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If an array name is passed as an argument and a subscript is used on the argument in the function, then modifying array elements in the function actually changes the elements of the array in the calling program. The function accesses the very same array that exists in the calling program; the array is not copied.
The behavior with array arguments seems to contradict the call by value rule. However there isnʹt a contradiction—the value of the array name is really a pointer, and a copy of the pointer is passed to the function. A subscript is really a form of indirection and applying indirection to the pointer accesses the locations that it points to. The argument (the pointer) is indeed a copy, but the indirection uses the copy to access the original array values.