CODEWITHSUNDEEP
rfor-loopdplyr
DN1
DN1

Reputation: 218

How to subtract a row from the row above using a for loop in R?

I am trying to learn how to use for loops in R, in particular to subtract a number from it's above number in a column in R.

I know can do this with b <- diff(df$a) or with:

library(dplyr)
df %>% 
  mutate(b = a - lag(a))

But I am trying to understand how I can also get the same result with something like:

for(i in 1:nrow(df)){
  result = df[2,] - df[i,]
  print (result)
}

How do I set this for loop so the df[2,] takes every following row, and not just the 2nd row, and subtracts from the row above?

For example I have data like this:

column a
1
10
20

and I want to eventually create a column with the subtractions:

column a    column b
1             10
11            9
20           ...

Upvotes: 0

Views: 667

Answers (1)

Ronak Shah
Ronak Shah

Reputation: 388907

You can use for loop like

df$columnB <- NA

for(i in 1:(nrow(df) - 1)) {
   df$columnB[i] = df$columnA[i+1] - df$columnA[i]
}
df
#  columnA columnB
#1       1      10
#2      11       9
#3      20       5
#4      25       9
#5      34      NA

data

Sample data used:

df <- data.frame(columnA  =  c(1, 11, 20, 25, 34))

Upvotes: 2

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