Cleaning Data - Some Important Tricks (Part 3)

Count the number of rows

dat1 = data.frame(ID = c(100,100,200), Name = c('Ha', NA, 'Ly'), Age = c(30,0,NA), stringsAsFactors = FALSE)
#Use: stringAsFactors for changing factor level in data frame into correct format (in this case: Name has chr type, Age has num type)
dat1

##    ID Name Age
## 1 100   Ha  30
## 2 100 <NA>   0
## 3 200   Ly  NA

length(unique(dat1[,c("ID")])) #Count the number of unique ID

## [1] 2

length(unique(dat1[,c("Name")])) #Count the number of unique name

## [1] 3

nrow(unique(dat1[,c("ID","Name")])) #Count the number of unique (ID and Name)

## [1] 3

length(unique(dat1[,c("ID","Name")])) #Count the number of column

## [1] 2

Notice: Difference between using length and nrow

• length: count the number of values in a vector.

• nrow: count the number of rows in a table. Because ID and Name in dat1 created a table, so we need to use nrow to count rows. If we use length, it will count the number of column in dat1.