Control Flow in R

Victor Ordu

Recap of the Use of Boolean Logic

Simply put, Boolean logic refers to binary values that can are akin to a switch:

OFF/ON
0/1
yes/no

In R, the inbuilt options for Boolean logic are TRUE/FALSE.

Logic Table

This is how combination of Boolean values evaluate with && (AND), || (OR), and xor.

( P )	( Q )	( P AND Q )	( P OR Q )	( P )
T	T	T	T	F
T	F	F	T	F
F	T	F	T	T
F	F	F	F	T

Conditionals

if, else if, else: These can be used to instruct a particular part of the code to be run or not run.
For example take this instruction:
If x is greater than y, print “Yes o!”, otherwise print “No way!”

Using R code, it would look like this:

# Note that x is 8 and y is 2
if (x > y) {
  "Yes o!"
} else {
  "No way!"
}

When the expression in if() is TRUE, the code in that part is executed. If FALSE, the else part is executed.

What is the result?

x <- '8'

if(x < 5) {
  "yes" 
  } else {
    "no"
  }

[1] "no"

if (is.integer(x)) {
  "Integer"
} else if (is.double(x)) {
  "Double"
} else if (is.character(x)) {
  "String"
} else {
  "What the heck are you?"
}

[1] "String"

Loops

Loops allow us to run a code block over and over again.

For a simple explanation of loops, read this article.

Motivation

Imagine having to write code that has the same statement repeated:

x <- 1L

x <- x + 1
x <- x + 1
x <- x + 1
# and on and on...

---

How about a construct that will allow us to just write the statement

x <- 1L

# Repeat this statement 3 times but write it once
x <- x + 1  # x 3
x

##  `repeat`

::: {.cell}

```{.r .cell-code}
# write repeat loop that prints all the even numbers from 2-10
# via incrementing the variable, i <- 0
i <- 0
repeat{
  i <- i + 2
  print(i)
  if(i >= 10)
    break
}

[1] 2
[1] 4
[1] 6
[1] 8
[1] 10

:::

# 2..Using the following variables
msg <- c("Hello")
i <- 1

# write a repeat{} loop that breaks of the incrementation of "i" after
# 5 loops, and prings "msg" at every increment.
repeat{
  i <- i + 1
  print(msg)
  if (i >= 6)
    break
}

[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"

`while`

# 3. Wwith i <- 1, write a while() loop that prints the odd numbers
# from 1 through 7
i <- 1
while (i <= 7) {
  if ((i %% 2) != 0)
    print(i)
  i <- i + 1
}

[1] 1
[1] 3
[1] 5
[1] 7

# 4. Using the following variable:
msg <- c("Hello")
i <- 1
# write a while() loop that increments the variabe "i" 6 times and prints "msg"
# at every iteration
while (i < 7) {
  i <- i + 1
  print(msg)
}

[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"

`for`

# 5. Print the first four numbers of this sequence
x <- c(7, 4, 3, 8, 9, 25)
for (i in 1:4)
  print(x[i])

[1] 7
[1] 4
[1] 3
[1] 8

# 6. write a for() loop that prints all the letters in
y <- c("q", "w", "e", "r", "z", "c")
for (i in 1:length(y))
  print(y[i])

[1] "q"
[1] "w"
[1] "e"
[1] "r"
[1] "z"
[1] "c"

Read data from a file

Task: Read a table from a CSV file called mockdata.csv.

Use read.csv() function to read the file into R and at the same time create an object called mydata.

mydata <- read.csv("mockdata.csv")

# Print out the data inside R
mydata

   no key age    sex height location
1   1   A  21   male   0.96    Abuja
2   2   B  22 female   1.86    Abuja
3   3   C  23 female   1.68    Abuja
4   4   D  24   male   1.13    Abuja
5   5   E  25 female   1.40    Lagos
6   6   F  26 female   1.52   Kaduna
7   7   G  27 female   1.06  Makurdi
8   8   H  28   male   1.25    Abuja
9   9   I  29   male   1.06    Lagos
10 10   J  30   male   1.12    Abuja
11 11   K  31 female   0.96   Sokoto
12 12   L  32 female   0.93    Lagos
13 13   M  33 female   1.17    Abuja
14 14   N  34 female   1.31  Makurdi
15 15   O  35   male   1.48  Makurdi
16 16   P  36   male   0.99    Lagos
17 17   Q  37 female   1.31  Makurdi
18 18   R  38   male   1.24   Sokoto
19 19   S  39 female   1.25    Lagos
20 20   T  40 female   0.96   Sokoto

Use a loop to check the types of the columns