R seq(), list(), matrix() - 2020/01/06

---

title: "test4"
output: html_document

---

knitr::opts_chunk$set(echo = TRUE)

#sequence seq(from,end,by)
seq(3,10,2)

?seq_along()

vec<-1:10
for (i in vec) {
  print(i)
}

for (i in seq_along(vec)) {
  print(i)
}


df<-data.frame(a=1:3,b=5:7)
df

?data.frame

for (i in seq_len(ncol(df))) {
  print(i)
}


rep(1:2, times=5)
rep(1:3, each=3)
rep(1:2,each=5,times=2)

#list : (key=value, key1=value1) can store different data type, scala or vector
x<-list(name="juhee", height=152)
x

y<-list(name="seungchan",height=c(130,180,182))
y

#list()  we can store another list() in list()

k=list(a=list(val=c(1,2,3,4)),b=list(val=c(1,2,3,4,5,6)))
k$a$val
k$b$val

#access data in list() by index
k[1]
#$a
#$a$val
#[1] 1 2 3 4
k[[1]]
#$val
#[1] 1 2 3 4
k$a

#Matrix can store 1 type of scala, must have the same data type in every column
matrix(c(1,2,3,4,5,6,7,8,9), nrow = 3, dimnames = list(c("r1","r2","r3"),c("c1","c2","c3")))

#   c1 c2 c3
#r1  1  4  7
#r2  2  5  8
#r3  3  6  9
m<-matrix(c(1,2,3,4,5,6,7,8,9), nrow = 3,byrow = TRUE, dimnames = list(c("r1","r2","r3"),c("c1","c2","c3")))
#   c1 c2 c3
#r1  1  2  3
#r2  4  5  6
#r3  7  8  9

m[1:2,] # m[row,column]
m[,2:3]
m[,-1]# remove the first column

?t()
#t() : transpose of a complex matrix A^*
t(m)
?solve
nrow(m)
ncol(m)
dim(m)

#to notice whether the matrix can be identity matrix 
(x<-matrix(c(1,2,3,4),ncol = 2))
#     [,1] [,2]
#[1,]    1    3
#[2,]    2    4

solve(x)
x %*% solve(x)


# we can alter dimension of matrix with dim()
dim(x)<-c(4,1)
x