Range Over Built in Swift

Range iterates over elements in a variety of built-in data structures. Let’s see how to use range with some of the data structures we’ve already learned.

Here we use range to sum the numbers in a slice. Arrays work like this too.

let nums = [2, 3, 4]
var sum = 0
for num in nums {
    sum += num
}
print("sum:", sum)

range on arrays and slices provides both the index and value for each entry. Above we didn’t need the index, so we ignored it with the blank identifier _. Sometimes we actually want the indexes though.

for (i, num) in nums.enumerated() {
    if num == 3 {
        print("index:", i)
    }
}

range on map iterates over key/value pairs.

let kvs = ["a": "apple", "b": "banana"]
for (k, v) in kvs {
    print("\(k) -> \(v)")
}

range can also iterate over just the keys of a map.

for k in kvs.keys {
    print("key:", k)
}

range on strings iterates over Unicode code points. The first value is the starting byte index of the rune and the second the rune itself.

for (i, c) in "go".enumerated() {
    print(i, c)
}

To run the program, put the code in main.swift and use swiftc to compile and run it.

$ swiftc main.swift -o main
$ ./main
sum: 9
index: 1
a -> apple
b -> banana
key: a
key: b
0 g
1 o

Next example: Pointers.