Subsetting rows by passing an argument to a function - r

Question

I have the following data frame which I imported into R using read.table() (I incorporated read.table() within read_data() which is a function I created that also throw messages in case the file name is not written appropriately):
> raw_data <- read_data("n44.txt")
[1] #### Reading txt file ####
> head(raw_data)
subject block trial_num soa target_identity prime_type target_type congruency prime_exposure target_exposure button_pressed rt ac
1 99 1 1 200 82 9 1 9 0 36 1 1253 1
2 99 1 2 102 95 2 1 2 75 36 1 1895 1
3 99 1 3 68 257 2 2 1 75 36 2 1049 1
4 99 1 4 68 62 9 1 9 0 36 1 1732 1
5 99 1 5 34 482 9 3 9 0 36 3 765 1
6 99 1 6 68 63 9 1 9 0 36 1 2027 1
Then I'm using raw_data within the early_prep() function I created (I copied only the relevant part of the function):
early_prep <- function(file_name, keep_rows = NULL, id = NULL){
if (is.null(id)) {
# Stops running the function
stop("~~~~~~~~~~~ id is missing. Please provide name of id column ~~~~~~~~~~~")
}
# Call read_data() function
raw_data <- read_data(file_name)
if (!is.null(keep_rows)) {
raw_data <- raw_data[keep_rows, ]
# Print to console
print("#### Deleting unnecesarry rows in raw_data ####", quote = FALSE)
}
print(dim(raw_data))
print(head(raw_data))
return(raw_data)
}
}
My problem is with raw_data <- raw_data[keep_rows, ].
When I enter keep_rows = "raw_data$block > 1" this is what I get:
> x1 <- early_prep(file_name = "n44.txt", keep_rows = "raw_data$block > 1", id = "subject")
[1] #### Reading txt file ####
[1] #### Deleting unnecesarry rows in raw_data ####
[1] 1 13
subject block trial_num soa target_identity prime_type target_type congruency prime_exposure target_exposure button_pressed rt ac
NA NA NA NA NA NA NA NA NA NA NA NA NA NA
How can I solve this so it will only delete the rows I want?
Any help will be greatly appreciated
Best,
Ayala

Answer 1

The problem is that you pass the condition as a string and not as a real condition, so R can't evaluate it when you want it to.
if you still want to pass it as string you need to parse and eval it in the right place for example:
cond = eval(parse(text=keep_rows))
raw_data = raw_data[cond,]
This should work, I think