You can read the original post in its original format on Rtask website by ThinkR here: Row-wise operations with the {tidyverse}

We are often asked how to perform row-wise operations in a data.frame (or a tibble) the answer is, as usual, “it depends” ?

Let’s look at some cases that should fit your needs.

library(tidyverse)

Let’s make an example dataset:

base <- tibble::tibble(
  a = 1:10,
  b = 1:10,
  c = 21:30
) %>% head()
base
## # A tibble: 6 × 3
##       a     b     c
##   <int> <int> <int>
## 1     1     1    21
## 2     2     2    22
## 3     3     3    23
## 4     4     4    24
## 5     5     5    25
## 6     6     6    26

Let’s say we want to add a new column whose value will depend on the content, per row, of columns a, b and c of our base example

Like this:

# A tibble: 6 x 4
      a     b     c new      
  <int> <int> <int> <chr>    
1     1     1    21 a equals 1 
2     2     2    22 other case
3     3     3    23 other case
4     4     4    24 other case
5     5     5    25 c equals 25
6     6     6    26 other case

With case_when()

base %>%
  mutate(
    new = case_when(
      a == 1 ~ "a equals 1",
      c == 25 ~ "c equals 25",
      TRUE ~ "other case"
    )
  )
## # A tibble: 6 × 4
##       a     b     c new        
##   <int> <int> <int> <chr>      
## 1     1     1    21 a equals 1 
## 2     2     2    22 other case 
## 3     3     3    23 other case 
## 4     4     4    24 other case 
## 5     5     5    25 c equals 25
## 6     6     6    26 other case

case_when() is nice, it’s much more readable than nested ifelse(), but it can quickly become more complex.
So let’s create a function which, depending on the values of a, b, c, returns the expected value.

Depending on the case (and your skills) you will sometimes have a vectorized function and sometimes a non-vectorized function. It is always better to create a vectorized function, but it is not always possible.
A vectorized function is a function that can be directly applied to a set of vectors and that returns a response vector.

An example of a vectorized function that repeats the operations of the previous case_when():

vectorised_function <- function(a, b, c, ...){
  ifelse(a == 1 , "a equals 1",
         ifelse(c == 25 , "c equals 25",
                "other case"
         ))
}
vectorised_function(a = 1, c = 25, b = "R")
## [1] "a equals 1"
vectorised_function(a = c(1, 1, 3), c = 27:25, b = "R")
## [1] "a equals 1"  "a equals 1"  "c equals 25"

Here is the “same” function, but not vectorized:

non_vectorised_function <- function(a, b, c, ...){
  if ( a == 1  ) { return("a equals 1") }
  if ( c == 25 ) { return("c equals 25") }
  return("autre")
}
non_vectorised_function(a = 1, c = 25, b = "R")
## [1] "a equals 1"
non_vectorised_function(a = c(1, 1, 3), c = 27:25, b = "R") # ne fonctionne pas
## Warning in if (a == 1) {: la condition a une longueur > 1 et seul le
## premier élément est utilisé
## [1] "a equals 1"

With a vectorized function

This is the simplest case, and the fastest too.
You can use it as is in a mutate() :

base %>%
  mutate(
    new = vectorised_function(a = a, b = b, c = c)
  )
## # A tibble: 6 × 4
##       a     b     c new        
##   <int> <int> <int> <chr>      
## 1     1     1    21 a equals 1 
## 2     2     2    22 other case 
## 3     3     3    23 other case 
## 4     4     4    24 other case 
## 5     5     5    25 c equals 25
## 6     6     6    26 other case

With a NON vectorized function

The result returned by a mutate() is not correct (the first value returned is repeated…)

base %>%
  mutate(
    new = non_vectorised_function(a = a, b = b, c = c)
  )
## Warning in if (a == 1) {: la condition a une longueur > 1 et seul le
## premier élément est utilisé
## # A tibble: 6 × 4
##       a     b     c new       
##   <int> <int> <int> <chr>     
## 1     1     1    21 a equals 1
## 2     2     2    22 a equals 1
## 3     3     3    23 a equals 1
## 4     4     4    24 a equals 1
## 5     5     5    25 a equals 1
## 6     6     6    26 a equals 1

So let’s change our strategy.

With rowwise()

rowwise() is back in the {dplyr} world and is specifically designed for this case:

base %>%
  rowwise() %>% 
  mutate(
    new = non_vectorised_function(a = a, b = b, c = c)
  )
## # A tibble: 6 × 4
## # Rowwise: 
##       a     b     c new        
##   <int> <int> <int> <chr>      
## 1     1     1    21 a equals 1 
## 2     2     2    22 autre      
## 3     3     3    23 autre      
## 4     4     4    24 autre      
## 5     5     5    25 c equals 25
## 6     6     6    26 autre

With pmap()

base %>%
  mutate(
    new = pmap_chr(list(a = a, b = b, c = c), non_vectorised_function)
  )
## # A tibble: 6 × 4
##       a     b     c new        
##   <int> <int> <int> <chr>      
## 1     1     1    21 a equals 1 
## 2     2     2    22 autre      
## 3     3     3    23 autre      
## 4     4     4    24 autre      
## 5     5     5    25 c equals 25
## 6     6     6    26 autre

Bonus with Vectorize()

The Vectorize() function allows to vectorize a function…
It’s a bit of a cheat, but it can help ?

base %>%
  mutate(
    new = Vectorize(non_vectorised_function)(a = a, b = b, c = c)
  )
## # A tibble: 6 × 4
##       a     b     c new        
##   <int> <int> <int> <chr>      
## 1     1     1    21 a equals 1 
## 2     2     2    22 autre      
## 3     3     3    23 autre      
## 4     4     4    24 autre      
## 5     5     5    25 c equals 25
## 6     6     6    26 autre

Row-wise operations are yours!

Experiment and tell us what your practices are!

To go further: https://dplyr.tidyverse.org/articles/rowwise.html

This post is better presented on its original ThinkR website here: Row-wise operations with the {tidyverse}