R语言入门（26）-函数（function）

> f <- function() {
+ ## This is an empty function
+ } 

> ## Functions have their own class
> class(f)
[1] "function"

> ## Execute this function  
> f()
NULL

> f <- function() {
+ cat("Hello, world!n")
+ } 
> f()
Hello, world!

this function = cut and print????

prints the message “Hello, world!” to the console a number of times indicated by the argument num.

> f <- function(num) {
+ for(i in seq_len(num)) {
+ cat("Hello, world!n") 
+ } 
+ } 
> f(3)
Hello, world!
Hello, world!
Hello, world!

this function returns the total number of characters printed to the console????

> f <- function(num) {
+     hello <- "Hello, world!n"
+     for(i in seq_len(num)) {
+         cat(hello)
+     } 
+     chars <- nchar(hello) * num
+         chars
+} 
> meaningoflife <- f(3)
Hello, world!
Hello, world!
Hello, world! 
> print(meaningoflife)
[1] 42

 function的调用如果没有arg，就会报错。为了防止使用时我们忘记加入arg，可以在一开始定义function的时候就set一个default value。????

has one formal argument named num with a default value of 1.

The formal arguments are the arguments included in the function definition.

The formals() function returns a list of all the formal arguments of a function.

> f <- function(num = 1) {                 # set the default value for num to be 1
+        hello <- "Hello, world!n"        
+        for(i in seq_len(num)) {
+        cat(hello)
+        } 
+        chars <- nchar(hello) * num
+        chars
+ } 

> f()               ## Use default value for 'num' 
Hello, world! [1] 14
> f(2)              ## Use user-specified value
Hello, world!
Hello, world! 
[1]28

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