Append

With the Swift append() method we can add characters or another string to a string. We must use var to have a string that can be modified.

With the plus operator, we can also append and concatenate strings. But sometimes using append() is clearer in code (it is obvious we are not adding ints).

Characters

This example creates a Character array and an empty string. It then adds those Characters to the string with append().

let chars: [Character] = ["A", "B", "C"]
var result = String()

// Loop over all Characters in the array.
for char in chars {
    for _ in 0..<3 {
        // Append a Character to the string.
        result.append(char)
    }
}

// Write the result String.
print(result)
AAABBBCCC

Append strings

Here we append a string to another existing string. The end result is printed to the console. Append() can handle characters or entire strings at once.

var result = "cat"

// The append method can append a string.
result.append(" and dog")
print(result)
cat and dog

ReserveCapacity

With this method we can increase the memory used for a string. Then using append() on the string can proceed without resizing the string—this improves performance.

// Use reserveCapacity to ensure enough memory.
// ... This reduces resizing.
var pattern = String()
pattern.reserveCapacity(100)

// Add characters to string.
for _ in 0..<100 {
    pattern += "a"
}

// Print first 20 characters of the string.
print(pattern[pattern.startIndex..<pattern.index(pattern.startIndex, offsetBy: 20)])
aaaaaaaaaaaaaaaaaaaa

With append we change strings to have more characters or strings on their ends. This is useful for building up strings from other data sources.