Learn R

Test of Missing Values

#Use is.na for testing for missing values

> y<-c(1,3,5,7,NA,11,NA,NA,17)

> is.na(y)

[1] FALSE FALSE FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE

Recode Values to Missing Values

# recode 99 to missing for variable v1

# select rows where v1 is 99 and recode column v1 

df$v1[df$v1==99] <- NA

Excluding Missing Values from Analyses

> x<-c(1,2,NA,3) 

> mean(x) # returns NA

[1] NA 

> mean(x,na.rm = TRUE) # returns 2

[1] 2

The function complete.cases() returns a logical vector indicating which cases are complete.