Table of Contents

• Overview
• The Recycling Rule of Mixed vector and array arithmetic
• Calculate the proportion of a subset of a vector
• Generate combinations of vectors' elements

Overview

1 + 2 - 3 * 4 / (5 ^ 6)
a <- c(1,2,3,4)
sqrt(a)
exp(a)
log(a)
a <- c(1,2,3)
b <- c(10,11,12,13)
# shows warninging: not a multiple of shorter one
a + b

mean(x)
median(x)
quantile(x)
min(x)
max(x)
range(x)  # c(min(x), max(x))
var(x)
sd(x)

1:3
3:1
seq(1, 3, by = 1)
rep(0, times = 3)

[1] 1 2 3
[1] 3 2 1
[1] 1 2 3
[1] 0 0 0

n <- 3
1:n-1    # === 1:3 - 1
1:(n-1)  # === 1:2

[1] 0 1 2
[1] 1 2

x <- 1:5
x > 3
x[x > 3]

[1] FALSE FALSE FALSE  TRUE  TRUE
[1] 4 5

x[1]
x[3]
x[-2]      # everything except the 2nd element
x[1:3]     # 1st - 3rd elements
x[c(1, 4)] # 1st, and 4th elements

z = c(TRUE, FALSE, TRUE, FALSE, TRUE)
x[z]  # corresponding TRUE elements

The Recycling Rule of Mixed vector and array arithmetic discussion

• The expression is scanned from left to right.
• Any short vector operands are extended by recycling their values until they match the size of any other operands.
• As long as short vectors and arrays only are encountered, the arrays must all have the same dim attribute or an error results.
• Any vector operand longer than a matrix or array operand generates an error.
• If array structures are present and no error or coercion to vector has been precipitated, the result is an array structure with the common dim attribute of its array operands.

Calculate the proportion of a subset of a vector howto

• Take advantage of type coercion, alongside mean()

mean(rnorm(10) > 0)

[1] 0.4

Generate combinations of vectors' elements howto

First, we can use expand.grid(). In this case, we need to it with apply(A, 1, f) to process each combinations

x = c("foo", "bar", "baz")
y = c(1, 2)
(A = expand.grid(x, y))

  Var1 Var2
1  foo    1
2  bar    1
3  baz    1
4  foo    2
5  bar    2
6  baz    2

apply(A, 1, function(row) {
  x = row[1]
  y = row[2]
  sprintf("%s+%s", x, y)
})

[1] "foo+1" "bar+1" "baz+1" "foo+2" "bar+2" "baz+2"

Or, if you need a single composed vector, you can use outer() as follows:

(B = outer(x, y, FUN = paste))

     [,1]    [,2]   
[1,] "foo 1" "foo 2"
[2,] "bar 1" "bar 2"
[3,] "baz 1" "baz 2"

as.vector(B)

[1] "foo 1" "bar 1" "baz 1" "foo 2" "bar 2" "baz 2"

Or, just use rep(..., each = N) for simple cases:

(xx = rep(x, each = 2))
paste(xx, y)

[1] "foo" "foo" "bar" "bar" "baz" "baz"
[1] "foo 1" "foo 2" "bar 1" "bar 2" "baz 1" "baz 2"