Issue while executing drop() function in R - r

Question

I am trying to find out usage of drop() function. I read the documentation that a matrix or array can be the input object for the function however the size of the matrix or object does not change. Can someone explain its actual usage and how it works?
I am using R version 3.2.1. Code snippet:
data1 <- matrix(data=(1:10),nrow=1,ncol=1)
drop(data1)

Answer 1

R has factors, which are very cool (and somewhat analogous to labeled levels in Stata). Unfortunately, the factor list sticks around even if you remove some data such that no examples of a particular level still exist.
# Create some fake data
x <- as.factor(sample(head(colors()),100,replace=TRUE))
levels(x)
x <- x[x!="aliceblue"]
levels(x) # still the same levels
table(x) # even though one level has 0 entries!
The solution is simple: run factor() again:
x <- factor(x)
levels(x)
If you need to do this on many factors at once (as is the case with a data.frame containing several columns of factors), use drop.levels() from the gdata package:
x <- x[x!="antiquewhite1"]
df <- data.frame(a=x,b=x,c=x)
df <- drop.levels(df)

Answer 2

R matrix is a two dimensional array. R has a lot of operator and functions that make matrix handling very convenient.
Matrix assignment:
>A <- matrix(c(3,5,7,1,9,4),nrow=3,ncol=2,byrow=TRUE)
>A
[,1] [,2]
[1,] 3 5
[2,] 7 1
[3,] 9 4
Matrix row and column count:
>rA <- nrow(A)
>rA
[1] 3
>cA <- ncol(A)
>cA
[1] 2
t(A) function returns a transposed matrix of A:
>B <- t(A)
>B
[,1] [,2] [,3]
[1,] 3 7 9
[2,] 5 1 4
Matrix multplication:
C <- A * A
C
[,1] [,2]
[1,] 9 25
[2,] 49 1
[3,] 81 16
Matrix Addition:
>C <- A + A
>C
[,1] [,2]
[1,] 6 10
[2,] 14 2
[3,] 18 8
Matrix subtraction (-) and division (/) operations ... ...
Sometimes a matrix needs to be sorted by a specific column, which can be done by using order() function.
Following is a csv file example:
,t1,t2,t3,t4,t5,t6,t7,t8
r1,1,0,1,0,0,1,0,2
r2,1,2,5,1,2,1,2,1
r3,0,0,9,2,1,1,0,1
r4,0,0,2,1,2,0,0,0
r5,0,2,15,1,1,0,0,0
r6,2,2,3,1,1,1,0,0
r7,2,2,3,1,1,1,0,1
Following R code will read in the above file into a matrix, and sort it by column 4, then write to a output file:
x <- read.csv("sortmatrix.csv",header=T,sep=",");
x <- x[order(x[,4]),];
x <- write.table(x,file="tp.txt",sep=",")
The result is:
"X","t1","t2","t3","t4","t5","t6","t7","t8"
"1","r1",1,0,1,0,0,1,0,2
"4","r4",0,0,2,1,2,0,0,0
"6","r6",2,2,3,1,1,1,0,0
"7","r7",2,2,3,1,1,1,0,1
"2","r2",1,2,5,1,2,1,2,1
"3","r3",0,0,9,2,1,1,0,1
"5","r5",0,2,15,1,1,0,0,0

Answer 3

The DROP function supports natively compiled, scalar user-defined functions.
Removes one or more user-defined functions from the current database
To execute DROP FUNCTION, at a minimum, a user must have ALTER permission on the schema to which the function belongs, or CONTROL permission on the function.
DROP FUNCTION will fail if there are Transact-SQL functions or views in the database that reference this function and were created by using SCHEMA BINDING, or if there are computed columns, CHECK constraints, or DEFAULT constraints that reference the function.
DROP FUNCTION will fail if there are computed columns that reference this function and have been indexed.
DROP FUNCTION { [ schema_name. ] function_name } [ ,...n ]