On 05/08/2015 06:26 PM, Alan Gauld wrote:
On 08/05/15 19:10, Jim Mooney Py3.4.3winXP wrote:
On 7 May 2015 at 18:42, Dave Angel <da...@davea.name> wrote:
Python doesn't have pointers
So what is the difference between a python name and a pointer?
OK, This could get deepo.
Lets start with the supoerficial...
A pointer (in most languiages) is a named alias for a memory address.
That is a name that is a specidic memory location. At least in C and
some other languages.
That means you can, in C , declare a variable (say p) to be a pointer
to some array of objects and it is simply the memory address of the
first object in the array. The second objects address is therefore
p+sizeof(o) where 'o' is the object type of the array.
You can in fact write code like this in C (and frequently do):
int p*; // declare p to be a pointer to an integer
int ia[] = {1,2,3,4,5,6,7,8,9,0}; // an array of 10 integers
p = ia; # p holds the address in ia, ie its first element
This is a shorthand for
p = & (ia[0])
printf("%d\n", p); // print the object, ie ia[0]
print("%d\n", *p) // print the first object ia[0]
print(("%d\n", p[0]); // print the first object, ie ia[0]
printf("%d",p+3); // print the 4th object ie ia[3]
printf("%d",*(p+3)); // print the 4th object ie ia[3]
printf("%d",p[3]); // print the 4th object ie ia[3]
and p[274] is a random pile of bits which might or might not happen to
be readable. Using it might read some other int, it might read a few
bytes of code, it might cause a segmentation fault.
So p is a pointer to an integer. And an array is a sequence
of integers in memory so you can access subsequent memory
locations by adding numbers to p. (The compiler multiplies
the number by the size of the type of p to get the actual
memory address.)
And all this is part of the language specification. C is after all, an
overblown assembly language, and all processors are expected to emulate
the appropriate Dec machine.
In other languages (eg Pascal) pointers are slightly more
abstract but not much. They are more like C++ references
than memory addresses, but the end result is much the same:
they are very tightly tied to the physical concepts of
memory versus variables.
Now in contrast...
In Python a name is a much more abstract concept and is
just a label that is attached to an object. How the label
gets attached is an entirely abstract and implementation
specific concept. In practice its usually via a dictionary
so that a variable is a name which is a key of a dictionary.
The corresponding value is an object or, (crucially) maybe
a pointer to an object. But the name is not the pointer
it's the corresponding value that (may be) a pointer.
So this Python code (compare to the C above) makes no sense:
aList = [1,2,3,4,5,6,7,8,9,0]
print aList_+ 3
aList is not a pointer to the start of a sequence of
objects in memory. aList is a key to a dictionary
that holds a reference to a list of objects. And adding
3 to a key in a dictionary is not a sensible operation.
I'd maintain that this is still more than what the language guarantees.
The model I have of the language is that a name is a key in some
namespace (but not necessarily a dict). The value that's associated
with that key is an abstraction that the interpreter knows how to decode
to identify one particular object. It might be a pointer, it might be
an integer, it might be a pair of values.
The object might have a fixed location, or it might move around. As
long as at any moment that the code is running, the interpreter can find
the object, it doesn't matter.
--
DaveA
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