How to convert a very long string to array of strings in a most effective way?

Question

I have text file consisting repeating strings elements like this

@SRR1582908.1 ILLUMINA_0154:8:1101:3556:1998/1        //1 line
CCTCGCTGGTCGATTTGTTTAACCGTTTTCTGTTCAGCGCCAAAATTATTTT  //2 line
+                                                     //3 line
BCCFFFFFHHHHHIIIIIHIHGHHGGIIIIIGHHDBGHHHIIGHIIGIHIII  //4 line

and 29 millions of such elements in the file. What I need is to extract second line from each element into new array of strings

I have a function which is doing following:

do
{
    let textToOpen = try String(contentsOf: chosenFile!, encoding: .utf8)
    var arrayOfTextLines = textToOpen.components(separatedBy: "\n")

    arrayOfReads = stride(from: 1, to: arrayOfTextLines.endIndex, by: 4).map{(arrayOfTextLines[$0])}
}
catch
{}

The code is working correctly but for the file with 29 millions elements is relatively slow.

The main drawback is

var arrayOfTextLines = textToOpen.components(separatedBy: "\n")

Is there a way to speed-up the function ?

try in a background thread or use `NSScanner` otherwise you can leverage streams... — Lorenzo B, Sep 13 '18 at 10:08
Holding a 29*4 million lines in memory can also become a memory problem. — Cristik, Sep 13 '18 at 10:08
You could read the file in chunks and keep a reference to the current line. — Cristik, Sep 13 '18 at 10:09
Try NSString methods. For example, `enumerateLines` is [very fast](https://stackoverflow.com/a/46491196/2227743) and could probably be helpful. At list I would try and compare. — Eric Aya, Sep 13 '18 at 10:09
Possibly helpful: [Read a file/URL line-by-line in Swift](https://stackoverflow.com/q/24581517/1187415). — Martin R, Sep 13 '18 at 10:53
@Moritz Great advice! I have tried the `enumerateLines`and now the function is faster - about 2 min 15 sec instead of about 3 min 45 sec. I have up voted your post. — VYT, Sep 13 '18 at 10:54

How to convert a very long string to array of strings in a most effective way?

0 Answers0