Wednesday, January 28, 2015

Basic R revision - Part 1

A random useful function

To get the data type of of variable in R, use the function class().

my_numeric = 42
my_character = "forty-two"
my_logical = FALSE

> class(my_numeric)
[1] "numeric"
> class(my_character)
[1] "character"
> class(my_logical)
[1] "logical"

Always remember: Python/C++ vector indices start with 0, R vector indices start with 1
 Subset a vector in R, use vectorname[c(starting index: ending index)]
If disparate (non-adjacent elements): vectorname[c(index1, index2, index3 ..)]

Compare to Python:
Subset a vector in Python, use vectorname[starting index: ending index + 1]
Note that index numbers will be defined as per Python convention
Suppose from a vector v = ['P','O','K','E','R'], we need to output ['O','K','E']
In Python, use v[1:4]
In R, use v[c(2:4)] or just v[2:4]

Also to get all elements in Python a way is to do Mymatrix[3, : ] (gets row 3)
To do the same exercise in R the way to do is Mymatrix[3,  ]

Comparison Operators in R vs VBA and Python

Comparison Operators in R and C++
<, >, >=, ==, !=

Comparison operators in VBA
<, >, <=, =, <>
Python supports both != and

For equality, Python supports == (like R and C++)

In R you can use comparison operator between a vector and a number and get a binary vector which compares each element of the vector to that number.
(Not sure if you can do that in Python. Will check later and update.) Also, you can use that binary vector as an index to get a subset of the original vector.

Matrix in R: To construct a matrix in R you need to add a matrix() wrapper to a vector. e.g. matrix(c(1:9), byrow=TRUE, nrow=3)

Naming elements of a vector and rows/cols of a matrix

Naming can often be useful later. Syntax is simple:
For vector:
vectorv = c(2,3,4)
Now, vectorv[“a”]=2
Now, vectorv[“a”]=2

For matrix:
new_hope = c( 460.998007, 314.4)
empire_strikes = c(290.475067, 247.900000)
return_jedi = c(309.306177,165.8)
# Construct the matrix
star_wars_matrix = matrix(c(new_hope,empire_strikes,return_jedi), nrow=3, byrow=TRUE)
# Add your code here such that rows and columns of star_wars_matrix have a name!
rownames(star_wars_matrix) = c("A New Hope", "The Empire Strikes Back", "Return of the Jedi")
colnames(star_wars_matrix)= c("US", "non-US")

Another way can be to include these in the matrix definition itself:
movie_names = c("A New Hope","The Empire Strikes Back","Return of the Jedi")
col_titles = c("US","non-US")
star_wars_matrix = matrix(box_office_all, nrow=3, byrow=TRUE, dimnames=list(movie_names,col_titles))
Summing all elements of entire rows or columns, or summing all elements of any vector

To do row sums or column sums in R for a matrix just use rowSums(matrixname) or colSums(matrixname). Note that it is important to capitalize S in rowSums or colSums. Another way can be to reference the needed vector by using something like Mymatrix[3, ] and then wrapping sum() around it.

Combining/Appending functions

cbind(vectorname) can append a vector to an existing matrix as a new column, provided vector's length is same as number of matrix rows. Similarly, rbind. Note the similarity to c() wrapper to construct any vector.

Arithmetic Operators

+,-,*,/ work in an elementwise way for both vectors and matrices
matrix1 * matrix2 does elementwise multiplication, not matrix multiplication as in Linear Algebra for which we use %*% in R

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