row_sd.R

Testdata <- data.frame(var_1 = c(1:4,NA), var_2 = 6:10,var_3 = 11:15)
Testdata

##   var_1 var_2 var_3
## 1     1     6    11
## 2     2     7    12
## 3     3     8    13
## 4     4     9    14
## 5    NA    10    15

apply(Testdata, 1, function(x) sd(x, na.rm = T))

## [1] 5.000000 5.000000 5.000000 5.000000 3.535534

sd(c(10,15))

## [1] 3.535534

apply(Testdata, 1, sd)

## [1]  5  5  5  5 NA