How to use sapply to divide different rows for each column

I'm a bit new at R, but I am trying to do something very simple using sapply because I will need to do it a lot. Say that you have many variables for 5 years, and want to divide the fifth row's values by the first row's for each of the columns at once.

a b c
184 20 55
100 32 563
18 12 88
5 99 52
32 36 22


So far I can either do it one by one:

df$a<-(df[5,]$a/df[1,]$a)


Or if I try to use sapply:

df2<-data.frame(sapply(names(df)[-1], function(x)
(df[x]/df[x])
))


The problem is that I don't know how to denote the rows with the sapply so above I'm just dividing vars by themselves. What's the quickest way of doing this? Thanks!

3 Answers
3

If you have mixed type columns here is a dplyr approach

dplyr

library(dplyr)
df %>% mutate_if(is.numeric, function(x) replace(x, length(x), x[length(x)] / x[1]))
# a b c d e
#1 184.000000 20.0 55.0 a A
#2 100.000000 32.0 563.0 b B
#3 18.000000 12.0 88.0 c C
#4 5.000000 99.0 52.0 d D
#5 0.173913 1.8 0.4 e E


# Sample data with mixed numeric and character columns
df <- read.table(text =
"a b c
184 20 55
100 32 563
18 12 88
5 99 52
32 36 22 ", header = T)
df <- cbind(df, d = letters[1:5], e = LETTERS[1:5])


This is something that could be good as a function:

library(dplyr)

div_row <- function(data, numerator, denominator)
data %>% mutate_if(is.numeric, funs(if_else(row_number() == numerator, .[numerator]/.[denominator], .)))


df %>% div_row(5,1)
# a b c d
# 1 184 20 55 a
# 2 100 32 563 a
# 3 18 12 88 c
# 4 5 99 52 e
# 5 0.174 1.8 0.4 t

df %>% div_row(2,1)
# a b c d
# 1 184 20 55 a
# 2 0.543 1.6 10.2 a
# 3 18 12 88 c
# 4 5 99 52 e
# 5 32 36 22 t


For this task you don't need sapply and instead do

sapply

df[5, ] <- df[5, ] / df[1, ]
df
# a b c
#1 184.000000 20.0 55.0
#2 100.000000 32.0 563.0
#3 18.000000 12.0 88.0
#4 5.000000 99.0 52.0
#5 0.173913 1.8 0.4


Referring to @Mako212's comment, if your data contains non-numeric columns you could create a logical vector first that is TRUE at positions where you're data contains numeric columns. Use it for column subsetting and then do the operation.

TRUE

col_idx <- sapply(df, is.numeric)
df[5, col_idx] <- df[5, col_idx] / df[1, col_idx]


iris[1,1:4]/iris[5,1:4]

df

col_idx <- sapply(df, is.numeric)

iris

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