Subsetting in R

# Contents
## SUBSETTING A VECTOR
## SUBSETTING A LIST 
## SUBSETTING A DATA FRAME

# Note: data.table package provides faster data manipulation in terms of subsetting with simpler code.
# Explained in Data Manipulation with R_part 3.r file
#____________________________________________________________________

## SUBSETTING A VECTOR

a <- c(1, 3, 5, 3, 5.5)

# To get a subset of a (only elements in the range 1<a<4)
a[a > 1 & a < 4] 

# To get an index (position) of numbers which are in the range 1<a<4
 which(a > 1 & a < 4) 

#When using brackets of the form parent[child], single brackets return an object the same class as the parent 
#and double brackets returns an object the same class as the child.

## SUBSETTING A LIST 

x <- list(a = list(10, 12, 14), b = c(3.14, 2.81))

# [ ] return an object from object x. So x['a'] returns a list. This list has only one member with name of 'a'.
x["a"]
# [[ ]] returns an element of x. [['a']] returns the first element of x, which has a name 'a'. It is a list of length 3, 
x[["a"]]
# x$a is equivalent to [['a']]. 
x$a

## SUBSETTING A DATA FRAME

# From a dataframe, subset where the values of two different columns meet certain criteria
x <- andy # x is assigned the data frame 'andy'
x[x[["Col3"]] > 135 & x[["col4"]] < 18 , ]   ## doubts in results


x = list(a = 1:5, b="hi")
y = x["a"]
y

# To create a data frame
n = c(2, 3, 5) 
s = c("aa", "bb", "cc") 
b = c(TRUE, FALSE, TRUE) 
df = data.frame(n, s, b)