v) and use (.) in paste0() or get()</p>
  
  <p>I also am considering reshaping so I can do one mutate. What is the best practice here?</p>