Python "why" questions

Dave Angel davea at ieee.org
Sun Aug 15 14:50:48 CEST 2010


Roald de Vries wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">On Aug 
> 15, 2010, at 2:16 PM, geremy condra wrote:
>> On Sun, Aug 15, 2010 at 4:55 AM, Roald de Vries <downaold at gmail.com> 
>> wrote:
>>> On Aug 15, 2010, at 1:00 PM, Lawrence D'Oliveiro wrote:
>>>>
>>>> It would be if pointers and arrays were the same thing in C. Only 
>>>> they’re
>>>> not, quite. Which somewhat defeats the point of trying to make them 
>>>> look
>>>> the
>>>> same, don’t you think?
>>>
>>> How are they not the same?
>>>
>>> The code snippet (in C/C++) below is valid, so arrays are just 
>>> pointers. The
>>> only difference is that the notation x[4] reserves space for 4 
>>> (consecutive)
>>> ints, and the notation *y doesn't.
>>>
>>> int x[4];
>>> int *y =x;
>>>
>>> Moreover, the following is valid (though unsafe) C/C++:
>>>
>>> int *x;
>>> int y = x[4];
>>
>> Just to demonstrate that they are different, the following code
>> compiles cleanly:
>>
>> int main() {
>> int *pointer;
>> pointer++;
>> return 0;
>> }
>>
>> While this does not:
>>
>> int main() {
>> int array[0];
>> array++;
>> return 0;
>> }
>
> Interesting! Thanks for the lesson ;-).
>
> Cheers, Roald
>
>
That particular example doesn't really matter; it just shows that array 
is a *const* pointer.

One that does matter, sometimes drastically, is sizeof(array) vs. 
sizeof(pointer).

One interesting other effect of the compiler (nearly always) treating 
pointers and arrays the same is expressions like:
int hexdigit = ...something...
char ch = "0123456789abcdef"[hexdigit];
char ch2 = hexdigit["0123456789abcdef";


both are valid assignments, and equivalent to
char ch3 = hexdigit + &("0123456789abcdef");





More information about the Python-list mailing list