Objective-C: Reading a file line by line

Question

This will work for general reading a String from Text. If you would like to read longer text (large size of text), then use the method that other people here were mentioned such as buffered (reserve the size of the text in memory space).

Say you read a Text File.

NSString* filePath = @""//file path...
NSString* fileRoot = [[NSBundle mainBundle] 
               pathForResource:filePath ofType:@"txt"];

You want to get rid of new line.

// read everything from text
NSString* fileContents = 
      [NSString stringWithContentsOfFile:fileRoot 
       encoding:NSUTF8StringEncoding error:nil];

// first, separate by new line
NSArray* allLinedStrings = 
      [fileContents componentsSeparatedByCharactersInSet:
      [NSCharacterSet newlineCharacterSet]];

// then break down even further 
NSString* strsInOneLine = 
      [allLinedStrings objectAtIndex:0];

// choose whatever input identity you have decided. in this case ;
NSArray* singleStrs = 
      [currentPointString componentsSeparatedByCharactersInSet:
      [NSCharacterSet characterSetWithCharactersInString:@";"]];

There you have it.

That's a great question. I think @Diederik has a good answer, although it's unfortunate that Cocoa doesn't have a mechanism for exactly what you want to do.

NSInputStream allows you to read chunks of N bytes (very similar to java.io.BufferedReader), but you have to convert it to an NSString on your own, then scan for newlines (or whatever other delimiter) and save any remaining characters for the next read, or read more characters if a newline hasn't been read yet. (NSFileHandle lets you read an NSData which you can then convert to an NSString, but it's essentially the same process.)

Apple has a Stream Programming Guide that can help fill in the details, and this SO question may help as well if you're going to be dealing with uint8_t* buffers.

If you're going to be reading strings like this frequently (especially in different parts of your program) it would be a good idea to encapsulate this behavior in a class that can handle the details for you, or even subclassing NSInputStream (it's designed to be subclassed) and adding methods that allow you to read exactly what you want.

For the record, I think this would be a nice feature to add, and I'll be filing an enhancement request for something that makes this possible. :-)

---

Edit: Turns out this request already exists. There's a Radar dating from 2006 for this (rdar://4742914 for Apple-internal people).

This should do the trick:

#include <stdio.h>

NSString *readLineAsNSString(FILE *file)
{
    char buffer[4096];

    // tune this capacity to your liking -- larger buffer sizes will be faster, but
    // use more memory
    NSMutableString *result = [NSMutableString stringWithCapacity:256];

    // Read up to 4095 non-newline characters, then read and discard the newline
    int charsRead;
    do
    {
        if(fscanf(file, "%4095[^\n]%n%*c", buffer, &charsRead) == 1)
            [result appendFormat:@"%s", buffer];
        else
            break;
    } while(charsRead == 4095);

    return result;
}

Use as follows:

FILE *file = fopen("myfile", "r");
// check for NULL
while(!feof(file))
{
    NSString *line = readLineAsNSString(file);
    // do stuff with line; line is autoreleased, so you should NOT release it (unless you also retain it beforehand)
}
fclose(file);

This code reads non-newline characters from the file, up to 4095 at a time. If you have a line that is longer than 4095 characters, it keeps reading until it hits a newline or end-of-file.

Note: I have not tested this code. Please test it before using it.

Mac OS X is Unix, Objective-C is C superset, so you can just use old-school fopen and fgets from <stdio.h>. It's guaranteed to work.

[NSString stringWithUTF8String:buf] will convert C string to NSString. There are also methods for creating strings in other encodings and creating without copying.

You can use NSInputStream which has a basic implementation for file streams. You can read bytes into a buffer (read:maxLength: method). You have to scan the buffer for newlines yourself.

The appropriate way to read text files in Cocoa/Objective-C is documented in Apple's String programming guide. The section for reading and writing files should be just what you're after. PS: What's a "line"? Two sections of a string separated by "\n"? Or "\r"? Or "\r\n"? Or maybe you're actually after paragraphs? The previously mentioned guide also includes a section on splitting a string into lines or paragraphs. (This section is called "Paragraphs and Line Breaks", and is linked to in the left-hand-side menu of the page I pointed to above. Unfortunately this site doesn't allow me to post more than one URL as I'm not a trustworthy user yet.)

To paraphrase Knuth: premature optimisation is the root of all evil. Don't simply assume that "reading the whole file into memory" is slow. Have you benchmarked it? Do you know that it actually reads the whole file into memory? Maybe it simply returns a proxy object and keeps reading behind the scenes as you consume the string? (Disclaimer: I have no idea if NSString actually does this. It conceivably could.) The point is: first go with the documented way of doing things. Then, if benchmarks show that this doesn't have the performance you desire, optimise.

A lot of these answers are long chunks of code or they read in the entire file. I like to use the c methods for this very task.

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.