What is the perfect way to avoid "beyond bounds" when using NSArray - ios

Question

I knows some ways to avoid this, for example
if (index >= [_data count] || index < 0) return nil;
return [_data objectAtIndex:index];
But should i aways do this? or are there any other solutions about this topic?

Answer 1

First, I'd like to echo #rmaddy's comment, which is spot on:
There is no general solution. Every case is different.
That said, there are other techniques you can use:
firstObject and lastObject
These methods will return the object, or nil if there is none. These methods will never throw an exception.
Fast Enumeration
You can use fast enumeration and never need to check the indices:
NSArray *myStrings = #[#"one", #"two"];
for (NSString *thisString in myStrings) {
NSLog(#"A string: %#", thisString);
}
Safe Category
You can add a category on NSArray if you find yourself doing this frequently:
- (id)safeObjectAtIndex:(NSUInteger)index {
if (index >= [self count] || index < 0) return nil;
return [self objectAtIndex:index];
}
See Customizing Existing Classes if you aren't familiar with categories.
One downside of this is it may be harder to find errors in your code.

Answer 2

In apple library NSArray objectAtIndex method:
objectAtIndex:
Returns the object located at the specified index.
- (id)objectAtIndex:(NSUInteger)index
Parameters
index
An index within the bounds of the array.
Return Value
The object located at index.
You can use "NSUInteger index ;" so "if (index < 0)" could be omitted.
In my opinion, if you need to provide some interface to others, you may need to add these code to avoid "beyond bounds".
But if your code just works for yourself, you need not to do the stuff because most of time what you needs is an object in the array but not the nil. If the index beyonds bound, there must be some logic error which you need to fix. Let exception goes and find your bug.

Answer 3

U can swizzle the objectAtIndexmethod,before call objectAtIndex, invoke method like 'custom_objectAtIndex' to check if out of bounds .
+ (void)load{
Method method1 = class_getInstanceMethod(objc_getClass("__NSArrayI"),#selector(objectAtIndex:));
Method method2 = class_getInstanceMethod(objc_getClass("__NSArrayI"),#selector(custom_objectAtIndex:));
method_exchangeImplementations(method1, method2);
}
- (id)custom_objectAtIndex:(NSUInteger)index{
NSLog(#"~~~~~~~:%ld",count);
if (index >= self.count) {
return #"out of bounds";
} else {
return [self custom_objectAtIndex:index];
}
}