replace multiple columns of data frame with indexing in r

Question

I'm trying to replace multiple columns of a large data frame based with indexing. What I want/have done so far combines this post and this post. Let me provide the example for clarity.

Here is much simplified sample data in dput format:

DF <-
structure(list(Fruits = structure(c(1L, 3L, 4L, 2L), 
.Label = c("Apples", "Avocado", "Oranges", "Pineapple"), 
class = "factor"), Weight2 = c(20L, 15L, 40L, 60L), 
Weight = c(2L, 4L, 8L, 5L), `Number` = c(10L, 
16L, 6L, 20L)), class = "data.frame", row.names = c(NA, -4L))

Fruits   Weight   Weight2   Number

  Apples      20      2          10
  Oranges     15      4          16
  Pineapple   40      8           6
  Avocado     60      5          20

What I want to do is for DF, given a list of Fruits, change the columns Weight and Weight2 to N. I'll mention that my DF is actually a list of data frames and my list is a list of lists so indexing is needed. Here is my code so far:

fruit.to.change <- c("Apples","Pineapple")  
DF$Weight[which(DF$Fruits == fruit.to.change)] <- "N" #change the first column but I want to change multiple columns.

colID <- grepl("Weight", names(DF))
which(DF$Fruits %in% fruit.to.change[1:length(fruit.to.change)]) #gets the positions matching

But not sure how to select and replace the columns in colID? I'm sure it's just another level of indexing but can't figure it out. I basically want something like DF$Weight:Weight2 [which(DF$Fruits %in% fruit.to.change )] <- "N" Thanks VERY much.

Answer 1

We can subset rows using fruit.to.change and columns with colID and replace where the condition is satisfied to "N".

DF[DF$Fruits %in% fruit.to.change,colID] <- "N"
DF
#     Fruits Weight2 Weight Number
#1    Apples       N      N     10
#2   Oranges      15      4     16
#3 Pineapple       N      N      6
#4   Avocado      60      5     20

Answer 2

An option with dplyr would be

library(dplyr)
DF %>%
   mutate_at(vars(colID), ~ replace(., Fruits %in% fruit.to.change,  "NA"))