# Logical subsetting
# Remember birth data
library(SMPracticals)
data(births)

# say I want times just for day 2
births$day
# which are day 2
births$day == 2
# data.frame rows for day 2
births[births$day == 2, ]
# just the times for day 2
births$time[births$day == 2]
# or
births[births$day == 2, "time"]

# equivalently
subset(births, day == 2)
subset(births, day == 2)$time

# more complicated expressions
# day 2 and labour longer than 8 hours
subset(births, day == 2 & time > 8)

# labour less than four hours or more than 8
subset(births, time < 4 | time > 8)

# if you want to do something with the subset you need to save it to a variable
short_long <- subset(births, time < 4 | time > 8)
hist(short_long$time)

# iris data
head(iris)
summary(iris)

# Q: try to get Petal.Width for obs with Petal.Length > 3

# Q: of those that have Petal.Length > 3 how many are virginica

# Q: how could I get every 2nd row? hint: remember recycling