Calculate the ratio between columns based on the condition in R

Question

Hi I am doing a very simple thing here.

I have 3 columns of data; a, b and c. I want to calculate the ratio of b and c when the value of b is only greater than 5.

The code I used is as follows:

a <- c(1,2,3,4,5,6,7,8)
b <- c(2,4,5,8,10,12,15,18)
c <- c (10,9,8,15,31,12,13,12)

df <- data.frame(a,b,c)
 ## Calculate the ratio 
df$d <- with(df,b/c)

I calculated the ratio but couldn't use the condition there. I know its trivial but this is taking a lot of time. I appreciate your help.

Answer 1

You can use ifelse. You don't say what you want the result to be in cases where b is less than 5, so I've assumed you want NA there.:

> df$d <- with(df, ifelse(b > 5, b/c, NA))
> df
  a  b  c         d
1 1  2 10        NA
2 2  4  9        NA
3 3  5  8        NA
4 4  8 15 0.5333333
5 5 10 31 0.3225806
6 6 12 12 1.0000000
7 7 15 13 1.1538462
8 8 18 12 1.5000000