common commands more efficiently or apply to certain specific objects such asmatrices. For example, we already have seen the operator : that creates a sequence. In R it is possible to define your own operators.
# +-+ # This function is a new operator # arguments: # x -- numeric # y -- numeric # returns: # x- y `+-+` <- function(x, y) x - y 5 +-+ 5 ## [1] 0 5 +-+ 1 ## [1] 4 # Remove the new operator: rm(`+-+`)
It is even possible to redefine elementary operators such as + with the aforementioned code. This is of course not a wise thing to do, but we understand how it can be a fun practical joke or a tricky job interview question.
The following are some common operators that help working with data.
operator – other
# create a list x <- c(10:20) x ## [1] 10 11 12 13 14 15 16 17 18 19 20 # %in% can find an element in a vector 2 %in% x # FALSE since 2 is not an element of x ## [1] FALSE 11 %in% x # TRUE since 11 is in x ## [1] TRUE x[x %in% c(12,13)] # selects elements from x ## [1] 12 13 x[2:4] # selects the elements with index ## [1] 11 12 13 # between 2 and 4
4.5 Flow Control Statements
R is Turing complete and hence offers a range of tools to make choices and repeat certain parts of code. Knowing the different ways to change the flow of a code by if-statements and loops is essential knowledge for each R-programmer.
flow control
4.5.1 Choices
4.5.1.1 The if-Statement
The workhorse to control the flow of actions is the if() function.
if()
The construct is both simple and efficient.
Function use for if()
if (logical statement) { executed if logical statement is true } else { executed if the logical statement if false }
Note that the else-statement is optional.
This basic construct can also be enriched with else if statements. For example, we draw a random number from the normal distribution and check if it is bigger than zero.
set.seed(1890) x <- rnorm(1) if (x < 0) { print(‘x is negative’) } else if (x > 0) { print(‘x is positive’) } else { print(‘x is zero’) } ## [1] “x is positive”
It is possible to have more than one else-if statement and/or use nested statements.
x <- 122 if (x < 10) { print(‘less than ten’) } else if (x < 100) { print(‘between 10 and 100’) } else if (x < 1000) { print(‘between 100 and 1000’) } else { print(‘bigger than 1000 (or equal to 1000)’) } ## [1] “between 10 and 1000”
Note that the statements do not necessarily have to be encapsulated by curly brackets if the statement only takes one line.
x <- TRUE y <- pi y <- if (x) 1 else 2 y # y is now 1 ## [1] 1
Note that hybrid forms are possible, but it gets confusing very fast. In the following piece of code the variable y will not get the value one, but rather six.
z <- y <- if (x) {1; z <- 6} else 2 y # y is now 6 ## [1] 6 z # z is also 6 ## [1] 6
4.5.1.2 The Vectorised If-statement
The function ifelse() is the vectorised version of the if-function. It allows to use vectors as input and output. While the if-statement is useful for controlling flow in code, the ifelse-function handy for data manipulation.
ifelse()
x <- 1:6 ifelse(x %% 2 == 0, ‘even’, ‘odd’) ## [1] “odd” “even” “odd” “even” “odd” “even”
The ifelse function can also use vectors as parameters in the output.
x <- 1:6 y <- LETTERS[1:3] ifelse(x %% 2 == 0, ‘even’, y) ## [1] “A” “even” “C” “even” “B” “even” # Note that y gets recycled!
4.5.1.3 The Switch-statement
An if-else construct that assigns one value to a variable based on one other variable can be condensed via the switch() function.
switch()
x <- ‘b’ x_info <- switch(x, ‘a’ = “One”, ‘b’ = “Two”, ‘c’ = “Three”, stop(“Error: invalid `x` value”) ) # x_info should be ‘two’ now: x_info ## [1] “Two”
The switch statement can always be written a an else-if statement. The following code does the same as the aforementioned code.
x <- ‘b’ x_info <- if (x == ‘a’ ) { “One” } else if (x == ‘b’) { “Two” } else if (x == ‘c’) { “Three” } else { stop(“Error: invalid `x` value”) } # x_info should be ‘two’ now: x_info ## [1] “Two”
The switch() statement can always be written as with the if-else-if construction, which in its turn can always be written based on with if-else statements. This is same logic also applies for loops (that
