Functions in R

class: center, middle, inverse, title-slide

# Functions in R
## Developing skills in R, DEEP, SU
### Andreas Novotny
### 14 November 2018

---

name: intro
class: spaced

## Do you use functions when coding?

```r
sqrt(4)
```

```
## [1] 2
```

```r
print("Hello World")
```

```
## [1] "Hello World"
```

```r
data.frame(x=numeric(), y=factor())
```

```
## [1] x y
## <0 rows> (or 0-length row.names)
```

```r
c(1,2,3,4,5)
```

```
## [1] 1 2 3 4 5
```

---
name: General structure

## What is needed?

* Input: X1, X2, X3, X3
* Function: f()
* Output: y <-

![](function.png)

---
name: Different types

## What is needed?

* output <- function(X1, ..., Xn)

```r
output <- sqrt(2)
```

![](function.png)
---
name: What goes in?

## What goes into the function?

### Who decides what can go in?
### Who descides what comes out?

![](function.png)

???

YOU Decide!

---
name: why?

## Why do we need to know how to write our own functions?

* Only possible solution?

* Writing your own package?

* Easier to interpret "Help" files.

* Good coding practice - Makes code readable

---
name: syntax1

## SYNTAX: How are functions defined?

* Definition

```r
name_of_function <- function(input1, input2, inputn) {
  
  "Do something"
  
}
```

---
name: example1

## SYNTAX: How are functions defined?

* Example

```r
sum_of_three <- function(a, b, c) {
  
  print( a + b + c)
  
}
```

```r
sum_of_three(a=3,b=4,c=5)
```

```
## [1] 12
```

---

## SYNTAX: How are functions defined?

* Example
* Default inputs

```r
sum_of_three <- function(a=3, b=3, c=3) {
  
  print( a + b + c)
  
}
```

```r
sum_of_three(a=3)
```

```
## [1] 9
```
---
name: exc1

## First excercise

[CONTINUE TO EXCERCISES](../functionsexc.html)

---
## Return objects, not printed to the console

```r
name_of_function <- function(input) {
  
  output <- input
  
  return(output)
  
}
```

---
## Error message

```r
normalizeR <- function(inputvector) {
  
  if (class(inputvector) != "numeric") {
    stop("Input is not a numeric vector")
    }
  
  normalized <- inputvector/mean(inputvector)
  
  return(normalized)
}
```

---
## Wrapper functions

* Change default settings of others functions

* Combine many functions in one

```r
mean_noNA <- function(x) {
  
    return(mean(x, na.rm = T))
  
}
```

---
## Wrapper functions

* Don't forget to add flexibility to the function

```r
mean_noNA <- function(x) {
    return(mean(x, na.rm = TRUE))
}
```

```r
mean_noNA <- function(x, na.rm= TRUE) {
    return(mean(x, na.rm = na.rm))
}
```

```r
mean_noNA <- function(x, na.rm= TRUE, ...) {
    return(mean(x, na.rm = na.rm, ...))
}
```

---
## Name spaces

* Functions uses their own namespaces
* Objects created inside the function cannot be available outside the function:

```r
x <- 10

helloR <- function() {
    
    x <- 15
    print("Hello World")

}
```

```r
helloR()
```

```
## [1] "Hello World"
```

```r
print(x)
```

```
## [1] 10
```
---
## Name spaces

* Functions uses their own namespaces
* Objects created inside the function cannot be available outside the function
*Object created outside the function can eventually be accessed from inside. AVOID THIS!

```r
x <- 10

helloR <- function() {
    
    print(x)
    print("Hello World")

}

helloR()
```

```
## [1] 10
## [1] "Hello World"
```
---

## Now - more excercices:

[CONTINUE TO EXCERCISES](../functionsexc.html)

---
name: report

## Session Info

```r
R.version
```

```
##                _                           
## platform       x86_64-pc-linux-gnu         
## arch           x86_64                      
## os             linux-gnu                   
## system         x86_64, linux-gnu           
## status                                     
## major          3                           
## minor          5.1                         
## year           2018                        
## month          07                          
## day            02                          
## svn rev        74947                       
## language       R                           
## version.string R version 3.5.1 (2018-07-02)
## nickname       Feather Spray
```

---
name: end-slide
class: end-slide

# Thank you