If-then-else

If-then-else statements select which code to run based on a condition. The simplest form is:

a <- 5.0
b <- 10.0
if (a < b) {
  print("a is smaller than b")
}

## [1] "a is smaller than b"

An if statement can be extended with else if branches and an optional final else branch. In this example, a more specific condition is checked first:

if (a < b/2) {
  print("a is much smaller than b")
} else if (a < b) {
  print("a is smaller than b")
} else if (a == b) {
  print("a equals b")
}

## [1] "a is smaller than b"

Conditions are checked in order. The first condition that is TRUE selects its branch; later branches are skipped. If none is TRUE, the final else branch runs, if present.

The example prints nothing when a is larger than b, because none of its conditions is TRUE. Add a final else branch to handle that case:

a <- 15.0
b <- 10.0
if (a < b/2) {
  print("a is much smaller than b")
} else if (a < b) {
  print("a is smaller than b")
} else if (a == b) {
  print("a equals b")
} else {
  print("a is larger than b.")
}

## [1] "a is larger than b."

You can combine comparisons with logical operators:

c <- 20.0
if (a < b & a == c) {
  print("a is smaller than b and equal to c")
} else if (a > b & a == c) {
  print("a is larger than b and equal to c")
} else {
  print("a non-equal to c.")
}

## [1] "a non-equal to c."

An if branch runs when its condition is TRUE. You can also test whether a stored logical value is FALSE:

test <- a < b
if (test == FALSE) {
  print("a is larger than b")
} else {
  print("a is smaller than or equal b.")
}

## [1] "a is larger than b"

If-then-else decisions are useful if your computation needs different workflows for different variable values.

# Pseudocode
if("<some logical (and boolean) expression>"){
    "<do something>"
} else {
    "<do something else>"
}

Test what you’ve learned so far!

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