Unable to replace object in NSMutableArray - objective-c

Question

Somehow the code below is throwing a NSInvalidArgumentException, although I followed step by step to make sure everything has proper value...
I have an array of CGPoints stored in NSValues, and this is part of a method to offset all points by (x,y).
for (int i = 0; i < [allPoints count]; i++) {
CGPoint pt = [[allPoints objectAtIndex:i] CGPointValue];
CGPoint newPt = CGPointMake(pt.x + x, pt.y + y);
NSValue *newEntry = [NSValue valueWithCGPoint:newPt];
[allPoints replaceObjectAtIndex:i withObject:newEntry];
}

Answer 1

try this ,
-----------------------------sample code---------------------------------
CGPoint pt =CGPointMake(20, 30);
NSValue *point = [NSValue valueWithCGPoint:pt];
CGPoint pt2 =CGPointMake(40, 40);
NSValue *point2 = [NSValue valueWithCGPoint:pt2];
NSMutableArray *allPoints =[[NSMutableArray alloc]initWithObjects:point,point2, nil];
NSLog(#"%#",allPoints);
// -------------output----------
// "NSPoint: {20, 30}",
// "NSPoint: {40, 40}"
int x=10; int y=20;
for (int i = 0; i < [allPoints count]; i++)
{
CGPoint pt = [[allPoints objectAtIndex:i] CGPointValue];
CGPoint newPt = CGPointMake(pt.x + x, pt.y + y);
NSValue *newEntry = [NSValue valueWithCGPoint:newPt];
[allPoints replaceObjectAtIndex:i withObject:newEntry];
}
NSLog(#"%#",allPoints);
// -------------output----------
// "NSPoint: {30, 50}",
// "NSPoint: {50, 60}"

Answer 2

NSMutableArray method replaceObjectAtIndex:withObject raises an NSInvalidArgumentException if object at given index is nil or new object is nil.
Double check if one of objects in allPoints NSMutableArray is nil.
Or allPoints NSMutableArray doesn't contain NSValue objects with CGPoint elements.

Answer 3

I do not know how, but it seems although I declared allPoints to be NSMutableArray and after a search in my entire code I cannot find in any place where I set the pointer of allPoints to a NSArray, but the system seems to think now that allPoints is now a NSArray and hence I cannot replace any objects.
Therefore I have to circumvent with a new NSMutableArray.
for (int i = 0; i < [allPoints count]; i++) {
CGPoint pt = [[allPoints objectAtIndex:i] CGPointValue];
CGPoint newPt = CGPointMake(pt.x + x, pt.y + y);
NSValue *newEntry = [NSValue valueWithCGPoint:newPt];
NSMutableArray *temp = [NSMutableArray arrayWithArray:allPoints];
[temp replaceObjectAtIndex:i withObject:newEntry];
allPoints = temp;
}