Using for() over variables that need to be changed

Question

I'd like to be able use for() loop to automate the same operation that runs over many variables modifying them.

Here's simplest example to could design:

varToChange = list( 1:10, iris$Species[1:10], letters[1:10]) # assume that it has many more than just 3 elements
varToChange
for (i in varToChange ) {
  if (is.character(y)) i <- as.integer(as.ordered(i))
  if (is.factor(y)) i <- as.integer(i)
}
varToChange # <-- Here I want to see my elements as integers now

Here's actual example that led me to this question - taken from: Best way to plot automatically all data.table columns using ggplot2

In the following function

f <- function(dt, x,y,k) {
  if (is.numeric(x)) x <-  names(dt)[x]
  if (is.numeric(y)) y <-  names(dt)[y]
  if (is.numeric(k)) k <-  names(dt)[k]

 ggplot(dt, aes_string(x,y, col=k)) + geom_jitter(alpha=0.1)
}
f(diamonds, 1,7,2)

instead of brutally repeating the same line many times, as a programmer, I would rather have a loop to repeat this line for me. Something like this one:

for (i in c(x,y,k)) {
  if (is.numeric(i)) i <-  names(dt)[i]
 }

In C/C++ this would have been done using pointers. In R - is it all possible?

UPDATE: Very nice idea to use Map below. However it does not work for this example

getColName <- function(dt, x) { 
  if (is.numeric(x)) { 
    x <-  names(dt)[x]
  }  
  x  
}

f<- function(dt, x,y,k) {
  list(x,y,k) <- Map(getColName, list(x,y,k), dt)
  # if (is.numeric(x)) x <-  names(dt)[x]
  # if (is.numeric(y)) y <-  names(dt)[y]
  # if (is.numeric(k)) k <-  names(dt)[k]

  ggplot(dt, aes_string(x,y, col=k)) + geom_jitter(alpha=0.1)
}
f(diamonds, 1,7,2) # Brrr..

MrFlick · Accepted Answer · 2017-06-15T20:39:53.017

No need for for loop, just Map a function over each of your list items

varToChange = list( 1:10, iris$Species[1:10], letters[1:10])     
myfun <- function(y) {
  if (is.character(y)) y <- as.integer(as.ordered(y))
  if (is.factor(y)) y <- as.integer(y)
  y
}
varToChange <- Map(myfun, varToChange)

UPDATE: Map never modifies variables in place, This is simply not done in R. Use the new values returned by Map

f<- function(dt, x, y, k) {
  args <- Map(function(x) getColName(dt, x), list(x=x,y=y,k=k))

  ggplot(dt, aes_string(args$x,args$y, col=args$k)) + geom_jitter(alpha=0.1)
}
f(diamonds, 1,7,2)

score 1 · Answer 2 · answered Jun 13 '17 at 20:35

You have two choices for iteration in R, iterate over variables themselves, or over their indices. I generally recommend iterating over indices. This case illustrates a strong advantage of that because your question is a non-issue if you are using indices.

varToChange = list( 1:10, iris$Species[1:10], letters[1:10])
for (i in seq_along(varToChange)) {
  if (is.character(varToChange[[i]])) varToChange[[i]] <- as.integer(as.factor(varToChange[[i]]))
  if (is.factor(varToChange[[i]])) varToChange[[i]] <- as.integer(varToChange[[i]])
}

I also replaced as.ordered() with as.factor() - the only difference between an ordered factor and a regular factor are the default contrasts used in modeling. As you are just coercing to integer, it doesn't matter.

Using for() over variables that need to be changed

2 Answers2