Question

I’m quite new to R and I would like to learn how to write a Loop to create and process several columns.

I imported a table into R that cointains data with 23 variables. For all of these variables I want to calculate the per capita valuem multiply this with 1000 and either write the data into a new table or in the same table as the old data.

So to this for only one column my operation looked like this:

<i>agriculture<-cbind(agriculture,"Total_value_per_capita"=agriculture$Total/agriculture$Total.Population*1000)</i>

Now I’m asking how to do this in a Loop for the 23 variables so that I won’t have to write 23 similar lines of code.

I think the solution might look quite similar to the code pasted in this thread:

loop to create several matrix in R (maybe using paste)

but I dind’t got it working on my code.

So any suggestion would be very helpful.

Answer 1

I would always favor an appropriate *ply function over loops in R. In this case sapply could be your friend:

df <- data.frame( a=sample(10), b=sample(10), c=sample(10) )
df.per.capita <– as.data.frame(
  sapply(
    df[ colnames(df) != "c" ], function(x){ x/df$c *1000 }
  )
)

For more complicated cases, you should definitely have a look at the plyr package.