Question

I’ve been reading the book Learn Objective C on the Mac, and I was looking at this bit of sample code:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSFileManager *manager;
    manager = [NSFileManager defaultManager];

    NSString *home;
    home = [@"~" stringByExpandingTildeInPath];

    NSDirectoryEnumerator *direnum;
    direnum = [manager enumeratorAtPath: home];

    NSMutableArray *files;
    files = [NSMutableArray arrayWithCapacity: 42];

    // “Slow” enumeration
    NSString *filename;
    while ( filename = [direnum nextObject] )
    {
            if ([[filename pathExtension] isEqualTo: @”jpg”])
            {
                    [files addObject: filename];
            } // end if
    } // end while


    NSEnumerator *filenum;
    filenum = [files objectEnumerator];

    while ( filename = [filenum nextObject] )
    {
            NSLog(@”%@”, filename);
    }

[pool drain];
return 0;
} // main

what stood out for me was while ( filename = [filenum nextObject] ) and I thought, ‘how this is actually a conditional statement?’ I suppose a call to [filenum nextObject] is an NSString with a filename, or nil, but how do we get to execute filename = while we are evaluating that? can you point me to the correct documentation that would explain more clearly just what is possible here?

Answer 1

This is a conditional statement since every assignation has a value, which is the value on the right hand side assigned to the var on the left.
In this case, the final value turns out to be, either a valid NSString object, or nil.
As long as nil represents “nothing”, the condition is interpreted as false, otherwise it’s true.