How can I catch a NSFileHandleOperationException in Swift?
I use fileHandle.readDataToEndOfFile() which calls (according to the documentation) fileHandle.readDataOfLength() which can throw (again according to the documentation) a NSFileHandleOperationException.
How can I catch this exception? I tried
do {
return try fH.readDataToEndOfFile()
} catch NSFileHandleOperationException {
return nil
}
but Xcode says
Warning: No calls to throwing functions occur within 'try' expression
Warning: 'catch' block is unreachable because no errors are thrown in 'do' block
How do I do this? 😄
Edit:
I just decided to use C's good old fopen, fread, fclose as a workaround:
extension NSMutableData {
public enum KCStd$createFromFile$err: ErrorType {
case Opening, Reading, Length
}
public static func KCStd$createFromFile(path: String, offset: Int = 0, length: Int = 0) throws -> NSMutableData {
let fh = fopen(NSString(string: path).UTF8String, NSString(string: "r").UTF8String)
if fh == nil { throw KCStd$createFromFile$err.Opening }
defer { fclose(fh) }
fseek(fh, 0, SEEK_END)
let size = ftell(fh)
fseek(fh, offset, SEEK_SET)
let toRead: Int
if length <= 0 {
toRead = size - offset
} else if offset + length > size {
throw KCStd$createFromFile$err.Length
} else {
toRead = length
}
let buffer = UnsafeMutablePointer<UInt8>.alloc(toRead)
defer {
memset_s(buffer, toRead, 0x00, toRead)
buffer.destroy(toRead)
buffer.dealloc(toRead)
}
let read = fread(buffer, 1, toRead, fh)
if read == toRead {
return NSMutableData(bytes: buffer, length: toRead)
} else {
throw KCStd$createFromFile$err.Reading
}
}
}
KCStd$ (abbreviation for KizzyCode Standard Library) is the prefix because extensions are module-wide. The above code is hereby placed in Public Domain 😊
I'll leave this open because it's nonetheless an interesting question.
The answer seems to be that you can't. If you look at the declarations in FileHandle, you will see comment:
/* The API below may throw exceptions and will be deprecated in a future version of the OS.
Use their replacements instead. */
Related
How do i get the result of of i to return from the function here? I can return it from inside the if { } but then I get an error there’s no global return (for the function). My goal is to write a function that accepts an Integer and returns the square root. My next step is to throw an error if it's not an Int or between 1 and 10,000, but please assume simple numbers for this question.
let userInputInteger = 9
func squareRootCheckpoint4(userInputInteger: Int) -> Int {
for i in 1...100 {
let userInputInteger = i * i
if i == userInputInteger {
break
}
}
return i
}
update:
To be able to return it, you will have to declared a variable outside the for-loop, then based on the logic assign the "i" counter to that outside variable.
func squareRootCheckpoint4(userInputInteger: Int) -> Int {
var result = 0 // this is to keep the result
for i in 1...100 {
let powerOfTwoResult = i * i
if powerOfTwoResult == userInputInteger {
result = i // assign result variable based on the logic
break
}
}
return result // return result
}
The error shown is because if the for-loop finishes without any hit on the if statement, the function will not be able to return any Int. That's why the compiler produces that "no global return" error.
One way to do it if you don't want to return a default value is to throw an error. You could throw an error from a function (https://docs.swift.org/swift-book/LanguageGuide/ErrorHandling.html). For example for your goal of returning square root of an integer and throw error if the input integer is not between 1 and 10000 (if I understand correctly), you could do something like this
enum InputError: Error {
case invalidInteger
}
func squareRootCheckpoint4(userInputInteger: Int) throws -> Int {
if userInputInteger < 1 || userInputInteger > 10000 {
throw InputError.invalidInteger
}
// calculate square root
return resultOfSquareroot
}
// and to handle the error, you could encapsulate the squareRootCheckpoint4 function in a do-catch statement
do {
let squareRootResult = try squareRootCheckpoint4(userInputInteger: 4)
} catch InputError.invalidInteger {
// handle your error here
}
Alternatively, you could do a validation on the input integer separately before calling the squareRootCheckpoint4 function. For example
func validate(_ input: Int) -> Bool {
if userInputInteger < 1 || userInputInteger > 10000 {
return false
}
return true
}
func squareRootCheckpoint4(userInputInteger: Int) -> Int {
// calculate square root
return resultOfSquareroot
}
var input: Int = 9
if validate(input) {
let squareRootResult = squareRootCheckpoint4(input)
} else {
// handle when input is invalid
}
A couple of things are swapped around, and something (anything) needs to be returned from the function. The framing of the question only allows solutions that don't work if the input is not an integer with a square root.
List of Specifics:
The function argument name userInputInteger should not have been used to instantiate a new object. That's confusing. I did it because I'm learning and thought it was necessary.
Instead, i times i (or i squared) should have been assigned to a new object.
The if statement should be equal to this new object, instead of i.
Functions should return something. When i is returned in the if statement, it's not available anywhere outside the for loop. The print statements in the code example help explain what is available in these different scopes, as well as what's not.
The for loop stops when i is returned. That happens when the guess (i) is equal to the input (userInputInteger).
The for loop will continue through 100 if the input doesn't have a square root.
The function should return the correct number that the if statement was trying to guess, but you have to tell it to (with a return statement) and do it in the right spot. This is the "global return" error. However, because this approach in the question is setup poorly without error handling, only an Integer with a sqare root will return correctly. And in this case, the function's global return doesn't matter; Swift just requires something be returned, regardless of whether or not it's used.
Code Example - Direct Answer:
import Cocoa
let userInputInteger = 25
let doesNotMatter = 0
print("sqrt(\(userInputInteger)) is \(sqrt(25))") //correct answer for reference
func squareRootCheckpoint4(userInputInteger2: Int) -> Int {
var j: Int
for i in 1...100 {
print("Starting Loop \(i)")
j = i * i
if j == userInputInteger2 {
print("i if-loop \(i)")
print("j if-loop \(j)")
return i
}
print("i for-loop \(i)")
print("j for-loop \(j)")
}
//print("i func-end \(i)") //not in scope
//print("j func-end \(j)") //not in scope
//return i //not in scope
//return j //not in scope
print("Nothing prints here")
// but something must be returned here
return doesNotMatter
}
print("Input was \(userInputInteger)")
print("The Square Root of \(userInputInteger) is \(squareRootCheckpoint4(userInputInteger2: userInputInteger))")
Code Example - Improved with Error Handling
import Cocoa
enum ErrorMsg : Error {
case outOfBoundsError, noRootError
}
func checkSquareRoot (Input userInputInteger2: Int) throws -> Int {
if userInputInteger < 1 || userInputInteger > 10_000 {
throw ErrorMsg.outOfBoundsError
}
var j : Int
for i in 1...100 {
print("Starting Loop \(i)")
j = i * i
if j == userInputInteger2 {
print("i if-loop \(i)")
print("j if-loop \(j)")
return i
}
print("i for-loop \(i)")
print("j for-loop \(j)")
}
throw ErrorMsg.noRootError
}
let userInputInteger = 25
print("Input was \(userInputInteger)")
do {
let result = try checkSquareRoot(Input: userInputInteger)
print("The Square Root of \(userInputInteger) is \(result)")
} catch ErrorMsg.outOfBoundsError {
print("out of bounds: the userInputInteger is less than 1 or greater than 10,000")
} catch ErrorMsg.noRootError {
print("no root")
}
print("Swift built-in function for reference: sqrt(\(userInputInteger)) is \(sqrt(25))") //correct answer for reference
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?
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])
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()
}
}
}
I've got a char-by-char function that returns a character from a file. It is pretty performant, as far as i now.
Xcode says that the characterPointer.initialize(from: s.characters) "will be removed in Swift 4.0." And I have to "use 'UnsafeMutableBufferPointer.initialize(from:)' instead".
But I can't get it. Can you please explain it to me. How to use a quick iterator over characters with a UnsafeMutableBufferPointer? Do you have an example?
This is my function:
/// Return next character, or nil on EOF.
func nextChar() -> Character? {
//precondition(fileHandle != nil, "Attempt to read from closed file")
if atEof {
return nil
}
if stored_cnt > (stored_idx + 1) {
stored_idx += 1
let char = characterPointer.pointee
characterPointer = characterPointer.successor()
return char
}
let tmpData = fileHandle.readData(ofLength: (chunkSize))
if tmpData.count == 0 {
atEof = true
return nil
}
if var s = NSString(data: tmpData, encoding: encoding.rawValue) as String! {
characterPointer.initialize(from: s.characters)
stored_idx = 0
stored_cnt = s.characters.count
}
let char = characterPointer.pointee
characterPointer = characterPointer.successor()
return char
}
By the way, if there is a faster solution, please do not hesitate to tell me.
Thanks a lot.
i have to read a file char by char in swift. The way I am doing it is to read a chunk from a FileHandler and returning the first character of a string.
This is my code so far:
/// Return next character, or nil on EOF.
func nextChar() -> Character? {
precondition(fileHandle != nil, "Attempt to read from closed file")
if atEof {
return nil
}
if self.stored.characters.count > 0 {
let c: Character = self.stored.characters.first!
stored.remove(at: self.stored.startIndex)
return c
}
let tmpData = fileHandle.readData(ofLength: (4096))
print("\n---- file read ---\n" , terminator: "")
if tmpData.count == 0 {
return nil
}
self.stored = NSString(data: tmpData, encoding: encoding.rawValue) as String!
let c: Character = self.stored.characters.first!
self.stored.remove(at: stored.startIndex)
return c
}
My problem with this is that the returning of a character is very slow.
This is my test implementation:
if let aStreamReader = StreamReader(path: file) {
defer {
aStreamReader.close()
}
while let char = aStreamReader.nextChar() {
print("\(char)", terminator: "")
continue
}
}
even without a print it took ages to read the file to the end.
for a sample file with 1.4mb it took more than six minutes to finish the task.
time ./.build/debug/read a.txt
real 6m22.218s
user 6m13.181s
sys 0m2.998s
Do you have an opinion how to speed up this part?
let c: Character = self.stored.characters.first!
stored.remove(at: self.stored.startIndex)
return c
Thanks a lot.
ps
++++ UPDATEED FUNCTION ++++
func nextChar() -> Character? {
//precondition(fileHandle != nil, "Attempt to read from closed file")
if atEof {
return nil
}
if stored_cnt > (stored_idx + 1) {
stored_idx += 1
return stored[stored_idx]
}
let tmpData = fileHandle.readData(ofLength: (chunkSize))
if tmpData.count == 0 {
atEof = true
return nil
}
if let s = NSString(data: tmpData, encoding: encoding.rawValue) as String! {
stored = s.characters.map { $0 }
stored_idx = 0
stored_cnt = stored.count
}
return stored[0];
}
Your implementation of nextChar is terribly inefficient.
You create a String and then call characters over and over and you update that set of characters over and over.
Why not create the String and then only store a reference to its characters. And then track an index into characters. Instead of updating it over and over, simply increment the index and return the next character. No need to update the string over and over.
Once you get to the last character, read the next piece of the file. Create a new string, reset the characters and the index.
I've been working with the code form this answer, provided by Martin R. The code is awesome and it is very useful. However, it doesn't work with the links, while working fine with files. After putting some NSLogs and breaks, I have actually found that problem is in this code block:
init?(path: String, delimiter: String = "\n", encoding: UInt = NSUTF8StringEncoding, chunkSize : Int = 4096) {
self.chunkSize = chunkSize
self.encoding = encoding
self.fileHandle = NSFileHandle(forReadingFromURL: NSURL(string: path)!, error: nil)
println("PATH IS \(path)")
println("FILE HANDLE IS \(fileHandle)")
if self.fileHandle == nil {
println("FILE HANDLE IS NIL!")
return nil
}
The code above actually contains some minor changes, compared with Martin's answer. Also Apple says that it is possible to use fileHandle with forReadingFromURL and it shouldn't return nil.
But here is the console output:
PATH IS http://smth.com
FILE HANDLE IS nil
FILE HANDLE IS NIL!!!!!
The question is what's wrong?
UPDATE
As Martin R has kindly explained to me, this code won't work with URLs, this answer states the same, so I have rewritten the code, guiding by previous answers:
import Foundation
import Cocoa
class StreamReader {
let encoding : UInt
let chunkSize : Int
var atEof : Bool = false
var streamData : NSData!
var fileLength : Int
var urlRequest : NSMutableURLRequest
var currentOffset : Int
var streamResponse : NSString
var fileHandle : NSFileHandle!
let buffer : NSMutableData!
let delimData : NSData!
var reponseError: NSError?
var response: NSURLResponse?
init?(path: NSURL, delimiter: String = "\n", encoding: UInt = NSUTF8StringEncoding, chunkSize : Int = 10001000) {
println("YOUR PATH IS \(path)")
self.chunkSize = chunkSize
self.encoding = encoding
self.currentOffset = 0
urlRequest = NSMutableURLRequest(URL: path)
streamData = NSURLConnection.sendSynchronousRequest(urlRequest, returningResponse:&response, error:&reponseError)
streamResponse = NSString(data:streamData!, encoding:NSUTF8StringEncoding)!
self.fileLength = streamData.length
//println("WHAT IS STREAMDATA \(streamData)")
//println("WHAT IS URLREQUEST \(urlRequest)")
if streamData == nil {
println("LINK HAS NO CONTENT!!!!!")
}
self.fileLength = streamResponse.length
println("FILE LENGTH IS \(fileLength)")
self.buffer = NSMutableData(capacity: chunkSize)!
// Create NSData object containing the line delimiter:
delimData = delimiter.dataUsingEncoding(NSUTF8StringEncoding)!
println("WHAT DOES THE DELIMITER \(delimiter)LOOK LIKE?")
println("WHAT IS DELIMDATA \(delimData)")
}
deinit {
self.close()
}
/// Return next line, or nil on EOF.
func nextLine() -> String? {
if atEof {
println("AT THE END OF YOUR FILE!!!")
return nil
}
// Read data chunks from file until a line delimiter is found:
if currentOffset >= fileLength {
return nil
}
var blockLength : Int = buffer.length
var range = buffer.rangeOfData(delimData, options: NSDataSearchOptions(0), range: NSMakeRange(currentOffset, blockLength))
//println("STREAM DATA \(streamData)")
println("RANGE IS \(range)")
while range.location == NSNotFound {
var nRange = NSMakeRange(currentOffset, chunkSize)
println("nRange is \(nRange)")
var tmpData = streamData.subdataWithRange(nRange)
//println("TMP data length \(tmpData.length)")
currentOffset += blockLength
//println("TMPDATA is \(tmpData)")
if tmpData.length == 0 {
// EOF or read error.
println("ERROR ????")
atEof = true
if buffer.length > 0 {
// Buffer contains last line in file (not terminated by delimiter).
let line = NSString(data: buffer, encoding: encoding);
buffer.length = 0
println("THE LINE IS \(line)")
return line
}
// No more lines.
return nil
}
buffer.appendData(tmpData)
range = buffer.rangeOfData(delimData, options: NSDataSearchOptions(0), range: NSMakeRange(0, buffer.length))
}
// Convert complete line (excluding the delimiter) to a string:
let line = NSString(data: buffer.subdataWithRange(NSMakeRange(0, range.location)),
encoding: encoding)
// Remove line (and the delimiter) from the buffer:
buffer.replaceBytesInRange(NSMakeRange(0, range.location + range.length), withBytes: nil, length: 0)
return line
}
/// Start reading from the beginning of file.
func rewind() -> Void {
//streamData.seekToFileOffset(0)
buffer.length = 0
atEof = false
}
/// Close the underlying file. No reading must be done after calling this method.
func close() -> Void {
if streamData != nil {
streamData = nil
}
}
}
extension StreamReader : SequenceType {
func generate() -> GeneratorOf<String> {
return GeneratorOf<String> {
return self.nextLine()
}
}
}
But actually this code is very far from being perfect and I would like to see any recommendations on improving it. Please, be kind. I am very amateur and very inexperienced( but sooner or later I will learn it)
Finally, it is working. And probably the last problem is left, the code doesn't stop, it continues to read file from the beginning.
So now the question is probably more close to 'What's wrong with my code?', compared with previous: 'What's wrong?'
UPDATE
I have rewritten last parts of code like this:
let line = NSString(data: buffer.subdataWithRange(NSMakeRange(0, range.location + 1)),
encoding: encoding)
buffer.replaceBytesInRange(NSMakeRange(0, range.location + range.length), withBytes: nil, length: 0)
println("COMPLETE LINE IS \(line)")
if line!.containsString("\n"){
println("CONTAINS NEW LINE")
//println("BUFFER IS \(buffer)")
//
println("COMPLETE LINE IS \(line)")
return line
}
else {
println("NO LINE!")
atEof == true
return nil
}
The idea is to go through all the lines which contain \n and to exclude one line, which is the last one and shouldn't have the \n. But! Despite the fact that I have checked non-printing characters and there was no \n, here is the surprising console output: Optional("lastline_blablabla\n")
Probably now the question is, how to stop at the last line even if it contains \n?
If you need to retrieve data from the URL my code above (the one under first update) will work
But my own problem with \n has several ways to be solved. One of which I used in my code. As it's very individual I won't post the solution to \n at the end of file issue. Also I am not sure that my both solutions for urlStreamReader and \n are the best, so if you advice any better solution it will be much appreciated by me and probably some other people.
Great thanks to Martin R, who explained a lot, wrote great code and was very nice