Substring.
In Swift 5.8 we can acquire a substring in many ways. Strings cannot be directly indexed by integers, but we can use ranges to take substrings.
With startIndex and endIndex,
we access character offsets in strings. We use index() to move past the start and end. With the resulting range, we get a substring.
First example.
This program takes a substring of a string. We first create a range. We use the half-open range operator on the endIndex of the "name" string.
Start
We call index() on the string and pass startIndex and an offsetBy int to get the start of the range. We advance 4 chars.
Result
We skip past the first 4 characters of "The Grapes of Wrath." The resulting substring starts with Grapes.
let name =
"The Grapes of Wrath"
// Get range based on the string index.
let r = name.index(name.startIndex, offsetBy: 4)..<name.endIndex
// Access substring from range.
let result = name[r]
print(result)
Grapes of Wrath
Example 2.
This example is similar, but uses a different end. We isolate the middle substring "two" of our input string. We use index() to omit the first 4 and last 6 characters.
Info
The exclusive end is based on the endIndex of the string. We move backwards 6 chars from the end of the string.
Result
The substring is returned by indexing the string with the range. It equals "two."
// This is the input string.
let s =
"one two three"
// Get range 4 places from the start, and 6 from the end.
let r = s.index(s.startIndex, offsetBy: 4)..<s.index(s.endIndex, offsetBy: -6)
// Access the string by the range.
let substring = s[r]
print(substring)
two
End substring.
Sometimes we want to base a substring on the end of the string. Here we call index() to adjust the start index of the range by 6 chars.
And
We leave the end index alone. The resulting substring is the end of the original string.
let value =
"bird, lizard and fish"
// Get range of all characters past the first 6.
let range = value.index(value.startIndex, offsetBy: 6)..<value.endIndex
// Access the substring.
let substring = value[range]
print(substring)
lizard and fish
RemoveSubrange.
This func can be used to create substrings. We specify the range of the string we want to remove. The string is modified to have only the remaining characters.
Here
We remove the first 6 characters from the string. We pass a range to the removeSubrange method.
Note
We base our range on the string's startIndex. It covers the start to six places past the start.
Note 2
We must use the var keyword declare the string "dotnetperls" here. The string is modified by removeSubrange.
var name =
"dotnetperls"
// Remove the first 6 characters from this string.
// ... We begin at startIndex.
// ... We continue until startIndex advanced by 6 chars.
name.removeSubrange(name.startIndex..<name.index(name.startIndex, offsetBy: 6))
print(name)
perls
Substring after char.
This program introduces a secondWord method. It searches for a space in the argument string. It then returns the substring after that space character.
Start
We use the firstIndex() method to search for a space in secondWord. We then use "if let" to see if the space was found.
Finally
We use the index we just computed as the start. We return a substring based on the range of the start to the string's endIndex.
Important
We cannot directly return a string from a subscript. We must return the String() with a special call.
func secondWord(value: String) -> String {
// Find index of space.
if let space = value.firstIndex(of:
" "
) {
// Return String.
// ... Use "after" to avoid including the space.
// Use String() to for Swift 4.
return String(value[value.index(after: space)..<value.endIndex])
return
""
// Test our func.
print(secondWord(value:
"orange cat"
))
print(secondWord(value:
"black dog"
))
print(secondWord(value:
"multicolored parrot"
))
cat
parrot
Index before, after.
We can use firstIndex() and index() to search for a character in a string. With index() we can get the index before or after another index.
Here
We use these index methods on a simple string. This shows how the index method changes its results based on its arguments.
let colors =
"blue,red,green"
// Get character indexes.
let indexE = colors.firstIndex(of:
"e"
)!
let indexComma = colors.firstIndex(of:
","
)!
// Get before and after indexes.
let indexBeforeE = colors.index(before: indexE)
let indexAfterE = colors.index(after: indexE)
let indexBeforeComma = colors.index(before: indexComma)
let indexAfterComma = colors.index(after: indexComma)
// Get substrings based on ranges.
print(
"IndexE: \(colors[indexE..<colors.endIndex])"
)
print(
"IndexComma: \(colors[indexComma..<colors.endIndex])"
)
print(
"IndexBeforeE: \(colors[indexBeforeE..<colors.endIndex])"
)
print(
"IndexAfterE: \(colors[indexAfterE..<colors.endIndex])"
)
print(
"IndexBeforeComma: \(colors[indexBeforeComma..<colors.endIndex])"
)
print(
"IndexAfterComma: \(colors[indexAfterComma..<colors.endIndex])"
)
Single char, StartIndex.
To access a character from a string, we must use an index (not an Int). Here we use startIndex to get the first char in the string "Puma."
let value =
"Puma"
// A string's chars can be accessed with startIndex or endIndex.
let char = value[value.startIndex]
print(char)
P
Int subscript.
A string cannot be indexed by an Int. Characters in a string may be different lengths. So we must access strings with ranges based on startIndex and endIndex.
let value =
"lion"
// A string cannot be accessed with Ints.
var char = value[0]
main.swift:3:12: 'subscript' is unavailable: cannot subscript String with an Int
Returning String error.
In Swift 4 and beyond we must return a String with a special call. We cannot just return the result of a subscript directly.
Note
The error is shown in this example, and the fix to this error is shown afterwards.
func removeFirst2Chars(value: String) -> String {
// We need to wrap this in a String call.
return value[value.index(value.startIndex, offsetBy: 2)..<value.endIndex]
