Swift Loops and Collections

Loops run code many times, and collections hold many values together.

for-in Loops

A for-in loop goes through a range or a collection:

Example

swift

for i in 1...3 {
    print("Round \(i)")
}

for i in 0..<3 {
    print(i, terminator: " ")
}
print()

for i in stride(from: 10, to: 0, by: -3) {
    print(i, terminator: " ")
}
print()

Output

Round 1
Round 2
Round 3
0 1 2 
10 7 4 1

If you do not need the counter, use _: for _ in 1...3 { print("Hi") }.

while and repeat-while

while checks the condition first. repeat-while runs the code once and then checks.

Example

swift

var balance = 1000
var months = 0
while balance < 1500 {
    balance += 200
    months += 1
}
print("Saved \(balance) in \(months) months")

var tries = 0
repeat {
    tries += 1
} while tries < 3
print("Tries: \(tries)")

Output

Saved 1600 in 3 months
Tries: 3

Use break to stop a loop and continue to skip to the next round.

Arrays

An array is an ordered list of values of the same type. Use var if you want to change it.

Example

swift

var fruits = ["Mango", "Apple", "Banana"]
print(fruits[0])
print(fruits.count)

fruits.append("Guava")
fruits.remove(at: 1)
print(fruits)

for fruit in fruits {
    print("I like \(fruit)")
}

for (index, fruit) in fruits.enumerated() {
    print(index, fruit)
}

Output

Mango
3
["Mango", "Banana", "Guava"]
I like Mango
I like Banana
I like Guava
0 Mango
1 Banana
2 Guava

Indexes start at 0. Asking for an index that does not exist (like fruits[10]) crashes the program, so check count first.

Dictionaries

A dictionary stores key: value pairs. Looking up a key gives an optional, because the key may not exist.

Example

swift

var marks = ["Ali": 85, "Ayesha": 92]
marks["Bilal"] = 74

print(marks["Ayesha"] ?? 0)
print(marks["Zara"] ?? 0)
print(marks.count)

for name in marks.keys.sorted() {
    print("\(name): \(marks[name]!)")
}

Output

92
0
3
Ali: 85
Ayesha: 92
Bilal: 74

Dictionaries have no fixed order. If you need the items in order, sort the keys as in the example above. Here ! is safe because each key came from the dictionary itself.

Sets

A set stores unique values with no order. It is good for checking if something exists.

Example

swift

var visited: Set = ["Karachi", "Lahore", "Karachi"]
print(visited.count)
visited.insert("Quetta")
print(visited.contains("Lahore"))
print(visited.contains("Multan"))

Output

2
true
false

Choosing a Collection

Collection	Ordered?	Duplicates?	Use it for
Array	Yes	Allowed	A list of items in order
Dictionary	No	Keys unique	Looking up a value by key
Set	No	Not allowed	Unique items, fast checks

Exercise

Which line adds "Peach" to the end of the array var fruits = ["Mango"]?

fruits.add("Peach")fruits.push("Peach")fruits.append("Peach")fruits.insert("Peach")