# Control Flow

## Use Cases & Syntax

Used to test if a specific case is true or false

**Short-circuit evaluation:**

* Test if all conditions are true
* Test if any conditions are true
* Test if a condition is not true

#### Conditional evaluation

* If statement: run code if this statement is true
  * Only used at the beginning of a conditional statement
* Else if statement: if previous statements aren't true, try this
  * Can be used an unlimited number of times in an if statement
* Else statement: catch-all for anything outside of prior statements
  * Only used to end a conditional statement

#### Inputs:

```r
#If statement
a <- 2
b <- 1
if (a > b){
print("a is greater than b")}

#Else if statement
x <- 10
y <- 10
if (x > y){
print("x is greater than y")
} else if (x <= y){
print("x is less than or equal to y")
}

#Else statement
d <- 3
if (d > 5){
print("d is greater than 5")
} else if (d == 5){
print("d is equal to 5")
} else {
print("d is less than or equal to 5")
}
```

#### Outputs:

```r
#If statement
[1] "a is greater than b"

#Else if statement
[1] "x is less than or equal to y"

#Else statement
[1] "d is less than or equal to 5"
```

## Loops

Repeats a block of code a specified number of times or until some condition is met

* While loop
* For loop
* Use break to terminate loop

#### Inputs:

```r
#While loop
i <- 1
while (i < 5){
print(i)
i <- i + 1
}

#While loop with break
j <- 1
while (j < 5){
print(j)
j <- j + 1
if (j == 4){
break
}}

#For loop
fruit <- list("apple", "banana", "peach")
for (x in fruit) {
  print(x)
}

#Nested for loop
adjectives <- list("scrumptious", "overripe", "delicious")
fruit <- list("apple", "banana", "peach")
for (x in adjectives) {
    for (y in fruit) {
      print(paste(x, y))
}}
```

#### Outputs:

```r
#While loop
[1] 1
[1] 2
[1] 3
[1] 4

#While loop with break
[1] 1
[1] 2
[1] 3

#For loop
[1] "apple"
[1] "banana"
[1] "peach"

#Nested for loop
[1] "scrumptious apple"
[1] "scrumptious banana"
[1] "scrumptious peach"
[1] "overripe apple"
[1] "overripe banana"
[1] "overripe peach"
[1] "delicious apple"
[1] "delicious banana"
[1] "delicious peach"
```

## Comparison Operators

| Operator | Description           |
| -------- | --------------------- |
| >        | Greater than          |
| <        | Less than             |
| >=       | Greater than or equal |
| <=       | Less than or equal    |
| ==       | Exactly equal         |
| !=       | Not equal to          |
| &        | Entry wise and        |

Input:

```r
# Demonstrate comparison operators

# Assign values to variables
c1 <- 25
c2 <- 50
c3 <- 75
c4 <- 50

# Testing equality
c1 == c3
c2 == c4

# Changing values using assignment operators
c1 <- c1 * 3  # shorthand for c1 = c1 * 3
c4 <- c4 + 1  # shorthand for c4 = c4 + 1

# Testing less than and greater than
c1 < c2
c4 <= c2
c1 > c2
c3 >= c2
```

Output:

```r
# Testing equality
# c1 == c3
[1] FALSE
# c2 == c4
[1] TRUE

# Testing less than and greater than
# c1 < c2
[1] FALSE
# c4 <= c2
[1] FALSE
# c1 > c2
[1] TRUE
# c3 >= c2
[1] TRUE
```

## Resources

* R Documentation: [Conditional Execution](https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Conditional-execution)
* R Documentation: [Repetitive Execution](https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Repetitive-execution)