V2

Question 1

name two options of control structures

Answer 1

• branching : if, else if, switch

- looping: while, for, repeat

Question 2

how to code if-statement

Answer 2

if(class(box) == "character"){
right(box)
}else{
left(box)
}

example:
if(all(x<5)) print - check if all numbers are smaller than 5 in vector
if(any(x<5)) print - check if at leat 1 is smaller than 5

Question 3

how to code switch-statement

Answer 3

switch(class(box)),
logical = left(x),
numeric = middle(x),
character= right (x)
)

Question 4

how to code else if-statement

Answer 4

code same as if statement as second or third line

Question 5

how to code for-statement

Answer 5

for(x in 1:10){ box = box -x} - take x elements out of the box

Question 6

how to code while-statement

Answer 6

box

Question 7

Comparisons operators (6)

Answer 7

== - equal to
!= - not equal to
< - smaller than
> - lager than 
>= - smaller or equal to
>= - larger or equal to

Question 8

how to use &

Answer 8

& is vectorized, can be used for logical vectors

&& not vectorized, takes the first element from a vector

Question 9

what happens at combination of if and & , |

Answer 9

• if needs single logical value

- so output from & and | cannot be used directly

Question 10

how to use pairwise statements

Answer 10

A&B - pairwise AND
A|B pairwise OR

• > can store in a subset
• > can also be used for matrices

Question 11

use lapply (code)

Answer 11

lapply(X, FUN,…) - repeat a function to each element in a vector or list

na.rm to use if there is NA

Question 12

use apply (code)

Answer 12

apply(X,Margin, FUN, …) repeat function to each row or column in a matrix
margin = 1 , rows
margin = 2 , columns
margin = c(1,2), rows and columns

Question 13

what happens with lapply / apply results

Answer 13

• normally give back list

- use unlist function to get vector

Question 14

how to code a function

Answer 14

define a function with the keyword function

example:

boxwood

Question 15

function parameter theory: pass-by-value

Answer 15

function parameters get copied into the function, changing them does not alter the state of the variable that was passed

Question 16

function parameter theory: pass-by-reference

Answer 16

function parameters are references to the variable passed into the function, changing them alters the state of the variable that was passed

Question 17

function parameter theory: pass-by-promised

Answer 17

function parameters are references to the variables passed into the function, when changing them, a copy is made. this way the state of the variable that was passed is not changed

Question 18

functions: parameters

Answer 18

• in R function parameters get copied when changed inside a function (pass-by-promise)
• so updating them is not visible from he outside

Question 19

functions: default parameters

Answer 19

• we can set reasonable default for some variables

Question 20

whats a variadic function

Answer 20

• a variadic function is a function which accepts a variable number of arguments
• we can use … to specify variadic functions in R

Question 21

Functions: scope

Answer 21

• intern is not visible from outside
• this is called the scope of the variable
• we can however access variables in our parent scope (is called bad practice but sometimes we have to. Reasons: being lazy, save RAM, plot functions use it to read the environment settings)

Question 22

Overview brackets

Answer 22

() : used when you call a function, used in control structure statements
[]: specify an index in a vector, matrix or data.frame
[[]]: specify an index in a list
{}: defines blocks of code, is used to surround expressions - what expressions belong to an if statement
- what expression belong to this function

Question 23

How to work with strings

Answer 23

• strings are enclosed using: “ or ‘
• combine strings using: paste
• print them to screen using: print
• print them to anywhere (screen and file) using: cat

Question 24

how to escape characters

Answer 24

Quotes: \" and \'
New line: \n
Tab: \t
Backslash: \\
backspace: \b

Question 25

how to use the seperate elements (code)

Answer 25

cat (“Hello”, “World\n”, sep=”,”) - seperate with comma

Hello, World

Question 26

runif()

Answer 26

• get a random number
• uniform distribution
• every value has the same chance of being drawn

Question 27

rnorm()

Answer 27

• get a random number in the gaussian distribution

- value near the mean have a higher chance of being drawn

Question 28

rpois()

Answer 28

• get a random number in the poisson distribution

- numbers at the low en d of the distribution have a higher chance of occurring

Question 29

how to get a repeatable randomness

Answer 29

• use set.seed() function

- same set.seed number produces same randomness