################################################################################
#
# Codewalk Base R
################################################################################


## FUNCTIONS
# no default value
# function will return the value for the last executed line
addNumber <- function (x, y) {
    x+y
}
addNumber(4, 2)
addNumber(4)

# you can add a default argument value
addNumber <- function (x, y=10) {
    x+y
}
addNumber(4)

# with return fucntion
addNumber <- function (x, y=10) {
    z <- x+y
    #return(z)
}
addNumber(4)

# be aware when R providesa simple vectorization solution
# do not provide a loop solution
x <- c( 3, 4, 5)
y <- x + 1 # simple solution

# never this
y <- c()
for (e in x) {
  y <- c(y, e + 1)
}

################################################################################
## BUILT IN HELPER FUNCTIONS
args(which)
?which
help(which)



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## COMMENT AND UNCOMMENT
# highlight block of code then press ctrl+shift+c or cmd+shift+c in Mac

# Peter Piper picked a peck of pickled peppers.
# A peck of pickled peppers Peter Piper picked.
# If Peter Piper picked a peck of pickled peppers,
# Where's the peck of pickled peppers Peter Piper picked?



################################################################################
## MISSING VALUES: NaN vs. NA
# NaN values are NA, but NA values are not NaN
# division by zero results in Inf NOT NaN
a <- c(0/0, 1, 2/0, 3, NA)
a
is.na(a)
is.nan(a)
# demonstrates that Inf is a number
a <- na.omit(a)
a


###############################################
## CONDITIONAL FLOW CONTROL
# hours range from 0-23
thisHour <- as.POSIXlt(Sys.time())$hour
?POSIXlt
if (thisHour<12) {
    print("It's morning")
} else if (thisHour==12) {
        print("It's noon time")
} else if (thisHour>12 & thisHour<5) {
    print("It's afternoon")
} else {
    print("It's evening")
}

# ifelse() function is like the ternary operator
# weekdays range from 0-6
today <- as.POSIXlt(Sys.time())$wday
ifelse( today==0 & today==6, "It's a weekend", "It's a weekday")


########################################################
# LOOPS WITH "apply" FAMILY OF FUNCTIONS

# apply()
# applies function on margins of arrays or matrices and returns a vector or array
# an array in R is a vector with the additional attributes dim() and dimnames()
# 2D array is a matrix
# MARGIN = 1 apply over rows, 2 appply over columns, c(1,2) apply over both
x <- matrix(seq(12), nrow=3, ncol=4)
x
sum_rows <- apply(X=x, MARGIN=1, FUN=sum)
sum_rows
is.vector(sum_rows)
sum_cols <- apply(X=x, MARGIN=2, FUN=sum)
sum_cols


# lapply()
# applies a function over each item in a list and returns a list
# x will be coerced into a list type if it isn't already one.
# lapply(X=, FUN=, ... ) ... optional arguments to the function
lapply(1:5, function(x) x^3 + 9)
unlist(lapply(1:5, function(x) x^3 + 9))


# sapply()
# similar to lapply() but returns a vector instead of a list
sapply(1:5, function(x) x^3 + 9)


# More advanced apply functions not covered: tapply() and mapply().
# If you are interested, explore those on your own.