NSScanner & Array Loops

Question

I am trying to use NSScanner is an NSArray enumeration loop. This however fails:

-[NSXMLElement length]: unrecognized selector sent to instance 0x280da30

No, I am not calling length anywhere. It seems like the value of found gets changed before the scanner completes, hence making it blank, causing the length error.

If I remove the loop and hardcode the string (i don't want that, it's just a test) the code below works. (excluding the enumeration of course)

It never gets as far as calling the NSLogs either...

Here's my code:

    for (NSString*found in arr)
    {
        NSLog(@"Found %@",found);

        NSString*search = [NSString stringWithString:found];

        NSString *separatorString = @"\"";                             
        NSString *container = nil;
        NSScanner *aScanner = [NSScanner scannerWithString:search]; 

        [aScanner setScanLocation:0];                                        
        [aScanner scanUpToString:@"src=\"" intoString:nil];          
        [aScanner scanString:@"src=\"" intoString:nil];                     
        [aScanner scanUpToString:separatorString intoString:&container]; 

        NSLog(@"scanned");

        NSLog(@"%@",container);

        NSImage*previewImage = [[NSImage alloc] initWithContentsOfFile:[relativeDir stringByAppendingPathComponent:container]];

        [preview setImage:previewImage];

        [previewImage release];

        progressed = progressed + 1;

        [convertProgress setDoubleValue:[convertProgress doubleValue] + progressed];
    }

How can I get this to work?

Edit:

Here's the input:

    NSXMLDocument*doc = [[NSXMLDocument alloc] initWithData:[NSData dataWithContentsOfFile:webPath] options:NSXMLDocumentTidyHTML error:nil];

    NSArray*arr = [doc nodesForXPath:@"//img[@src]" error:nil];

It might help to know what the input looks like... – jlehr Jan 28 '11 at 18:31 — jlehr, Jan 28 '11 at 18:31
By the way, the scanning code works fine for simple inputs. – jlehr Jan 28 '11 at 18:32 — jlehr, Jan 28 '11 at 18:32
I've already said that it works fine for simple inputs. – Pripyat Jan 28 '11 at 18:32 — Pripyat, Jan 28 '11 at 18:32

bbum · Accepted Answer · 2011-01-28T21:47:15.653

From the documentation:

The nodesForXPath:error: method returns an array of NSXMLNode objects

Your array is not full of strings. It is full of various NSXML* objects. They aren't garbage. This is exactly how it is supposed to work.

You may not be calling length, but stringWithString: is probably calling length to figure out how much space to allocate in the new string.

Conversion to a string can mean a lot of things. An XML node may be a standalone tag, it may be some inline CDATA, it may be a tag with various attributes. As well, an XML document is highly structured. It is rarely meaningful to walk through the contents of a document as a straight enumeration of tags/contents without applying some amount of structural meaning.

You are likely going to need to recurse or otherwise walk the tags and interpret their contents appropriately.

The first step would be to remove everything but that initial NSLog() and see what you got. Then figure out how to walk the tree of nodes. Then figure out how to parse 'em appropriately.

how can I convert it to a string? – Pripyat Jan 28 '11 at 19:12 — Pripyat, Jan 28 '11 at 19:12

paulbailey · Answer 2 · 2011-01-28T19:04:55.713

1

Are you sure that your arr contains NSString objects? The fact that NSXMLElement is throwing an error looks fishy to me.

Edit

Yep, looks like the docs say the array from nodesForXPath doesn't contain NSString objects, but NSXMLElement objects.

http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSXMLNode_Class/Reference/Reference.html#//apple_ref/occ/instm/NSXMLNode/nodesForXPath:error:

edited Jan 28 '11 at 19:04

answered Jan 28 '11 at 18:30

paulbailey

5,328
22
35

They are NSStrings. I have explained why it calls `NSXMLElement` in the log - its outputting garbage, it's happened a few times. As the loop moves on, it seems like the string its supposed to search is cleared... – Pripyat Jan 28 '11 at 18:33
I'm with Paul, in all likelihood some of the items in the array are not instances of NSString. How are you creating the array? (Code would be helpful). – jlehr Jan 28 '11 at 18:38
They are strings. I've edited the post to show how I create the array. – Pripyat Jan 28 '11 at 18:44
1

According to Apple's docs: "The nodesForXPath:error: method returns an array of NSXMLNode objects". – jlehr Jan 28 '11 at 18:45
1

I am certain that [doc nodesForXpath:] is not returning strings but NSXMLElement – Joe Jan 28 '11 at 18:51
Thanks - how can I get the string value though? – Pripyat Jan 28 '11 at 19:09
Have you tried `XMLString`? (And BTW, saying that `stringValue` "doesn't work" is a little misleading.) – jlehr Jan 28 '11 at 19:12

NSScanner & Array Loops

2 Answers2