# if
if (condition is true) {
perform action
}
# if ... else
if (condition is true) {
perform actionelse { # that is, if the condition is false,
}
perform alternative action }
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:
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:
< 8
x
1if (x >= 10) {
2print("x is greater than or equal to 10")
else {
} 3print("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"
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:
< 8:10
x
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
< 11
x
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:
> 10 && x < 20 x
[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:
< 7
x
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."
&
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 returnTRUE
if at least oneTRUE
value is found within a vector, otherwise it will returnFALSE
.The function
all()
, as the name suggests, will only returnTRUE
if all values in the vector areTRUE
.
< c("apple", "banana")
x
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 ifelse 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:
1< rnorm(100, mean = 10, sd = 2)
x
2< "mean"
type 3switch(type,
mean = mean(x),
median = median(x),
max = max(x),
NA)
4< "max"
type switch(type,
mean = mean(x),
median = median(x),
max = max(x),
NA)
5< "random"
type 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 ofx
depending on the value oftype
. The first time we run it the mean is calculated.  4

We change the value of
type
to"max"
and useswitch()
again. This time the max is calculated.  5

We change the value of
type
to"random"
and useswitch()
again. This time the defaultNA
is returned as there are no matches of the value oftype
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
, andthe third statement is the statement that is evaluated when the condition is
FALSE
.
< 3
y ifelse(y < 0, "y is a negative number", "y is either positive or zero")
[1] "y is a negative number"
< c(3, 0, 3)
y 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"