Reading a file line by line using for..in

Question

I'm trying to to read lines one by one from an external txt file (about 180kb) using Objective-C. I have found this piece of code here on SO. The way I understand it this code will put every line in an element right?

- (IBAction)start:(id)sender; {
    NSString *fh = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:NULL];
    for (NSArray *line in [fh componentsSeparatedByString:@"\n"]) {
        NSLog(@"output= %@",line[5]);
    }
}

I try to check if it is working by printing a line to my output but nothing happens, what am I doing wrong here?

Thanks,

Update: I made the suggested changes and as it turnout fh is indeed nil. If I make fh a string containing a piece of the text file (including a few \n) it works fine. I checked the file path but that seems allright, So the question now would be how come fh = nil?

NSString *fh = [NSString stringWithContentsOfFile:@"file:///Users/Mubanga/Documents/Blender/rough2.txt" encoding:NSUTF8StringEncoding error:nil];
if([fh length] == 0) {
    NSLog(@"fh=nil");
}
for (NSString *line in [fh componentsSeparatedByString:@"\n"]) {
    NSLog(@"output= %@",line);
    }

}

@MartinR you were right, fh turned out te be nil. How can I prevent this from happening? — Mubanga, Jan 30 '14 at 15:28
You might also want to not use `fh` as the name of a variable that does not contain an NSFileHandle. Since it contains a string, I would name it `string`, `str`, or `fileContents` or something like that. — Peter Hosey, Jan 30 '14 at 23:26

trojanfoe · Answer 1 · 2014-01-30T15:01:29.073

3

The loop should be:

for (NSString *line in [fh componentsSeparatedByString:@"\n"]) {
    NSLog(@"output= %@",line);
}

(line will be an NSString and the output should be line, not line[5]).

edited Jan 30 '14 at 15:01

answered Jan 30 '14 at 14:56

trojanfoe

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Indeed. `componentsSeparatedByString:` returns an array, and the `for` loop *iterates through the objects in the array*. The objects in the array are strings—each string being one of the lines from the original string. – Peter Hosey Jan 30 '14 at 23:28

Martin R · Accepted Answer · 2014-01-30T15:39:40.067

In addition to the required changes from @trojanfoe's answer, your fh is nil because the path is wrong. stringWithContentsOfFile: expects a simple path, not a file-URL. It should be:

 NSString *fh = [NSString stringWithContentsOfFile:@"/Users/Mubanga/Documents/Blender/rough2.txt" encoding:NSUTF8StringEncoding error:nil];

Generally, you should use the error parameter:

NSError *error;
NSString *fh = [NSString stringWithContentsOfFile:@"/Users/Mubanga/Documents/Blender/rough2.txt"
                                         encoding:NSUTF8StringEncoding
                                            error:&error];
if (fh == nil) {
    NSLog(@"Could not read file: %@", error);
} else {
    // ...
}

The other reason fh could be nil is that the file is not valid UTF-8 data. Check the encoding of your file. — Greg Parker, Jan 30 '14 at 19:57

score 1 · Answer 3 · answered Jan 30 '14 at 15:27

1

I would do the character input by hand using native C calls. Just read() into a char[] buffer until you hit a newline, then process the buffer. No I won't write the code for you, check out the still-relevant K&R.

answered Jan 30 '14 at 15:27

QED

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I have to agree. When it comes to parsing files I still prefer standard C. There is nothing wrong with using C as Objective-C is just a superset of C. – Freddy Jan 30 '14 at 16:11
1

And yet K&R lived in the ASCII age where all users spoke English, or else. This is no longer the case and `NSString` is better equipped to deal with non-latin characters than plain C is. – trojanfoe Jan 31 '14 at 01:24

Reading a file line by line using for..in

3 Answers3