How do I change C file operation to Objective C?

Question

char buf[32];

FILE *fp = fopen("test.txt", "r");

fscanf(fp, "%s", buf);

fclose(fp);

I want to change above C file operations to Objective-C.

How do I change?

Is not there fscanf kind of thing in Objective-C?

For what it's worth, you can keep this code if you want to. Objective-C is a superset of C, and all the POSIX file stuff is available. See @Micah's answer for the equivalent, though. — Ben Zotto, Jun 03 '11 at 20:43
I know Objective-C is a superset of C. But I have to change buf to NSString to do something. Do I have to use [NSString stringWithCString...] after each fscanf? — user698200, Jun 03 '11 at 20:54

score 2 · Accepted Answer · answered Jun 03 '11 at 20:37

2

You don't. You can't scan from FILE * directly into an NSString. However, once you've read into the buffer, you can create an NSString from it:

NSString *str = [NSString stringWithCString: buf encoding: NSASCIIStringEncoding]; // or whatever

NOTE, however, that using fscanf as you have here will cause a buffer overflow. You must specify the size of the buffer, minus one for the trailing null character:

fscanf(fp, "%31s", buf);

answered Jun 03 '11 at 20:37

Jonathan Grynspan

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Isn't there something like fscanf in Objective C? – user698200 Jun 03 '11 at 20:48
Do I have to use [NSString stringWithCString...] after each fscanf? Actually there are many fscanfs between fopen and fclose. – user698200 Jun 03 '11 at 21:03
`NSScanner` lets you scan from a string into another string; `NSInputStream` lets you read a file. But there's no exact equivalent to `fscanf()` that operates on the Objective-C level, no. – Jonathan Grynspan Jun 03 '11 at 21:03
To convert from a C string to an `NSString`, yes, you must call the factory method for each string you're converting. – Jonathan Grynspan Jun 03 '11 at 21:04

score 2 · Answer 2 · edited May 23 '17 at 10:00

2

Your best bet is to use NSData to read in your file, and then create a string from that.

NSData *dataContents = [NSData dataWithContentsOfFile: @"test.txt"];
NSString *stringContents = [[NSString alloc] initWithData: dataContents 
                                       encoding: NSUTF8StringEncoding];

Assuming your file is UTF8.

No sense messing around with buffers. Let the framework do the dirty work.

EDIT: Responding to your comment, if you really, truly need to read it line by line, check out Objective-C: Reading a file line by line

edited May 23 '17 at 10:00

Community

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answered Jun 03 '11 at 20:37

Micah Hainline

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I want to do something while I read file line by line. I don't want do that after I read to the end of file. How can I do that? – user698200 Jun 03 '11 at 20:46

score 1 · Answer 3 · answered Jun 03 '11 at 20:56

1

You just want to read a text file into a string?

NSError *error = nil;
NSString *fileContents = [NSString stringWithContentsOfFile:@"test.txt" withEncoding:NSUTF8StringEncoding error:&error];

if(!fileContents) {
    NSLog(@"Couldn't read file because of error: %@", [error localizedFailureReason]);
}

This assumes UTF8 encoding, of course.

answered Jun 03 '11 at 20:56

Abizern

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I simplified my code too much. There are a lot of stuffs between fopen and fclose. I have to change buf to NSString right after I read line. I don't want to do that after I read whole file. – user698200 Jun 04 '11 at 14:28

How do I change C file operation to Objective C?

3 Answers3