Read a text file line by line in Swift? - ios

Question

I just started learning Swift. I have got my code to read from the text file, and the App displays the content of the entire text file. How can I display line by line and call upon that line multiple times?
TextFile.txt contains the following:
1. Banana
2. Apple
3. pear
4. strawberry
5. blueberry
6. blackcurrant
The following is what currently have..
if let path = NSBundle.mainBundle().pathForResource("TextFile", ofType: "txt"){
var data = String(contentsOfFile:path, encoding: NSUTF8StringEncoding, error: nil)
if let content = (data){
TextView.text = content
}
If there is another way of doing this please let me know. It would be much appreciated.

Answer 1

Swift 3.0
if let path = Bundle.main.path(forResource: "TextFile", ofType: "txt") {
do {
let data = try String(contentsOfFile: path, encoding: .utf8)
let myStrings = data.components(separatedBy: .newlines)
TextView.text = myStrings.joined(separator: ", ")
} catch {
print(error)
}
}
The variable myStrings should be each line of the data.
The code used is from:
Reading file line by line in iOS SDK written in Obj-C and using NSString
Check edit history for previous versions of Swift.

Answer 2

Swift 5.5
The solution below shows how to read one line at a time. This is quite different from reading the entire contents into memory. Reading line-by-line scales well if you have a large file to read. Putting an entire file into memory does not scale well for large files.
The example below uses a while loop that quits when there are no more lines, but you can choose a different number of lines to read if you wish.
The code works as follows:
create a URL that tells where the file is located
make sure the file exists
open the file for reading
set up some initial variables for reading
read each line using getLine()
close the file and free the buffer when done
You could make the code less verbose if you wish; I have included comments to explain what the variables' purposes are.
Swift 5.5
import Cocoa
// get URL to the the documents directory in the sandbox
let home = FileManager.default.homeDirectoryForCurrentUser
// add a filename
let fileUrl = home
.appendingPathComponent("Documents")
.appendingPathComponent("my_file")
.appendingPathExtension("txt")
// make sure the file exists
guard FileManager.default.fileExists(atPath: fileUrl.path) else {
preconditionFailure("file expected at \(fileUrl.absoluteString) is missing")
}
// open the file for reading
// note: user should be prompted the first time to allow reading from this location
guard let filePointer:UnsafeMutablePointer<FILE> = fopen(fileUrl.path,"r") else {
preconditionFailure("Could not open file at \(fileUrl.absoluteString)")
}
// a pointer to a null-terminated, UTF-8 encoded sequence of bytes
var lineByteArrayPointer: UnsafeMutablePointer<CChar>? = nil
// see the official Swift documentation for more information on the `defer` statement
// https://docs.swift.org/swift-book/ReferenceManual/Statements.html#grammar_defer-statement
defer {
// remember to close the file when done
fclose(filePointer)
// The buffer should be freed by even if getline() failed.
lineByteArrayPointer?.deallocate()
}
// the smallest multiple of 16 that will fit the byte array for this line
var lineCap: Int = 0
// initial iteration
var bytesRead = getline(&lineByteArrayPointer, &lineCap, filePointer)
while (bytesRead > 0) {
// note: this translates the sequence of bytes to a string using UTF-8 interpretation
let lineAsString = String.init(cString:lineByteArrayPointer!)
// do whatever you need to do with this single line of text
// for debugging, can print it
print(lineAsString)
// updates number of bytes read, for the next iteration
bytesRead = getline(&lineByteArrayPointer, &lineCap, filePointer)
}

Answer 3

If you have a huge file and don't want to load all data to memory with String, Data etc. you can use function readLine() which reads content from standard input line by line until EOF is reached.
let path = "path/file.txt"
guard let file = freopen(path, "r", stdin) else {
return
}
defer {
fclose(file)
}
while let line = readLine() {
print(line)
}

Answer 4

This is not pretty, but I believe it works (on Swift 5). This uses the underlying POSIX getline command for iteration and file reading.
typealias LineState = (
// pointer to a C string representing a line
linePtr:UnsafeMutablePointer<CChar>?,
linecap:Int,
filePtr:UnsafeMutablePointer<FILE>?
)
/// Returns a sequence which iterates through all lines of the the file at the URL.
///
/// - Parameter url: file URL of a file to read
/// - Returns: a Sequence which lazily iterates through lines of the file
///
/// - warning: the caller of this function **must** iterate through all lines of the file, since aborting iteration midway will leak memory and a file pointer
/// - precondition: the file must be UTF8-encoded (which includes, ASCII-encoded)
func lines(ofFile url:URL) -> UnfoldSequence<String,LineState>
{
let initialState:LineState = (linePtr:nil, linecap:0, filePtr:fopen(fileURL.path,"r"))
return sequence(state: initialState, next: { (state) -> String? in
if getline(&state.linePtr, &state.linecap, state.filePtr) > 0,
let theLine = state.linePtr {
return String.init(cString:theLine)
}
else {
if let actualLine = state.linePtr { free(actualLine) }
fclose(state.filePtr)
return nil
}
})
}
Here is how you might use it:
for line in lines(ofFile:myFileURL) {
print(line)
}

Answer 5

Probably the simplest, and easiest way to do this in Swift 5.0, would be the following:
import Foundation
// Determine the file name
let filename = "main.swift"
// Read the contents of the specified file
let contents = try! String(contentsOfFile: filename)
// Split the file into separate lines
let lines = contents.split(separator:"\n")
// Iterate over each line and print the line
for line in lines {
print("\(line)")
}
Note: This reads the entire file into memory, and then just iterates over the file in memory to produce lines....
Credit goes to: https://wiki.codermerlin.com/mediawiki/index.php/Code_Snippet:_Print_a_File_Line-by-Line

Answer 6

Update for Swift 2.0 / Xcode 7.2
do {
if let path = NSBundle.mainBundle().pathForResource("TextFile", ofType: "txt"){
let data = try String(contentsOfFile:path, encoding: NSUTF8StringEncoding)
let myStrings = data.componentsSeparatedByCharactersInSet(NSCharacterSet.newlineCharacterSet())
print(myStrings)
}
} catch let err as NSError {
//do sth with Error
print(err)
}
Also worth to mention is that this code reads a file which is in the project folder (since pathForResource is used), and not in e.g. the documents folder of the device

Answer 7

You probably do want to read the entire file in at once. I bet it's very small.
But then you want to split the resulting string into an array, and then distribute the array's contents among various UI elements, such as table cells.
A simple example:
var x: String = "abc\ndef"
var y = x.componentsSeparatedByString("\n")
// y is now a [String]: ["abc", "def"]

Answer 8

One more getline solution:
Easy to use. Just copy past.
Tested on real project.
extension URL
{
func foreachRow(_ mode:String, _ rowParcer:((String, Int)->Bool) )
{
//Here we should use path not the absoluteString (wich contains file://)
let path = self.path
guard let cfilePath = (path as NSString).utf8String,
let m = (mode as NSString).utf8String
else {return}
//Open file with specific mode (just use "r")
guard let file = fopen(cfilePath, m)
else {
print("fopen can't open file: \"\(path)\", mode: \"\(mode)\"")
return
}
//Row capacity for getline()
var cap = 0
var row_index = 0
//Row container for getline()
var cline:UnsafeMutablePointer<CChar>? = nil
//Free memory and close file at the end
defer{free(cline); fclose(file)}
while getline(&cline, &cap, file) > 0
{
if let crow = cline,
// the output line may contain '\n' that's why we filtered it
let s = String(utf8String: crow)?.filter({($0.asciiValue ?? 0) >= 32})
{
if rowParcer(s, row_index)
{
break
}
}
row_index += 1
}
}
}
Usage:
let token = "mtllib "
var mtlRow = ""
largeObjFileURL.foreachRow("r"){ (row, i) -> Bool in
if row.hasPrefix(token)
{
mtlRow = row
return true // end of file reading
}
return false // continue file reading
}

Answer 9

Here is an example of writeing and reading a text file one line at a time
in Swift version 5. Reads one line in at a time and includes EOF detection
//
// main.swift
// IO
//
// Created by Michael LeVine on 8/30/22.
//
import Foundation
let file = "file.txt" //this is the file. we will write to and read from it
let text = "some text\n" //just a text
// test file will be placed on deasktop
let dir = FileManager.default.urls(for: .desktopDirectory, in: .userDomainMask).first
let fileURL = dir!.appendingPathComponent(file).path
let fileURL2 = dir!.appendingPathComponent(file)
let fileManager = FileManager.default
// the following variable used by eof detection which also use var fileManager internally
var eofOffset: UInt64 = 0
if fileManager.fileExists(atPath: fileURL) {
do { try fileManager.removeItem(atPath: fileURL)}
catch {
print("Error removeing old \(fileURL)")
exit(1)
}
}
// create the new file
fileManager.createFile(atPath: fileURL, contents:Data(" ".utf8), attributes: nil)
var fileHandle = FileHandle(forWritingAtPath: fileURL)
//writing
for _ in 1...10 {
fileHandle!.write(text.data(using: .utf8)!)
}
do {
try fileHandle!.close()
}
catch { print("write close error \(error)")
exit(1)
}
// now to read text file by 2 methods
// first use String to read whole file in one gulp
let contents = try! String(contentsOfFile: fileURL)
let lines = contents.split(separator: "\n")
var i: Int = 0
// print out one way
for line in lines {
print("\(i) \(line)")
i=i+1
}
// printout another way
for j in 0...9 {
print("\(i) \(j) \(lines[j])")
i = i + 1
}
//Open up to see about reading line at a time
fileHandle = FileHandle(forReadingAtPath: fileURL)
eofInit() // must be called immediately after fileHandle init
var outputLine: String = ""
i = 0
// read a line and print it out as recieved
while true {
outputLine = getLine()
if eofTest(){
if outputLine.count > 0 {
print("\(i) \(outputLine)")
}
exit(1)
}
print("\(i) \(outputLine)")
i = i + 1
}
// function reads one character at each call and returns it as a 1 character string
// is called only by "getLine"
func getChar() -> String {
var ch: Data
if eofTest() {
return ""
}
do {
try ch = fileHandle!.read(upToCount: 1)! // read 1 character from text file
} catch { print("read 1 char \(error)")
exit(1)
}
let ch2: UnicodeScalar = UnicodeScalar(ch[0]) // convert to unicode scaler as intermediate value
let ch3: String = String(ch2) // Now create string containing that one returned character
return ch3 // and pass to calling function
}
// read in whole line one character at a time -- assumes line terminated by linefeed
func getLine() -> String {
var outputLine : String = ""
var char : String = ""
// keep fetching characters till line feed/eof found
lineLoop:
while true { // its an infinite loop
if eofTest() {
break lineLoop
}
char = getChar() // get next character
if char == "\n" { // test for linefeed
break lineLoop // if found exit loop
}
outputLine.append(char) // lf not found -- append char to output line
}
return outputLine // got line -- return it to calling routine
}
//eof handleing
//init routine must be called immediately after fileHandle inited to get current position
// at start of file
func eofInit()
{ var beginningOffset: UInt64 = 0
do {
try beginningOffset = fileHandle!.offset()
try eofOffset = fileHandle!.seekToEnd()
try fileHandle!.seek(toOffset: beginningOffset)
} catch {
print("Init eof detection error \(error)")
}
}
func eofTest() -> Bool{
var current: UInt64 = 0
do {
current = try fileHandle!.offset()
} catch {
print("eof test get current \(error)")
exit(1)
}
if current < eofOffset {
return false
} else {
return true
}
}

Answer 10

Based on Jason Cross answer simplified version line by line reader(gist).
import Darwin
class FileLineReader {
init?(path: String, removeNewLineOnEnd: Bool = true) {
file = fopen(path, "r")
self.removeNewLineOnEnd = removeNewLineOnEnd
if file == nil {
return nil
}
}
deinit {
fclose(file)
}
var iterator: AnyIterator<String> {
return AnyIterator(self.getNextLine)
}
func getNextLine() -> String? {
var line: UnsafeMutablePointer<CChar>!
var linecap: Int = 0
defer { free(line) }
if getline(&line, &linecap, file) > 0 {
if removeNewLineOnEnd {
var i = 0
while line[i] != 0 { i += 1 }
if i > 0 && line[i-1] == 10 { // new line symbol
line[i-1] = 0
}
}
return String(cString: line)
} else {
return nil
}
}
private let file: UnsafeMutablePointer<FILE>!
private let removeNewLineOnEnd: Bool
}
iUrii approach may not work if you need to open several files.