prop.table doesn't work in a for-loop? - r

Question

This may be a very simple question, but I don't see how to answer it.
I have the following reproducible code, where I have two small dataframes that I use to calculate a percentage value based on each column total:
#dataframe x
x <- structure(list(PROV = structure(c(1L, 1L), .Label = "AG", class = "factor"),
APT = structure(1:2, .Label = c("AAA", "BBB"), class = "factor"),
PAX.2013 = c(5L, 4L), PAX.2014 = c(4L, 2L), PAX.2015 = c(4L,0L)),
.Names = c("PROV", "APT", "PAX.2013", "PAX.2014", "PAX.2015"),
row.names = 1:2, class = "data.frame")
#dataframe y
y <- structure(list(PROV = structure(c(1L, 1L), .Label = "AQ", class = "factor"),
APT = structure(1:2, .Label = c("CCC", "AAA"), class = "factor"),
PAX.2013 = c(3L, 7L), PAX.2014 = c(2L, 1L), PAX.2015 = c(0L,3L)),
.Names = c("PROV", "APT", "PAX.2013", "PAX.2014", "PAX.2015"),
row.names = 1:2, class = "data.frame")
#list z (with x and y)
z <- list(x,y)
#percentage value of x and y based on columns total
round(prop.table(as.matrix(z[[1]][3:5]), margin = 2)*100,1)
round(prop.table(as.matrix(z[[2]][3:5]), margin = 2)*100,1)
as you can see, it works just fine.
Now I want to automate for all the list, but I can't figure out how to get the results. This is my simple code:
#for-loop that is not working
for (i in length(z))
{round(prop.table(as.matrix(z[[i]][3:5]), margin = 2)*100,1)}

Answer 1

You have two problems.
First, you have not put a range into your for loop so you are just trying to iterate over a single number and second, you are not assigning your result anywhere on each iteration.
Use 1:length(z) to define a range. Then assign the results to a variable.
This would work:
my_list <- list()
for (i in 1:length(z)){
my_list[[i]] <- round(prop.table(as.matrix(z[[i]][3:5]),
margin = 2)*100,1)
}
my_list
But it would be more efficient and idiomatic to use lapply:
lapply(1:length(z),
function(x) round(prop.table(as.matrix(z[[x]][3:5]), margin = 2)*100,1))

Answer 2

Barring discussions whether for-loops is the best approach, you had two issues. One, your for loop only iterates over 2 (which is length(z)) instead of 1:2. Two, you need to do something with the round(....) statement. In this solution, I added a print statement.
for (i in 1:length(z)){
print(round(prop.table(as.matrix(z[[i]][3:5]), margin = 2)*100,1))
}