##Chapter 19 ### Intro ### When should you write a function?

#for reproducible work
set.seed(1234)

#create a dataframe

df <- tibble::tibble(
  a = rnorm(10),
  b = rnorm(10),
  c = rnorm(10),
  d = rnorm(10))
#rescale each column
df$a <- (df$a - min(df$a, na.rm = TRUE)) / 
  (max(df$a, na.rm = TRUE) - min(df$a, na.rm = TRUE))
df$b <- (df$b - min(df$b, na.rm = TRUE)) / 
  (max(df$b, na.rm = TRUE) - min(df$a, na.rm = TRUE))
df$c <- (df$c - min(df$c, na.rm = TRUE)) / 
  (max(df$c, na.rm = TRUE) - min(df$c, na.rm = TRUE))
df$d <- (df$d - min(df$d, na.rm = TRUE)) / 
  (max(df$d, na.rm = TRUE) - min(df$d, na.rm = TRUE))

df
## # A tibble: 10 × 4
##        a      b     c     d
##    <dbl>  <dbl> <dbl> <dbl>
##  1 0.332 0.216  0.782 1    
##  2 0.765 0      0.473 0.519
##  3 1     0.0919 0.498 0.448
##  4 0     0.440  0.943 0.511
##  5 0.809 0.810  0.373 0.168
##  6 0.831 0.368  0     0.308
##  7 0.516 0.202  1     0    
##  8 0.524 0.0361 0.210 0.256
##  9 0.519 0.0667 0.708 0.575
## 10 0.424 1.41   0.253 0.522
rescale <- function(x) {
    #body
    x <- (x - min(x, na.rm = TRUE)) / 
  (max(x, na.rm = TRUE) - min(x, na.rm = TRUE))
    
    #return values
    return(x)

}
df$a <- rescale(df$a)
df$b <- rescale(df$b)
df$c <- rescale(df$c)
df$d <- rescale(df$d)

#conditional execution

detect_sign <- function(x) {
    
    if(x>0) {
        message("Value is positive")
        print(x)
    } else if(x ==0) {
        warning("Value is not positive, but it can be accepted")
        print(x)
    } else {
        stop("Value is negative, the function must stop")
        print(x)
    }
}

#3 %>% detect_sign()
#0 %>% detect_sign()
#-1 %>% detect_sign()

###function arguments

?mean
## starting httpd help server ... done
x <- c(1:10, 100, NA)
x
##  [1]   1   2   3   4   5   6   7   8   9  10 100  NA
x %>% mean()
## [1] NA
x %>% mean(na.rm = TRUE)
## [1] 14.09091
x %>% mean(na.rm = TRUE, trim =0.1)
## [1] 6
mean_remove_na <- function(x, na.rm =TRUE, ...) {
    
    avg <- mean(x, na.rm =na.rm, ...)
    
    return(avg)
}

x %>% mean_remove_na()
## [1] 14.09091
x %>% mean_remove_na(na.rm = FALSE)
## [1] NA
x %>% mean_remove_na(trim =0.1)
## [1] 6

two types of functions

vector inputs data frame inputs