12.1 Introduction to Functions

Let’s look at the example for generating Fibonacci sequence to motivate functions.

To get a Fibonacci sequence up to the value 100, you can use the following code.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq[1]
fib_cur_value <- fib_seq[2]
fib_next_value <- fib_last_value + fib_cur_value
while (fib_next_value < 100) {
    fib_seq <- c(fib_seq, fib_next_value)
    fib_cur_value <- fib_next_value
    fib_last_value <- fib_cur_value
    fib_next_value <- fib_last_value + fib_cur_value
}
fib_seq  #print the sequence
#> [1]  0  1  1  2  4  8 16 32 64

Now, to get a Fibonacci sequence up to the value 200, you just need to change the 100 in the previous code to 200.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq[1]
fib_cur_value <- fib_seq[2]
fib_next_value <- fib_last_value + fib_cur_value
while (fib_next_value < 200) {
    fib_seq <- c(fib_seq, fib_next_value)
    fib_cur_value <- fib_next_value
    fib_last_value <- fib_cur_value
    fib_next_value <- fib_last_value + fib_cur_value
}
fib_seq  #print the sequence
#>  [1]   0   1   1   2   4   8  16  32  64 128

If this is some task we want to do repeatedly , it would be better to wrap the whole process into a function.

Let’s first review the general syntax of a function.

fun_name <- function(arg1, arg2) {
    expr_1
    expr_2
    return(return_val)
}

First, we need to observe what objects are needs inside the function, which will be the arguments (arg1, arg2) of the function. Here, the upper bound of the Fibonacci sequence (value 100 and 200) is the only argument.

Second, we need to identify what to output from the function, i.e., return a value. In this example, the Fibonacci sequence would be a reasonable output for return_val.

Third, we need to name the function, following the naming rule of an object introduced back in Section 1.3.3. Here, we will name it as get_fibo.

Let’s look at our function.

get_fibo <- function(upper_bound) {
    fib_seq <- c(0, 1)
    fib_last_value <- fib_seq[1]
    fib_cur_value <- fib_seq[2]
    fib_next_value <- fib_last_value + fib_cur_value
    while (fib_next_value < upper_bound) {
        fib_seq <- c(fib_seq, fib_next_value)
        fib_cur_value <- fib_next_value
        fib_last_value <- fib_cur_value
        fib_next_value <- fib_last_value + fib_cur_value
    }
    return(fib_seq)  #print the sequence
}

Now, let’s try to evaluate this function with some examples.

get_fibo(upper_bound = 50)
#> [1]  0  1  1  2  4  8 16 32
get_fibo(upper_bound = 80)
#> [1]  0  1  1  2  4  8 16 32 64
get_fibo(upper_bound = 300)
#>  [1]   0   1   1   2   4   8  16  32  64 128 256