Count and Indices

You've used str.count in fundamentals to get the number of characters. Going further means understanding that a Swift String is a sequence of Characters, not a flat array of bytes.

let greeting = "Hello"
print(greeting.count)            // 5
print(greeting.first!)           // "H"
print(greeting.last!)            // "o"
print(greeting.isEmpty)          // false

You can't index a string with an integer. To grab a character at a specific position, ask the string for an index offset from its start:

let i = greeting.index(greeting.startIndex, offsetBy: 1)
print(greeting[i])               // "e"

The first, last, and isEmpty properties are the everyday tools. Reach for indices only when you really need a specific position.