### A sample session
# set working directory
setwd("~/Documents/Projects/200B/lab1/")

# R works just like a calculator A "#" 
# signifies a comment and will be ignored by R
# Basic math
3 + 5
3 * 10 
4 ^ 10
exp(2)
sin(1)
?exp

# Vectors
# use "c" to define a vector
c(1,2,3,4)
# "<-" assigns a value to a variable
x <- c(1,2,3,4)
# transpose
t(x)

length(x)
sum(x)
# element wise mulitplication
x * t(x)

# vector multiplication
x %*% t(x)

# some sequences
1:10
seq(0, 10, 0.5)
seq(to = 0, from = 10, by = -0.5)

# looking for a function
?product # doesn't exist
help.search("product") # find prod
?prod
prod(x)

# list objects in workspace
ls()

#character
name <- c("charlotte", "wickham")
class(name)

# convert character to numbers
numbers <- c("1","2","3")
numbers + 4
as.numeric(numbers)
as.numeric(numbers) + 4

# Matrices
# fills matrix by columns using vector
z <- matrix(c(1,2,3,4,5,6,7,8), nrow = 2)
matrix(c(1,2,3,4,5,6,7,8,9), nrow = 2)
# examples of recycling
matrix(c(1,2,3,4,5,6,7,8,9), nrow = 2, ncol = 6)
matrix(3, nrow = 4, ncol = 4)

# Subsetting matrices
# 1st column
z[,1]
# 1st row
z[1,]
# item in 1st row and 1st column
z[1,1]
# column 1 and 2
z[,c(1,2)]

# Dataframes
x <- 1:10
y <- rep(c("a","b"),5)
z <- x > 5
class(z) 	# logical class - T/F
# vectors must be the same length
# but can be different types
obs <- data.frame(x, y, z)
names(obs)

# Subsetting a dataframe
# a column
obs[,2]
# a row
obs[3,]
#column called x
obs$x
# columns x and y
obs[,c("x","y")]