R 語言的基本功［數與向量］Part 2

絕對值(abs)、開根號(sqrt)、指數(exp)、對數(log)

abs(-500)

## [1] 500

abs(300)

## [1] 300

sqrt(144)

## [1] 12

exp(2)

## [1] 7.389056

log(6)

## [1] 1.791759

log(exp(10))

## [1] 10

亦可以算出三角函數(sin、cos、tan)

n <- c(sin(1),cos(1),tan(1))
n

## [1] 0.8414710 0.5403023 1.5574077

查詢或定義向量名稱(names)

x <- 1:5
x

## [1] 1 2 3 4 5

names(x) <- c('a','b','c','d','e')
x

## a b c d e 
## 1 2 3 4 5

顯示物件的長度(length)

x:物件

value:強制轉換為整數值

x <- 101:2000
length(x)

## [1] 1900

y <- length(diag(5))
y

## [1] 25

數值總和(sum)、累計加總(cumsum)、數值乘積(prod)

x <- 1:10
x1 <- sum(x)
x2 <- cumsum(x)
x3 <- prod(x)
x1

## [1] 55

x2

##  [1]  1  3  6 10 15 21 28 36 45 55

x3

## [1] 3628800

最大數值(max)、最小數值(min)

x <- c(1234,5236,5222,5985,1236)
max(x)

## [1] 5985

min(x)

## [1] 1234

產生不同維度的陣列(array)

x:向量

dim:維度

x <- 1:10
array(x,dim = c(2,5)) #產生二維陣列

##      [,1] [,2] [,3] [,4] [,5]
## [1,]    1    3    5    7    9
## [2,]    2    4    6    8   10

array(x,dim = c(3,3)) #產生三維陣列

##      [,1] [,2] [,3]
## [1,]    1    4    7
## [2,]    2    5    8
## [3,]    3    6    9

產生矩陣(matrix)

nrow:行

ncol:列

byrow:矩陣按照行排列

dimnames:矩陣名稱

matrix(c(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16),
       nrow = 4,
       ncol = 4,
       byrow = TRUE,
       dimnames = list(c("a1","b2","c3","d4"),c("z1","z2","z3","z4")))

##    z1 z2 z3 z4
## a1  1  2  3  4
## b2  5  6  7  8
## c3  9 10 11 12
## d4 13 14 15 16

陣列的合併與轉換

逐列(cbind)

逐行(rbind)

x <- c(1:5)
y <- c(11:15)
cbind(x,y)

##      x  y
## [1,] 1 11
## [2,] 2 12
## [3,] 3 13
## [4,] 4 14
## [5,] 5 15

rbind(x,y)

##   [,1] [,2] [,3] [,4] [,5]
## x    1    2    3    4    5
## y   11   12   13   14   15

改變變數或元素

(as.Date、as.array、as.character、as.numeric…)
x:物件 format:格式 origin:起始時間 tz:時區

dates <- c("02/27/94","02/27/98","01/15/98","03/06/95")
as.Date(dates,"%m/%d/%y")

## [1] "1994-02-27" "1998-02-27" "1998-01-15" "1995-03-06"

format(Sys.time(),"%Y-%m-%d %H:%M:%S")

## [1] "2018-05-03 23:47:25"