Adding/Removing elements from Collection classes on current iteration data

LIST
The list will not directly allow adding/remove elements on the current iteration. Below code will throw an exception:
Example:1

List<Integer> intNummbers = new List<Integer>{12,13};
for(Integer numbers : intNummbers)
{
    intNummbers.add(0,49);
}
System.debug('--'+intNummbers);

SET & MAP: 
These classes will allow us to add/remove elements on the current iteration.
Example: 2

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Map<Integer,Integer> intMap = new Map<Integer,Integer>{}; intMap.put(1,1); intMap.put(2,2); intMap.put(3,3); for(Integer intval : intMap.values()) { intMap.put(4,4); } System.debug('--- '+intMap);

Here is the output:

16:15:23:004 USER_DEBUG [10]|DEBUG|--- {1=1, 2=2, 3=3, 4=4}

Similarly, we can also modify keys
Example:3

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Map<Integer,Integer> intMap = new Map<Integer,Integer>{}; intMap.put(1,1); intMap.put(2,2); intMap.put(3,3); for(Integer intval : intMap.keySet()) { intMap.put(4,4); } System.debug('--- '+intMap);

output remains the same, as above:

16:15:23:004 USER_DEBUG [10]|DEBUG|--- {1=1, 2=2, 3=3, 4=4}

Adding elements to the current iteration set
Example:4

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set<Integer> intSet = new set<Integer>{1,2,3}; for(Integer intval : intSet) { intSet.add(4); } System.debug('--- '+intSet);

here is the output:

16:25:49:004 USER_DEBUG [6]|DEBUG|--- {1, 2, 3, 4}

For Array also, we can add elements to the current iteration data, below example explains the same:
Example:5

Integer[] intArray= new Integer[]{13,34};
for(Integer a: intArray)
{   
    intArray[1] =33; 
}
System.debug('--- '+intArray);

Note: Trigger.new is also a list but while iterating, apex runtime engine will allow. To conclude this post, we can say Example2,3,4,5 are exceptional cases. I don't see anywhere more information about these design decisions.

tHiNk gooD and dO thE bEsT.........MANJU NATH 🌝