How to read a specific file's line in swift4? - swift

Question

Testing in Playground I read a whole file in an array of String, one string per line.
But what I need is a specific line only:
let dir = try? FileManager.default.url(for: .documentDirectory,
in: .userDomainMask, appropriateFor: nil, create: true)
let fileURL = dir!.appendingPathComponent("test").appendingPathExtension("txt")
let text: [String] = try String(contentsOf: fileURL).components(separatedBy: NSCharacterSet.newlines)
let i = 2 // computed before, here to simplify
print(text[i])
There is a way to avoid reading the complete big file?

Answer 1

I'm guessing you mean that you want to retrieve the index without manually searching the array with, say, a for-in loop.
In Swift 4 you can use Array.index(where:) in combination with the StringProtocol's generic contains(_:) function to find what you're looking for.
Let's imagine you're looking for the first line containing the text "important stuff" in your text: [String] array.
You could use:
text.index(where: { $0.contains("important stuff") })
Behind the scenes, Swift is looping to find the text, but with built-in enhancements, this should perform better than manually looping through the text array.
Note that the result of this search could be nil if no matching lines are present. Therefore, you'll need to ensure it's not nil before using the result:
Force unwrap the result (risking the dreaded fatal error: unexpectedly found nil while unwrapping an Optional value):
print(text[lineIndex!)
Or, use an if let statement:
if let lineIndex = stringArray.index(where: { $0.contains("important stuff") }) {
print(text[lineIndex])
}
else {
print("Sorry; didn't find any 'important stuff' in the array.")
}
Or, use a guard statement:
guard let lineIndex = text.index(where: {$0.contains("important stuff")}) else {
print("Sorry; didn't find any 'important stuff' in the array.")
return
}
print(text[lineIndex])

Answer 2

To find a specific line without reading the entire file in, you could use this StreamReader answer. It contains code that worked in Swift 3. I tested it in Swift 4, as well: see my GitHub repo, TEST-StreamReader, for my test code.
You would still have to loop to get to the right line, but then break the loop once you've retrieved that line.
Here's the StreamReader class from that SO answer:
class StreamReader {
let encoding : String.Encoding
let chunkSize : Int
var fileHandle : FileHandle!
let delimData : Data
var buffer : Data
var atEof : Bool
init?(path: String, delimiter: String = "\n", encoding: String.Encoding = .utf8,
chunkSize: Int = 4096) {
guard let fileHandle = FileHandle(forReadingAtPath: path),
let delimData = delimiter.data(using: encoding) else {
return nil
}
self.encoding = encoding
self.chunkSize = chunkSize
self.fileHandle = fileHandle
self.delimData = delimData
self.buffer = Data(capacity: chunkSize)
self.atEof = false
}
deinit {
self.close()
}
/// Return next line, or nil on EOF.
func nextLine() -> String? {
precondition(fileHandle != nil, "Attempt to read from closed file")
// Read data chunks from file until a line delimiter is found:
while !atEof {
if let range = buffer.range(of: delimData) {
// Convert complete line (excluding the delimiter) to a string:
let line = String(data: buffer.subdata(in: 0..<range.lowerBound), encoding: encoding)
// Remove line (and the delimiter) from the buffer:
buffer.removeSubrange(0..<range.upperBound)
return line
}
let tmpData = fileHandle.readData(ofLength: chunkSize)
if tmpData.count > 0 {
buffer.append(tmpData)
} else {
// EOF or read error.
atEof = true
if buffer.count > 0 {
// Buffer contains last line in file (not terminated by delimiter).
let line = String(data: buffer as Data, encoding: encoding)
buffer.count = 0
return line
}
}
}
return nil
}
/// Start reading from the beginning of file.
func rewind() -> Void {
fileHandle.seek(toFileOffset: 0)
buffer.count = 0
atEof = false
}
/// Close the underlying file. No reading must be done after calling this method.
func close() -> Void {
fileHandle?.closeFile()
fileHandle = nil
}
}
extension StreamReader : Sequence {
func makeIterator() -> AnyIterator<String> {
return AnyIterator {
return self.nextLine()
}
}
}