STA 517 3.0 Programming and Statistical Computing with R

# STA 517 3.0 Programming and Statistical Computing with R
## 🔩 Control Structures in R
### 
### Dr Thiyanga Talagala

---

}
</style>

### Today's menu

**Control structures (loops/ conditional executions)**

if, else

for

while

repeat

break

switch

]

]

---
class: inverse, center, middle

# Conditional executions control the flow of the execution.
---
background-image: url(ifloop.jpeg)
background-size: cover

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[
Draw a star on star-shaped cookies and draw a heart on heart-shaped cookies.

]]

---
background-image: url(ifloop.jpeg)
background-size: cover

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## if-else

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[
Draw a star on star-shaped cookies and draw a heart on heart-shaped cookies.

]]

---

# if-else

```r
if (condition) {
    # do something
} else {
    # do something else
}

```

]

Example

```r
test_even_odd <- function(x){
  if (x %% 2 == 0){
    print("even number")
  } else {
    print("odd number")
  }
}
```

```r
test_even_odd(5)
```

```
[1] "odd number"
```

```r
test_even_odd(6)
```

```
[1] "even number"
```

]
---

# `ifelse`: vectorization with `ifelse`

```r
ifelse(condition, TRUE condition output, FALSE condition output)

```

Example

```r
test_even_odd_v2 <- function(x){
  
  ifelse(x %% 2 == 0, "even number", "odd number")
  
}
```

```r
test_even_odd_v2(5)
```

```
FALSE [1] "odd number"
```

```r
test_even_odd_v2(c(1,6))
```

```
FALSE [1] "odd number"  "even number"
```

---

## Difference between `if, else` and `ifelse`

```r
test_even_odd <- function(x){
  if (x %% 2 == 0) {
    print("even number")
  } else {
    print("odd number")
  }
}
```

```r
test_even_odd(5)
```

```
FALSE [1] "odd number"
```

```r
test_even_odd(c(1,6))
```

```
FALSE [1] "odd number"
```

]

**`ifelse`**

```r
test_even_odd_v2 <- function(x){
  ifelse (x %% 2 == 0, "even number", "odd number")
}

test_even_odd_v2(5)
```

```
FALSE [1] "odd number"
```

```r
test_even_odd_v2(c(1,6))
```

```
FALSE [1] "odd number"  "even number"
```

]

---

# Nested if-else

- Multiple conditions

```r
grade_marks <- function(marks){
  
  if (marks < 20) {
    "D"
  } else if (marks <= 50) {
    "C"
  } else if (marks <= 60) {
    "B"
  } else {
    "A"
  }
}

grade_marks(75)
```

```
[1] "A"
```

---

# Your turn

---

R for Data Science-Exercises 19.4.4 - Q2

Help:

`lubridate::now()` and `lubridate::hour()`

---

## if

```r
check.negative <- function(x){

if (x < 0 ) {
  print("X is negative")
  }

}
```

```r
check.negative(-10)
```

```
[1] "X is negative"
```

```r
check.negative(10)
```

---

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

For the first 50 cookies, fill the center of cookies with jam.

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## for

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

For the first 50 cookies, fill the center of cookies with jam.

]]

---

# `for` loop

- execute a block of code a specific number of times or until the end of a sequence.

```r
for (i in 1:5) {
  print(i*100)
}
```

```
[1] 100
[1] 200
[1] 300
[1] 400
[1] 500
```

---

```r
continents <- c("Asia", "EU", "AUS", "NA", "SA", "Africa")
```

```r
for (i in 1:4) {
  print(continents[i])
}
```

```
[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"
```

```r
for (i in 1:4)  print(continents[i])
```

```
[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"
```

]

```r
for (i in seq(continents)) {
  print(continents[i])
}
```

```
[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"
[1] "SA"
[1] "Africa"
```

]
---

# Your turn

---

Write a for loop that iterates over the numbers 1 to 4 and prints the squared of each number using `print()`.

---

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[
For the first 50 cookies, draw a star on star-shaped cookies and draw a heart on heart-shaped cookies.

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## nested loops

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

For the first 50 cookies, draw a star on star-shaped cookies and draw a heart on heart-shaped cookies.

]]

---

## Nested loops

```r
a <- 1:10

testfun <- function(x){
for (x in 1:5) {
  
   if (x %% 2 == 0) {
    print("even number")
  } else {
    print("odd number")
  }
  
}}

testfun(a)
```

```
[1] "odd number"
[1] "even number"
[1] "odd number"
[1] "even number"
[1] "odd number"
```

---

# Nested loops

```r
mat <- matrix(1:6, ncol=2)
mat
```

```
     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6
```

```r
for (i in 1:3) {
  for (j in 1:2) {
    print(mat[i, j])
  }
}
```

```
[1] 1
[1] 4
[1] 2
[1] 5
[1] 3
[1] 6
```

---

# Your turn

---

Write a function to count the number of even numbers in a vector.

---

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Taste the cookies one by one until you find the star-shaped cookie.

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## while

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Taste the cookies one by one until you find the star-shaped cookie.

]]

---

# while

```r
i <- 1 # initial value

while (i != 10) {
  
  print(i)
  i <- i + 1 # increment
  
}
```

```
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
```
]

```r
i <- 1 # initial value

while (i < 10) {
  
  print(i)
  i <- i + 1 # increment
  
}
```

```
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
```
]

---

# Your turn

---

Can you guess the output?

```r
i <- 1 # initial value

while (i == 10) {
  
  print(i)
  i <- i + 1 # increment
  
}
```

---

# Your turn

---

Generate the first `n` numbers of the Fibonacci Sequence using while loop.

0 1 1 2 3 5 ...

---
background-image: url(cookie9.jpeg)
background-size: cover

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Taste the cookies one by one until you find the strawberry flavoured cookie.

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## repeat-break

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Taste the cookies one by one until you find the strawberry flavoured cookie.

]]

---

# repeat and break

- Iterate over a block of code multiple number of times.

-  No condition check in repeat loop to exit the loop.

- The only way to exit a repeat loop is to call break.

---

## repeat and break

Example 1

```r
x <- 5

repeat {
  print(x)
  x = x+1
  if (x == 10){
    break
  }
}
```

```
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9
```
]

--
.pull-right[

Example 2

```r
set.seed(1)

repeat {
 x<-runif(1, 5, 10)
 print(x)
 if(x < 6.1){
   break
 }
}
```

```
[1] 6.327543
[1] 6.860619
[1] 7.864267
[1] 9.541039
[1] 6.00841
```
]

---

##  Your turn

1. Write a function to print random numbers from the standard  normal distribution but stops (breaks) if you get a number bigger than 1.

2. Using next adapt the loop from last exercise so that doesn’t print negative numbers.

---
background-image: url(decorating-2702974_1920.jpg)
background-size: cover

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Skip cracked muffins and decorate the rest.

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## next

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Skip cracked muffins and decorate the rest.

]]

---

# next

```r
for(i in 1:10) {
  if(i <= 5) {
  next # Skip the first 5 iterations
  }
print(i)
}
```

```
[1] 6
[1] 7
[1] 8
[1] 9
[1] 10
```

---

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Green base - roses, small - dots, medium - stars, ...

]]

---

.bg-yellow.b--dark-yellow.ba.bw2.br3.shadow-5.ph4.mt5[

## switch

]]

.bg-washed-green.b--dark-green.ba.bw2.br3.shadow-5.ph4.mt5[

Green base - roses, small - dots, etc..

]]

---

# switch

When you want a function to do different things in different
circumstances, then the switch function can be useful.

```r
feelings <- c("sad", "afraid")

for (i in feelings){
  print(
  switch(i,
  happy = "I am glad you are happy",
  afraid = "There is nothing to fear",
  sad = "Cheer up",
  angry = "Calm down now"
  ))
}
```

```
[1] "Cheer up"
[1] "There is nothing to fear"
```

---
.pull-left[

## Single element - working

```r
x <- "a"
v <- switch(x, "a" = "apple",
               "b" = "banana",
               "c" = "cherry")
v
```

```
[1] "apple"
```

## Multiple elements - not working

```r
switch(c("a", "b"), "a" = "apple", # not working
       "b" = "banana",
       "c" = "cherry")
```

]

## Multiple element - use `for` loop

```r
vect <- c("a", "b")
for(i in seq(vect)){
print(switch(i, "a" = "apple", # not working
       "b" = "banana",
       "c" = "cherry"))
}
```

]

---
class: center, middle

## Thank you!

Slides available at: hellor.netlify.app