6.2 Conditional programming with if...else statements

The conditional if...else statement is used to test an expression similar to the if statement. However, rather than nothing happening if the test_expression is FALSE, the else part of the function will be evaluated.

# syntax of if...else statement
if (test_expression) {
        statement 1
} else {
        statement 2
}

The following extends the previous example illustrated for the if statement in which the if statement tests if any values in a vector are negative; if TRUE it produces one output and if FALSE it produces the else output.

# this test results in statement 1 being executed
x <- c(8, 3, -2, 5)

if(any(x < 0)){
        print("x contains negative numbers")
} else{
        print("x contains all positive numbers")
}
[1] "x contains negative numbers"

# this test results in statement 2 (or the else statement) being executed
y <- c(8, 3, 2, 5)

if(any(y < 0)){
        print("y contains negative numbers")
} else{
        print("y contains all positive numbers")
}
[1] "y contains all positive numbers"

Simple if...else statements, as above, in which only one line of code is being executed in the statements can be written in a simplified alternative manner. These alternatives are only recommended for very short if...else code:

x <- c(8, 3, 2, 5)

# alternative 1
if(any(x < 0)) print("x contains negative numbers") else print("x contains all positive numbers")
[1] "x contains all positive numbers"

# alternative 2 using the ifelse function
ifelse(any(x < 0), "x contains negative numbers", "x contains all positive numbers")
[1] "x contains all positive numbers"

We can also nest as many if...else statements as required (or desired). For example:

# this test results in statement 1 being executed
x <- 7

if(x >= 10){
        print("x exceeds acceptable tolerance levels")
} else if(x >= 0 & x < 10){
        print("x is within acceptable tolerance levels")
} else {
         print("x is negative")
}
[1] "x is within acceptable tolerance levels"