How to add objects to an NSArray using for loop?

NSArray is immutable. Use the mutable version, NSMutableArray.

NSMutableArray * customTickLocations = [NSMutableArray new];
for (int idx = 0; idx < 12; ++idx) {
    [customTickLocations addObject:[NSDecimalNumber numberWithInt:idx]];
}

...

you cannot add Objects at run time to NSArray.For adding or removing objects at Run time you have to use NSMutableArray.

NSMutableArray *mutableArray=[[NSMutableArray alloc] init];
for (int i=0; i<10; i++) {
    [mutableArray addObject:[NSDecimalNumber numberWithInt:i]];
}

NSMutableArray *customTickLocations = [NSMutableArray array];
for (int i=0; i<totalImagesOnXaxis; i++)
{
    [customTickLocations addObject:[NSDecimalNumber numberWithInt:i]];
}

The NSMutableArray class declares the programmatic interface to objects that manage a modifiable array of objects. This class adds insertion and deletion operations to the basic array-handling behavior inherited from NSArray

NSMutableArray Class Reference

I found that using this technique is a great way to easily add a few more elements to an NSArray, this was the answer I was looking for when I came to this thread so am posting it as it is a great easy addition.

If I want to add a new array to my current array

currentArray = [currentArray arrayByAddingObjectsFromArray: newArray]; 

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

for(int i = 0; i<WhateverNoYouWant;i++)
{
  NSDecimalNumber * x = [NSDecimalNumber numberWithInt:i]
  [customTickLocations addObject:x]
}

NSArray add object like this:

NSArray *arr = @["1","2","3","4"];

I think NSArray can not addObject like NSMutableArray. You should try it:

NSMutableArray *mulArr = [NSMutableArray new];
[mulArr addObject:[NSDecimalNumber numberWithInt:number]];