Foundations

Conditionals

An if statement chooses which block runs after checking a Boolean condition.

Choose a label

temperature
conditionals.swift
Replay: real traced execution (multi-file project)
let temperature = 72
let label: String

if temperature >= 80 {
    label = "hot"
} else if temperature >= 60 {
    label = "warm"
} else {
    label = "cool"
}

print("temperature=\(temperature)")
print("label=\(label)")

let temperature = 48
let label: String

if temperature >= 80 {
    label = "hot"
} else if temperature >= 60 {
    label = "warm"
} else {
    label = "cool"
}

print("temperature=\(temperature)")
print("label=\(label)")

let temperature = 91
let label: String

if temperature >= 80 {
    label = "hot"
} else if temperature >= 60 {
    label = "warm"
} else {
    label = "cool"
}

print("temperature=\(temperature)")
print("label=\(label)")

  1. temperature ← 72

    1let temperature→ 72 = 72  //@temperature=48, 912let label: String

  2. if temperature >= 60

    5    label = "hot"6} else if temperature72 >= 60 {7    label = "warm"8} else {

  3. print("temperature=\(temperature)")

    12print("temperature=\(temperature72)")13print("label=\(labelwarm)")
    outputtemperature=72
label=warm

  1. temperature ← 48

    1let temperature→ 48 = 482let label: String

  2. print("temperature=\(temperature)")

    12print("temperature=\(temperature48)")13print("label=\(labelcool)")
    outputtemperature=48
label=cool

  1. temperature ← 91

    1let temperature→ 91 = 912let label: String

  2. if temperature >= 80

    4if temperature91 >= 80 {5    label = "hot"6} else if temperature >= 60 {

  3. print("temperature=\(temperature)")

    12print("temperature=\(temperature91)")13print("label=\(labelhot)")
    outputtemperature=91
label=hot

Follow the Branch

  1. temperature starts at 72.
  2. Swift first checks whether temperature >= 80.
  3. 72 is below 80, so it checks temperature >= 60.
  4. That check is true, so label becomes warm.
  5. The program prints temperature=72 and label=warm. | temperature | path | label | | --- | --- | --- | | 48 | below 60 | cool | | 72 | at least 60 | warm | | 91 | at least 80 | hot |
if else Use `if` for the true branch and `else` for the fallback branch.

Exercise: conditionals.swift

Reproduce label=warm for temperature 72, then try 48 and 91 and predict the label before running each one.