Controlling flow

Control flow using logical statements

Often when we’re coding we want to control the flow of our actions. This can be done by setting actions to occur only if a condition or a set of conditions are met.

There are several ways you can control flow in R.

For conditional statements, the most commonly used approaches is to use if() and else statements:

# if
if (condition is true) {
  perform action
}

# if ... else
if (condition is true) {
  perform action
} else {  # that is, if the condition is false,
  perform alternative action
}

Say, for example, that we want R to print a different message depending on the value of variable x.

We could encode that with the following if() statement:

x <- 8 

1if (x >= 10) {
2  print("x is greater than or equal to 10")
} else {
3  print("x is less than 10")
}
1
The condition to be met is that x is greater than or equal to 10.
2
If the condition is met, the message “x is greater than or equal to 10” is printed.
3
If the condition is not met, the message “x is less than 10” is printed. 
[1] "x is less than 10"
Caution

Important: when R evaluates the condition inside if() statements, it is looking for a single logical value, i.e., TRUE or FALSE.

This can cause some headaches for beginners. For example:

x <- 8:10 

if (x >= 10) { 
  print("x is greater than or equal to 10") 
} else {
  print("x is less than 10") 
}
Error in if (x >= 10) {: the condition has length > 1

This produces an error because x >= 10 is a vector of logical values, not a single logical value.

Combine multiple conditions

Use && and ||

If we want to combine multiple conditions in our if statements, we can use the && and || operators.

For example, let’s say we want to print a message if x is greater than 10 and less than 20 and another if the condition is not met

x <- 11

if (x > 10 && x < 20) {
  print("x is greater than 10 and less than 20.")
} else {
  print("x is not greater than 10 and less than 20.")
}
[1] "x is greater than 10 and less than 20."

The && operator combines the two conditions:

x > 10 && x < 20
[1] TRUE

and returns TRUE because both conditions are met, printing the appropriate message.

If we change the value of x to 7, the message will change:

x <- 7

if (x > 10 && x < 20) {
  print("x is greater than 10 and less than 20.")
} else {
  print("x is not greater than 10 and less than 20.")
}
[1] "x is not greater than 10 and less than 20."
Tip

& and | are vectorised. This means that they can be used to compare vectors of logical values, and will return a vector of logical values.

&& and || are scalar versions, and will only compare vectors of length 1. Because if() statements require a single logical value, you should use && and || in if() statements.

Aggregate logical tests with any() and all()

Sometimes we might want to get an overall sense of whether a condition is met across a vector of values.

We can use the any() and all() functions to do this.

  • The any() function will return TRUE if at least one TRUE value is found within a vector, otherwise it will return FALSE.

  • The function all(), as the name suggests, will only return TRUE if all values in the vector are TRUE.

x <- c("apple", "banana")

any(x == "banana")
[1] TRUE
all(x == "banana")
[1] FALSE

Switch between multiple alternatives

Let’s say we want to be able to calculate either the mean, median, or max of a vector x depending on the value of a variable called type.

We can use the switch() function to do this.

The switch() function is used to select one of several alternatives based on the value of a single expression. It is similar to a series of if-else statements but provides a more concise and readable way to handle multiple conditions.

switch(EXPR, CASE1 = RESULT1, CASE2 = RESULT2, ..., DEFAULT)

The most straight forward way to use the function is to provide a single character string to the EXPR (the first) argument, and then a series of named arguments, each of which is a possible value or expression to be executed.

switch() then matches the value of EXPR to name of an option and executes the code.

A default value can be provided as the last unnamed argument, which is executed if no match is found.

Let’s have a look at it in action:

1x <- rnorm(100, mean = 10, sd = 2)

2type <- "mean"
3  switch(type,
         mean = mean(x),
         median = median(x),
         max = max(x),
         NA)

4type <- "max"
  switch(type,
         mean = mean(x),
         median = median(x),
         max = max(x),
         NA)

5type <- "random"
  switch(type,
         mean = mean(x),
         median = median(x),
         max = max(x),
         NA)
1
We create a vector x of 100 random numbers with a mean of 10 and a standard deviation of 2.
2
We create a variable type and set it to "mean".
3
We use switch() to calculate the mean, median, or max of x depending on the value of type. The first time we run it the mean is calculated.
4
We change the value of type to "max" and use switch() again. This time the max is calculated.
5
We change the value of type to "random" and use switch() again. This time the default NA is returned as there are no matches of the value of type to the named options. 
[1] 10.22283
[1] 14.91728
[1] NA

Vectorised if statements

As we just saw, if statements are designed to respond to a single logical condition.

To be able to perform if else statements on vectors, we can use the built in ifelse() function.

ifelse() function accepts both singular and vector inputs and is structured as follows:

# ifelse function
ifelse(condition is true, perform action, perform alternative action)

where:

  • the first argument is the condition or a set of conditions to be met

  • the second argument is the statement that is evaluated when the condition is TRUE, and

  • the third statement is the statement that is evaluated when the condition is FALSE.

y <- -3
ifelse(y < 0, "y is a negative number", "y is either positive or zero")
[1] "y is a negative number"
y <- c(-3, 0, 3)
ifelse(y < 0, "y is a negative number", "y is either positive or zero")
[1] "y is a negative number"       "y is either positive or zero"
[3] "y is either positive or zero"
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