In TobieSurette/gulf.utils: Southern Gulf of Saint Lawrence Utility Functions

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

Vectors:

Integer indexing

We retrieve values in a vector by declaring an index inside a single square bracket "[]" operator.

For example, the following shows how to retrieve a vector member. Since the vector index is 1-based, we use the index position 3 for retrieving the third member.

> s = c("aa", "bb", "cc", "dd", "ee") 
> s[3] 
[1] "cc"

The square bracket operator returns more than just individual members. In fact, the result of the square bracket operator is another vector, and s[3] is a vector slice containing a single member "cc". We can retrieve multiple elements by using a index that is itself a vector:

> s = c("aa", "bb", "cc", "dd", "ee") 
> s[3:5]
[1] "cc" "dd" "ee"

or retrieve the first, third and fifth elements:

> s = c("aa", "bb", "cc", "dd", "ee") 
> s[c(1,3,5)]
[1] "aa" "cc" "ee"

If an index is out-of-range, a missing value will be reported via the symbol NA.

> s[10] 
[1] NA

Negative integer indexing

If the index is negative, it would strip the member whose position has the same absolute value as the negative index. For example, the following creates a vector slice with the third member removed.

> s[-3] 
[1] "aa" "bb" "dd" "ee"

Multiple elements can also be removed this way. Say we wanted to remove the first, second and third elements:

> s[-1:-3] 
[1] "dd" "ee"

Logical indexing

Alternatively, vectors of logical values can also be used for indexing. Retrieving the first, third and fifth elements can be done by:

ix <- c(TRUE, FALSE, TRUE, FALSE, TRUE)
> s[ix] 
[1] "aa" "cc" "ee"

Logical vectors can be easily generated. Say we have a vector of ten uniform random variates:

x <- runif(10)

A vector of logical values which identify the values of 'x' which are larger than 0.5 can be generated thus:

x > 0.5 

which is a ten-element logical vector. This vector can be used as an index vector:

x[x > 0.5]

The 'which' function can be used to convert from a logical index vector to an integer index vector.

which(x > 0.5)

TobieSurette/gulf.utils documentation built on Dec. 1, 2024, 6:01 a.m.