7 Control Flow

Stolen without changes from Krista’s lesson. A stub, a work-in-progress.

7.1 For Loops

# General form

for (variable in sequence){
    Do something
}

for (i in (1:10)) {
  x <- i^2
  print(x)
}

## [1] 1
## [1] 4
## [1] 9
## [1] 16
## [1] 25
## [1] 36
## [1] 49
## [1] 64
## [1] 81
## [1] 100

7.2 While Loops

# General form:

while (condition){
    Do something
}

i <- 1

while (i < 11){
  print(i)
  i <- i + 1
}

## [1] 1
## [1] 2
## [1] 3
## [1] 4
## [1] 5
## [1] 6
## [1] 7
## [1] 8
## [1] 9
## [1] 10

7.3 If/Then Statements

# General form:

if (condition){
    Do something
} else {
    Do something different
}

if (i > 5) {
  print("This number is greater than 5.")
} else {
  print("This number is less than 5.")
}

## [1] "This number is greater than 5."

7.4 Apply functions

R-bloggers, lapply() and sapply()

FUNCTION	INPUT	OUTPUT
apply	matrix	vector or matrix
sapply	vector or list	vector or matrix
lapply	vector or list	list

apply() functions are a type functional, i.e. a function that takes a function as an input and returns a vector or a list as output. This can be used as an alternative to for loops.

X <- matrix(data = c(1,2,3, 1,2,3, 1,2,3), nrow = 3, ncol = 3, byrow = T)
X

##      [,1] [,2] [,3]
## [1,]    1    2    3
## [2,]    1    2    3
## [3,]    1    2    3

# Sum the values of each column with `apply()`
apply(X, 2, sum) # the second argument refers to a vector giving the subscripts which the function will be applied over, e.g. for a matrix 1 indicates rows, 2 indicates columns. The third argument specifies the function to be applied

## [1] 3 6 9

# Now sum the values of each row
apply(X, 1, sum)

## [1] 6 6 6

lapply() takes a function, applies it to each element in a list, and returns the results in the form of a list. Recall the for loop from the above example:

for (i in (1:10)){
  x <- i^2
  print(x)
}

We can accomplish the same result without using a for loop by instead using lapply(). Notice that lapply() returns a list.

lapply(1:10, function(x) x^2)

## [[1]]
## [1] 1
## 
## [[2]]
## [1] 4
## 
## [[3]]
## [1] 9
## 
## [[4]]
## [1] 16
## 
## [[5]]
## [1] 25
## 
## [[6]]
## [1] 36
## 
## [[7]]
## [1] 49
## 
## [[8]]
## [1] 64
## 
## [[9]]
## [1] 81
## 
## [[10]]
## [1] 100

If we want to get back an atomic vector instead of a list, we can use sapply().

sapply(1:10, function(x) x^2)

##  [1]   1   4   9  16  25  36  49  64  81 100