How do I read a specific line from a large text file with Objective-C?

Question

Say I have text file my.txt like this

this is line 1
this is line 2
....
this is line 999999
this is line 1000000

In Unix I can get the line of "this is line 1000" by issuing command like "head -1000 my.txt | tail -1". What is the corresponding way to get this in Objective-C?

Answer 1

If it's not too inefficient to have the whole thing in memory at once then the most compact sequence of calls (which I've expanded onto multiple lines for simpler exposition) would be:

NSError *error = nil;
NSString *sourceString = [NSString stringWithContentsOfFile:@"..."
                                    encoding:NSUTF8StringEncoding error:&error];
NSArray *lines = [sourceString componentsSeparatedByCharactersInSet:
                                     [NSCharacterSet newlineCharacterSet]];

NSString *relevantLine = [lines objectAtIndex:1000];

You should check the value of error and the count of lines for validation.

EDIT: to compare to Nathan's answer, the benefit of splitting by characters in set is that you'll accept any of the five unicode characters that can possibly delimit a line break, with anywhere where several of them sit next to each other counting as only one break (as per e.g. \r\n).

NSInputStream is probably what you're going to have to deal with if memory footprint is an issue, which is barely more evolved than C's stdio.h fopen/fread/etc so you're going to have to write your own little loop to dash through.

Answer 2

The answer does not explain how to read a file too LARGE to keep in memory. There is not nice solution in Objective-C for reading large text files without putting them into memory (which isn't always an option).

In these case I like to use the c methods:

FILE* file = fopen("path to my file", "r");

size_t length;
char *cLine = fgetln(file,&length);

while (length>0) {
    char str[length+1];
    strncpy(str, cLine, length);
    str[length] = '\0';

    NSString *line = [NSString stringWithFormat:@"%s",str];        
    % Do what you want here.

    cLine = fgetln(file,&length);
}

Note that fgetln will not keep your newline character. Also, We +1 the length of the str because we want to make space for the NULL termination.

Answer 3

The simplest is to just load the file using one of the NSString file methods and then use the -[NSString componentsSeparatedByString:] method to get an array of every line.

Or you could use NSScanner, scan for newline/carriage return characters counting them until you get to you line of interest.

If you are really concerned about memory usage you could look at NSInputStream use that to read in the file, keeping count of the number of newlines. It a shame that NSScanner doesn't work with NSInputStream.

Answer 4

I don't think this is an exact duplicate, because it sounds like you want to skip some lines in the file, but you could easily use an approach like the one here:

Objective-C: Reading a file line by line (Specific answer that has some sample code)

Loop on the input file, reading in a chunk of data, and look for newlines. Count them up and when you hit the right number, output the data after that one and until the next.

Your example looks like you might have hundreds of thousands of lines, so definitely don't just read in the file into a NSString, and definitely don't convert it to an NSArray.

If you want to do it the fancier NSInputStream way (which has some key advantages in character set decoding), here is a great example that shows the basic idea of polling to consume all of the data from a stream source (in a file example, its somewhat overkill). Its for output, but the idea is fine for input too: Polling versus Run Loop Scheduling