CODEWITHSUNDEEP
rfor-loopdplyr
MaddieS
MaddieS

Reputation: 71

Using mutate with function that includes nth row value

I am wondering if it is possible to use dplyr's mutate function to add a new column to a data frame that is a function of multiple row values from other columns. For example, if I had a data frame df with columns a and b and wanted to make a new column c, where c = a[n] + b[n +1], can I use mutate with a for loop?

I tried the below code but it didn't work:

  for(n in 1:nrow(df)){
        df %>%
              mutate(c = a[n] + b[n + 1])    
  }

Upvotes: 1

Views: 446

Answers (1)

user295691
user295691

Reputation: 7248

The function you're probably looking for it lead, to get a view of b offset by one record. Then the command would be

df %>% mutate(c=a + lead(b, 1))

For example,

> data.frame(a=1:10, b=1:10) %>% mutate(c = a + lead(b, 1))
    a  b  c
1   1  1  3
2   2  2  5
3   3  3  7
4   4  4  9
5   5  5 11
6   6  6 13
7   7  7 15
8   8  8 17
9   9  9 19
10 10 10 NA

Upvotes: 4

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