- vector
    - x<-c(88,5,12,13)
    - x<-c(x[1:3],168,x[4])    # 88 5 12 168 13
    - length(x)    # 5
    - m<-rbind(c(1,2),c(3,4))
    - m+10:13    # 11=1+10 14=3+11 14=2+12 17=4+13
    - y<-vector(length=2)    # declare 2-length vector
    - y[1]<-5
    - y[2]<-12
    - y<-c(5,12)
    - c(1,2,4)+c(6,0,9,20,22)    # 7=1+6 2=2+0 13=4+9 21=1+20 24=2+22 with warning?
    - x<-rbind(c(1,4),c(2,5),c(3,6))
    - x+c(1,2)    # [[2,6],[4,6],[4,8]] add c(1,2,1,2,1,2) 2=1+1 4=2+2 4=3+1 6=4+2 6=5+1 8=6+2
    - "+"(2,3)    # 2+3
    - x<-c(1,2,4)
    - x * c(5,0,-1)    # 5 0 -4 dot product
    - x / c(5,4,-1)    # 0.2 0.5 -4.0
    - x %% c(5,4,-1)    # 1 2 0
    - y<-c(1.2,3.9,0.4,0.12)
    - y[c(1,3)]    # 1.2 0.4
    - y[2:3]    # 3.9 0.4
    - v<-3:4
    - y[v]    # 0.4 0.12
    - x<-c(4,2,17,5)
    - y<-x[c(1,1,3)]    # 4 4 17
    - x[-1]    # 2 17 5 exclude 4
    - x[-1:-2]    # 17 5 exclude 4 and 2
    - x[-length(x)]    # 4 2 17 exclude 5
    - x[1:(length(x)-1)]    # same as above
    - 5:8    # 5 6 7 8
    - i<-2
    - 1:i-1    # 0 1 which means 1:i first, then -1

- seq
    - seq(from=12,to=30,by=3)    # 12 15 18 21 24 27 30
    - seq(from=1.1,to=2,length=10)    # 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
    - x<-c(5,12,13)
    - seq(x)    # 1 2 3
    - x<-NULL
    - seq(x)    # integer(0)
- rep
    - x<-rep(8,4)    # 8 8 8 8 make 8 with 4 times
    - rep(1:3,2)    # 1 2 3 1 2 3
    - rep(c(5,12,13),each=2)    # 5 5 12 12 13 13

- any
    - x<-1:10
    - any(x>8)    # TRUE
    - any(x>88)    # FALSE
    - all(x>88)    # FALSE
    - all(x>0)    # TRUE

- vector operation
    runs<-NULL
    for (i in 1:10) {
        runs<-c(runs,i)    # not good for performance
    runs<-vector(length=n)
    for (i in 1:n) {
        runs[i]<-i    # good
    return(runs)
    - y<-c(5,2,-3,8)
    - cumsum(y)    # 57 4 12
    - u<-c(5,2,8)
    - v<-c(1,3,9)
    - u>v    # TRUE FALSE FALSE
    - w<-function(x) return(x+1)
    - w(u)    # 6 3 9
    - sqrt(1:9)    # 1.000000 1.414214 ... 3.000000
    - y<-c(1.2,3.9,0.4)
    - round(y)    # 1 4 0
    - '+'(u,4)    # 9 6 12
    - x<-1:8
    - z12<-function(z) return (c(z,z^2))
    - z12(x)    # 1 2 3 4 5 6 7 8 1 4 9 16 25 36 49 64

- NA
    - x<-c(88,NA,12,168,13)
    - mean(x)    # NA
    - mean(x, na.rm=T)    # 70.25
    - x<-c(88,NULL,12,168,13)
    - mean(x)    # 70.25
    - mode(x[2])    # "numeric"
    - y<-c("abc","def",NA)
    - mode(y[3])    # "character"
    - z<-NA
    - for (i in 1:10) if (i%%2==0) z<-c(z,i)    # NA 2 4 6 8 10
    - z<-NULL
    - for (i in 1:10) if (i%%2==0) z<-c(z,i)    # 2 4 6 8 10

- filter
    - z<-c(5,2,-3,8)
    - w<-z[z*z>8]    # 5 -3 8
    - ">"(2,1)    # TRUE
    - z*z>8    same as ">"(z*z,8)
    - j<-z*z>8
    - y<-c(1,2,30,5)
    - y[j]    # 1 30 5 TRUE FALSE TRUE TRUE
    - x<-c(1,3,8,2,20)
    - x[x>3]<-0    # 1 3 0 2 0
    - which(z*z>8)    # 1 3 4 index not value
    - x<-c(6,1:3,NA,12)
    - x[x>5]    # 6 NA 12
    - subset(x,x>5)    # 6 12

- ifelse
    - x<-1:10
    - y<-ifelse(x%%2==0,5,12)    # 12 5 12 5 12 5 12 5 12 5
    - x<-c(5,2,9,12)
    - ifelse(x>6,2*x,3*x)    # 15 6 18 24
    - args(ifelse)    # function(test,yes,no)    check function definition

- Test
    - x<-1:3
    - y<-c(1,3,4)
    - x==y    # TRUE FALSE FALSE
    - all(x==y)    # FALSE
    - x<-1:2
    - y<-c(1,2)
    - identical(x,y)    # FALSE
    - typeof(x)    # "integer"
    - typeof(y)    # "double"
    - names(x)    # NULL
    - x<-c(1,2,4)
    - names(x)<-c("a","b","ab")
    - x["ab"]    # 4 using as index
    - names(x)<-NULL    # remove names
    - c(5,2,c(1.5,6))    # 5.0 2.0 1.5 6.0 make flat