R – iteratively changing column classes

April 4, 2017

(This article was first published on R – scottishsnow, and kindly contributed to R-bloggers)

When you write an ESRI Shapefile from R, writeOGR assumes your numeric fields are real numbers and so you get a lot of redundant zeros appended. This is misleading and distracting when you present data to third parties. You can fix this by changing column class from numeric to integer:

class(df$col) = "integer"

However, this only changes one column at a time. You can’t change the whole data frame (S4 spatial, or whatever) using the same method, or you’ll no longer have a data frame. Usually I’d solve this problem using an *apply method, in this case mapply or apply, but these don’t work in this case because the data frame is coerced into a matrix. This is one of the rare occasions when you need to use a for loop in R:

cols_to_change = c(1, 3, 5:9)
for(i in cols_to_change){
   class(df[, i]) = "integer"

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