dplyr - using mutate() like rowmeans()

Question

I can't find the answer anywhere.

I would like to calculate new variable of data frame which is based on mean of rows.

For example:

data <- data.frame(id=c(101,102,103), a=c(1,2,3), b=c(2,2,2), c=c(3,3,3))

I want to use mutate to make variable d which is mean of a,b and c. And I would like to be able to make that by selecting columns in way d=mean(a,b,c), and also I need to use range of variables (like in dplyr) d=mean(a:c).

And of course

mutate(data, c=mean(a,b)) 

or

mutate(data, c=rowMeans(a,b)) 

doesn't work.

Can you give me some tip?

Regards

Answer 1

You're looking for

data %>% 
    rowwise() %>% 
    mutate(c=mean(c(a,b)))

#      id     a     b     c
#   (dbl) (dbl) (dbl) (dbl)
# 1   101     1     2   1.5
# 2   102     2     2   2.0
# 3   103     3     2   2.5

or

library(purrr)
data %>% 
    rowwise() %>% 
    mutate(c=lift_vd(mean)(a,b))

Answer 2

dplyr is badly suited to operate on this kind of data because it assumes tidy data format and — for the problem in question — your data is untidy.

You can of course tidy it first:

tidy_data = tidyr::gather(data, name, value, -id)

Which looks like this:

   id name value
1 101    a     1
2 102    a     2
3 103    a     3
4 101    b     2
5 102    b     2
6 103    b     2
    …

And then:

tidy_data %>% group_by(id) %>% summarize(mean = mean(value))

    name  mean
  (fctr) (dbl)
1      a     2
2      b     2
3      c     3

Of course this discards the original data. You could use mutate instead of summarize to avoid this. Finally, you can then un-tidy your data again:

tidy_data %>%
    group_by(id) %>%
    mutate(mean = mean(value)) %>%
    tidyr::spread(name, value)

     id     mean     a     b     c
  (dbl)    (dbl) (dbl) (dbl) (dbl)
1   101 2.000000     1     2     3
2   102 2.333333     2     2     3
3   103 2.666667     3     2     3

Alternatively, you could summarise and then merge the result with the original table:

tidy_data %>%
    group_by(id) %>%
    summarize(mean = mean(value)) %>%
    inner_join(data, by = 'id')

The result is the same in either case. I conceptually prefer the second variant.

Answer 3

I think the answer suggesting using data.frame or slicing on . is the best, but could be made simpler and more dplyr-ish like so:

data %>% mutate(c = rowMeans(select(., a,b)))

Or if you want to avoid ., with the penalty of having two inputs to your pipeline:

data %>% mutate(c = rowMeans(select(data, a,b)))

Answer 4

And yet another couple of ways, useful if you have the numeric positions or vector names of the columns to be summarised:

data %>% mutate(d = rowMeans(.[, 2:4]))

or

data %>% mutate(d = rowMeans(.[, c("a","b","c")]))

Answer 5

Another simple possibility with few code is:

data %>%
    mutate(c= rowMeans(data.frame(a,b)))

 #     id a b   c
 #  1 101 1 2 1.5
 #  2 102 2 2 2.0
 #  3 103 3 2 2.5

As rowMeans needs something like a matrix or a data.frame, you can use data.frame(var1, var2, ...) instead of c(var1, var2, ...). If you have NAs in your data you'll need to tell R what to do, for example to remove them: rowMeans(data.frame(a,b), na.rm=TRUE)

Answer 6

I think this is the dplyr-ish way. First, I'd create a function:

my_rowmeans = function(...) Reduce(`+`, list(...))/length(list(...))

Then, it can be used inside mutate:

data %>% mutate(rms = my_rowmeans(a, b))

#    id a b c rms
# 1 101 1 2 3 1.5
# 2 102 2 2 3 2.0
# 3 103 3 2 3 2.5

# or

data %>% mutate(rms = my_rowmeans(a, b, c))

#    id a b c      rms
# 1 101 1 2 3 2.000000
# 2 102 2 2 3 2.333333
# 3 103 3 2 3 2.666667

---

To deal with the possibility of NAs, the function must be uglified:

my_rowmeans = function(..., na.rm=TRUE){
  x = 
    if (na.rm) lapply(list(...), function(x) replace(x, is.na(x), as(0, class(x)))) 
    else       list(...)

  d = Reduce(function(x,y) x+!is.na(y), list(...), init=0)

  Reduce(`+`, x)/d
} 

# alternately...

my_rowmeans2 = function(..., na.rm=TRUE) rowMeans(cbind(...), na.rm=na.rm)

# new example

data$b[2] <- NA  
data %>% mutate(rms = my_rowmeans(a,b,na.rm=FALSE))

   id a  b c rms
1 101 1  2 3 1.5
2 102 2 NA 3  NA
3 103 3  2 3 2.5

data %>% mutate(rms = my_rowmeans(a,b))

   id a  b c rms
1 101 1  2 3 1.5
2 102 2 NA 3 2.0
3 103 3  2 3 2.5

The downside to the my_rowmeans2 is that it coerces to a matrix. I'm not certain that this will always be slower than the Reduce approach, though.

Answer 7

If you'd like to use a pivot_longer()-style solution:

data%>%
pivot_longer(cols=-id)%>%
group_by(id)%>%
mutate(mean=mean(value))%>%
pivot_wider(names_from=name, values_from=value)

Note that this requires the tidyr package.

This is my preference for the fact that I only need to type the name of my ID column, and don't have to worry about column indices or names otherwise. Good for a quick copy-and-point-this-at-different-data solution, though the same can be said of other answers here. Also good for cases where you might have more than one column with categorical information and haven't created a single unique identifier column.

For what it's worth, I found that this solution is very easily modified to ignore NA values with simple addition of na.rm=TRUE in the mean calculation.

For example:

data <- data.frame(id=c(101,102,103), a=c(NA,2,3), b=c(2,2,2), c=c(3,3,3))


data%>%
pivot_longer(cols=-id)%>%
group_by(id)%>%
mutate(mean=mean(value,na.rm=TRUE))%>%
pivot_wider(names_from = name, values_from=value)