introduction

when should you write a function?

# for reproducible work
set.seed(1234)

#create a data frame
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$b, na.rm = TRUE) - min(df$b, na.rm = TRUE))
df$c <- (df$c - min(df$c, na.rm = TRUE)) / 
  (max(df$c, na.rm = TRUE) - min(df$c, na.rm = TRUE))
df$a <- (df$a - min(df$a, 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 0.101 0.153  0.782  1.10 
##  2 0.233 0      0.473 -0.476
##  3 0.305 0.0651 0.498 -0.709
##  4 0     0.311  0.943 -0.501
##  5 0.246 0.573  0.373 -1.63 
##  6 0.253 0.260  0     -1.17 
##  7 0.157 0.143  1     -2.18 
##  8 0.160 0.0255 0.210 -1.34 
##  9 0.158 0.0472 0.708 -0.294
## 10 0.129 1      0.253 -0.466
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)

df
## # A tibble: 10 × 4
##        a      b     c     d
##    <dbl>  <dbl> <dbl> <dbl>
##  1 0.332 0.153  0.782 1    
##  2 0.765 0      0.473 0.519
##  3 1     0.0651 0.498 0.448
##  4 0     0.311  0.943 0.511
##  5 0.809 0.573  0.373 0.168
##  6 0.831 0.260  0     0.308
##  7 0.516 0.143  1     0    
##  8 0.524 0.0255 0.210 0.256
##  9 0.519 0.0472 0.708 0.575
## 10 0.424 1      0.253 0.522

functions are for humans and computers

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()
## value is positive
## [1] 3
0 %>% detect_sign()
## Warning in detect_sign(.): value is not positive, but it can be accepted
## [1] 0

function arguments

?mean


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

return values