NSMutableArray insertObject：atIndex的异常行为

Question

i try to make an array that has the numbers from 1 to 30 using..

NSMutableArray *numberOfdaysInAMonth = [[NSMutableArray alloc] init ] ;

for ( i=0; i< 30; i++){
    [numberOfdaysInAMonth  addObject:[NSNumber numberWithInteger:i+1]];
    NSLog (@"Element_numberofDaysinAMonth %i = %@", i, [numberOfdaysInAMonth objectAtIndex: i]); 
}

and I have an array with 35 elements and they have the value of null:

NSMutableArray *latestArrayForMonth = [[NSMutableArray alloc]init ];

for ( i=0; i<36; i++){        
    [latestArrayForMonth addObject:[NSNull null]];         
}

Now I want to insert the values from numberOfDaysInAMonth into the LatestArrayWithMonth statrting from index:1 using:

for (i = 0; i < 30; i++){  
    [latestArrayForMonth insertObject:[numberOfdaysInAMonth objectAtIndex:i] atIndex:1];
    NSLog (@"Element_latestArrayForMonth %i = %@", i, [latestArrayForMonth objectAtIndex: i]);  
}

Output on the Console is:

Element_latestArrayForMonth 0 = <null>
Element_latestArrayForMonth 1 = 2
Element_latestArrayForMonth 2 = 2
Element_latestArrayForMonth 3 = 2
Element_latestArrayForMonth 4 = 2
Element_latestArrayForMonth 5 = 2
.
.
.
Element_latestArrayForMonth 28 = 2
Element_latestArrayForMonth 29 = 2

Why am I getting all 2s on the output?

Answer 1

It's because of this line:

[latestArrayForMonth insertObject:[numberOfdaysInAMonth objectAtIndex:i] atIndex:1];

Because you're inserting the item atIndex:1 continuously, as the 'i' variable is increasing in the loop, so is the number of items in the array - so the logging of the item at index 'i' is always the same item.

Please also check that inserting the object at index 1 is what you are wanting to do - Objective-C is a zero-indexed language, meaning that the first item in the array is at index 0.

You probably want to remove all the objects in the latestArrayForMonth array and simply use the addObject method (or use addObjectsFromArray to quickly add all the items from the array).

Also might be worthy investigating alternative approaches to having an array full of 'empty' values - I've never seen a use case for this approach yet, better to have an empty array than an array full of 'empty' values.

One other thing to note is that since you're just storing primitive integer values, you may find NSIndexSet more applicable to use than NSArray (depends on the application of the posted code, but worth investigation).

Answer 2

Using insertObject:... atIndex:1 you are inserting month 1 at index 1 (no elements so it goes to index 0), then month 2 (which is actually at index 1) then you keep inserting before 2. After 5 elements your array will look like this 1,5,4,3,2. What you should do is just access the array by subtracting 1 from the index, but if you want to access starting at index 1 add an NSNull first.

[latestArrayForMonth addObject:[NSNull null]];

for (i = 0; i < 30; i++){

    [latestArrayForMonth addObject:[numberOfdaysInAMonth objectAtIndex:i]];

    NSLog (@"Element_latestArrayForMonth %d = %@", i + 1, [latestArrayForMonth objectAtIndex: i + 1]);  

}