## 10.1 Conditional Statements via if, else, and switch()

The first type of control statement is conditional statements, which execute a portion of codes depending on the value of a condition.

### 10.1.1 Conditional Statements

The general structure of a simple conditional statement is as below.

if(condition) {
true_code_1
true_code_2
} else {
false_code_1
false_code_2
false_code_3
}

Here, the condition is usually a logical statement, the true_code_1 and true_code_2 represent the codes to run when condition == TRUE, and the false_code_1, false_code_2, false_code_3 represent the codes to run when condition == FALSE. Note that the else keyword is optional.

Let’s first see an example where we check whether x is an even number of an odd number, and print out the corresponding messages.

x <- 3
if(x %% 2 == 0) {
cat("x is an even number")
} else {
cat("x is an odd number")
}
#> x is an odd number

We can also use the ifelse() function to choose a value depending on whether the condition is TRUE or FALSE.

x_type <- ifelse(x%%2, "even", "odd")
x_type
#> [1] "even"

In addition to a single if and else pair, we can have more than two options. Let’s see the following example.

x <- 9
if(x %% 3 == 1) {
cat("the remainder of x divided by 3 is 1.")
} else if(x %% 3 == 2) {
cat("the remainder of x divided by 3 is 2.")
} else {
cat("x is divisible by 3.")
}
#> x is divisible by 3.

Next, let’s introduce the switch() function which can switch to a particular value depending on the value of the expression, which is the first argument.

If the first argument is an integer, the function will return the value corresponding to the location of the options.

switch(2, "sheep", "pig", "monkey")
#> [1] "pig"
switch(4, "sheep", "pig", "monkey")

The first statement returns the second option with value "pig", and the second statement returns nothing as there is no 4th option.

If the first argument is a character, it will be matched to the names of the elements in the options, and return the corresponding value.

switch("pig", sheep = 2, pig = 3, monkey = 4)
switch("pi", sheep = 2, pig = 3, monkey = 4)
switch("monkey", sheep = 2, pig = 3, monkey = "mona")

### 10.1.2 Logical Operators in if Statement

Recall we have learned logical operators on logical vectors in Section 2.7. The most commonly used logical operators are the AND operation with & and OR operation with |. Both operators conducts element-wise operations. In the if statement, we usually use another set of AND and OR operations, namely && and ||. Let’s use some examples to understand how they work.

logi_1 <- c(T, F, T, F)
logi_2 <- c(F, T, T, F)
logi_1 & logi_2
#> [1] FALSE FALSE  TRUE FALSE
logi_1 && logi_2
#> [1] FALSE
logi_1 | logi_2
#> [1]  TRUE  TRUE  TRUE FALSE
logi_1 || logi_2
#> [1] TRUE

Compare to the & and |, the && and || examine only the first element of the input logical vectors and the results are also a single logical value.

Another important difference is that && and || are short-circuit, skipping the evaluation of arguments if the result is determined.

Let’s see an example. Let’s use an undefined object named undefined_object.

undefined_object
TRUE || undefined_object
#> [1] TRUE
TRUE | undefined_object
#> Error in eval(expr, envir, enclos): object 'undefined_object' not found

Here, we can see that even though undefined_object is not found, TRUE || undefined_object still returns TRUE, due to that the result of || operator between TRUE and any other value is always TRUE, hence determined. In particular, both TRUE | FALSE and TRUE | TRUE have the value TRUE. Note that the | operator will throw an error as it will always faithfully perform the logical operations.

Let’s see another example of the AND operator between FALSE and another argument.

FALSE && undefined_object
#> [1] FALSE
FALSE & undefined_object
#> Error in eval(expr, envir, enclos): object 'undefined_object' not found

Similarly, the && operator between FALSE and an undefined object is FALSE since the evaluation stops once the result is determined since we know both FALSE & FALSE and FALSE & TRUE have the value FALSE. The & operator will again throw an error since undefined_object is not found.