How do I get the index of an object in an NSArray using string value?

NSArray does not guarantee that you can only store one copy of a given object, so you have to make sure that you handle that yourself (or use NSOrderedSet).

That said, there are a couple approaches here. If your category objects implement isEqual: to match category_title, then you can just use -indexOfObject:.

If you can't do that (because the category objects use a different definition of equality), use -indexOfObjectPassingTest:. It takes a block in which you can do whatever test you want to define your "test" - in this case, testing category_title string equality.

Note that these are all declared for NSArray, so you won't see them if you are only looking at the NSMutableArray header/documentation.

EDIT: Code sample. This assumes objects of class CASCategory with an NSString property categoryTitle (I can't bring myself to put underscores in an ivar name :-):

    CASCategory *cat1 = [[CASCategory alloc] init];
    [cat1 setCategoryTitle:@"foo"];
    CASCategory *cat2 = [[CASCategory alloc] init];
    [cat2 setCategoryTitle:@"bar"];
    CASCategory *cat3 = [[CASCategory alloc] init];
    [cat3 setCategoryTitle:@"baz"];

    NSMutableArray *array = [NSMutableArray arrayWithObjects:cat1, cat2, cat3, nil];
    [cat1 release];
    [cat2 release];
    [cat3 release];

    NSUInteger barIndex = [array indexOfObjectPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
        if ([[(CASCategory *)obj categoryTitle] isEqualToString:@"bar"]) {
            *stop = YES;
            return YES;
        }
        return NO;
    }];

    if (barIndex != NSNotFound) {
        NSLog(@"The title of category at index %lu is %@", barIndex, [[array objectAtIndex:barIndex] categoryTitle]);
    }
    else {
        NSLog(@"Not found");
    }

Not sure that I understand the question but something like this might work (assuming the Mutable Array contains objects of Class "Category"):

int indx;
bool chk;
for (Category *aCategory in theArray) 
{
    chk = ([[aCategory category_title] isEqualToString:@"valOfCategoryTitle"])
    if ( chk )
        indx = [theArray indexOfObject:aCategory];
}