If Statements, Loops, Custom Functions

If statements and loops help implement logical operations in any programming language. We’ll learn how to write these operations in R through basic examples. A function groups operations into a single line of code which can be used repeatedly with different inputs.

If Statements
- An if statement in R has the following syntax:
```
if (<condition>) {
  <statement1>
}
```
- If the condition is True, then statement1 is executed. If the condition is False, statement1 is ignored and nothing happens. Try running this block of code which will print out a message if x is smaller than 10:
```
x <- 5
if (x < 10) {
  print("x is smaller than 10!")
}
```
- In this case, we can extend this statement to include an else clause to execute another statement if the condition x < 5 is False.
```
x <- 10
if (x < 5) {
  print("x is smaller than 10!")
} else {
  print("x is larger than or equal to 10!")
}
```
- How about checking more conditions? Use else if! Any guesses which message the code will print out?
```
x <- 12
if (x < 5) {
  print("small x")
} else if (x < 10) {
  print("medium x")
} else if (x < 15) {
  print("large x")
} else {
  print("very large x")
}
```
Loops
- A for loop runs some specific lines of code for a number of times.
  
  c(1:3) is a vector containing integers [1,2,3]. The loop below will hypothetically execute statement for a total of 3 times, one time each for i=1, 2, 3. Don’t try to run this loop; it’s illustrative!
```
for (i in c(1:3)) {
  <statement1>
}
```
- From Activity 3, we have a score variable containing test scores of 8 students. Suppose that we want to add 20 more points for each student. This can be achieved efficiently with a for loop. length(score) calculates the length of vector score which is the number of times we need to iterate through the loop (8 times). i serves as an index and is a variable that is local/valid to only this for loop. We can also use i within the loop.
```
# score <- c(70, 62, 67, 69, 90, 72, 75, 80)
lenscore <- length(score)
for (i in c(1:lenscore)) {
  score[i] <- score[i] + 20
}
score
```
- Exercise: how would you change the above code to add 20 points for only the first 3 scores and add 10 points for the last 5 scores? Hint: you may need one loop for the first 3 scores and another loop for the last 5.
Custom Functions
- We’ve used many functions built into R and from installed packages but what if there is no off-the-shelf function that does exactly what we want or deals with our specific dataset? Let’s take matters in our own hands and build a function ourselves!
- Defining a function in R follows the syntax:
```
function_name <- function(input1, input2, ...) {
   <statement1>
   <statement2>
   do something with the inputs
   return <result>
}
```
- A function usually takes in a number of inputs (also called arguments), performs some operations with the inputs, and return a result. Take a look at this function addScore that does the same thing as the for loop we saw previously.
```
addScore <- function(score) {
    lenscore <- length(score)
    for (i in c(1:lenscore)) {
        score[i] <- score[i] + 20
    }
    return(score)
}
```
- Running the lines above in R doesn’t do anything yet other than defining the function. To run the function on the data score:
```
addScore(score)
```
- Exercise: Fancy adding an if statement to it? Try modifing the addScore function to also print out a warning when someone’s new score is above 100 and if that happens, assign the score to the maximum value of 100. Feel free to ask the instructor for help!

NEXT STEP: Earn a Workshop Badge