indirectly addressing vars in Python
Forgive my newbieness - I want to refer to some variables and indirectly alter them. Not sure if this is as easy in Python as it is in C. Say I have three vars: oats, corn, barley I add them to a list: myList[{oats}, {peas}, {barley}] Then I want to past that list around and alter one of those values. That is I want to increment the value of corn: myList[1] = myList[1] + 1 Is there some means to do that?. Here's my little session trying to figure this out: oats = 1 peas = 6 myList=[] myList [] myList.append(oats) myList [1] myList.append(peas) myList [1, 6] myList[1]= myList[1]+1 myList [1, 7] peas 6 So I don't seem to change the value of peas as I wished. I'm passing the values of the vars into the list, not the vars themselves, as I would like. Your guidance appreciated... Ross. -- http://mail.python.org/mailman/listinfo/python-list
Re: indirectly addressing vars in Python
Ross wrote: myList[1]= myList[1]+1 The problem is this makes myList[1] point to a new integer, and not the one that peas points to. Python 2.5.1 (r251:54863, Jul 10 2008, 17:25:56) [GCC 4.1.2 20070925 (Red Hat 4.1.2-33)] on linux2 Type help, copyright, credits or license for more information. oats=[1] peas=[6] mylist = [oats, peas] mylist[1][0] = mylist[1][0]+1 mylist [[1], [7]] peas [7] This is because integers are immutable, but lists are mutable. -- Jeremy Sanders http://www.jeremysanders.net/ -- http://mail.python.org/mailman/listinfo/python-list
Re: indirectly addressing vars in Python
On 2008-10-01, Ross [EMAIL PROTECTED] wrote: Forgive my newbieness - I want to refer to some variables and indirectly alter them. Not sure if this is as easy in Python as it is in C. Python doesn't have variables. It has names bound to objects. When you do an assignment, that binds (or rebinds) a name to an object. There is no such thing as a C-like variable (a named region of memory with a fixed location into which you can write different values). Some objects (e.g. lists, dictionaries) are mutable (you can change their value or contents), some objects (e.g. strings, integers, floats) are not mutable. Say I have three vars: oats, corn, barley I add them to a list: myList[{oats}, {peas}, {barley}] Then I want to past that list around and alter one of those values. That is I want to increment the value of corn: myList[1] = myList[1] + 1 Is there some means to do that?. Here's my little session trying to figure this out: oats = 1 peas = 6 Those two lines created two integer objects with values 1 and 6 and bound the names oats and peas to those two objects. myList=[] myList [] myList.append(oats) That line finds the object to which the name oats is currently bound and appends that object to the list. myList [1] myList.append(peas) Likewise for the object to which the name peas is currently bound. myList [1, 6] myList[1]= myList[1]+1 That line creates a new integer object (whose value happens to be 7) and replaces the object at position 1 in the list with the new object. myList [1, 7] peas 6 So I don't seem to change the value of peas as I wished. Correct. The name peas is still bound to the same object it was before I'm passing the values of the vars into the list, not the vars themselves, as I would like. There are no vars as the word is used in the context of C programming. Just names and objects. Here's an article explaining it: http://rg03.wordpress.com/2007/04/21/semantics-of-python-variable-names-from-a-c-perspective/ A couple other good references: http://starship.python.net/crew/mwh/hacks/objectthink.html http://effbot.org/zone/python-objects.htm -- Grant Edwards grante Yow! What GOOD is a at CARDBOARD suitcase ANYWAY? visi.com -- http://mail.python.org/mailman/listinfo/python-list
Re: indirectly addressing vars in Python
On Wed, Oct 1, 2008 at 7:53 AM, Ross [EMAIL PROTECTED] wrote: Forgive my newbieness - I want to refer to some variables and indirectly alter them. Not sure if this is as easy in Python as it is in C. Say I have three vars: oats, corn, barley I add them to a list: myList[{oats}, {peas}, {barley}] You mean: myList = [oats, peas, barley] And you're not adding *variables* to a list, you're adding *values* to the list. The list elements (and the Python runtime) have *no idea* what variables they are/were bound to. Then I want to past that list around and alter one of those values. That is I want to increment the value of corn: myList[1] = myList[1] + 1 This won't do what you're expecting. Integers in Python are immutable, so instead of changing the value of 'corn', you're calculating a new int object and overwriting the first element of the list with it. Is there some means to do that?. Here's my little session trying to figure this out: oats = 1 peas = 6 myList=[] myList [] myList.append(oats) myList [1] myList.append(peas) myList [1, 6] myList[1]= myList[1]+1 myList [1, 7] peas 6 So I don't seem to change the value of peas as I wished. I'm passing the values of the vars into the list, not the vars themselves, as I would like. Your guidance appreciated... Python uses *call-by-object*, not call-by-value or call-by-reference semantics, so unlike C/C++ but like Java you can't make a reference to a variable and use that to non-locally rebind the variable to a new value. To do what you want, you need to create a mutable value that can be updated. You could code a MutableInt class wrapping 'int', or you could use a dictionary to hold the values and then always refer to the values using the dictionary. There are a few other ways to do it. Hope that elucidates it for you somewhat. Then again I am a little short on sleep :) Cheers, Chris -- Follow the path of the Iguana... http://rebertia.com Ross. -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: indirectly addressing vars in Python
Chris Rebert a écrit : On Wed, Oct 1, 2008 at 7:53 AM, Ross [EMAIL PROTECTED] wrote: Forgive my newbieness - I want to refer to some variables and indirectly alter them. Not sure if this is as easy in Python as it is in C. Say I have three vars: oats, corn, barley I add them to a list: myList[{oats}, {peas}, {barley}] You mean: myList = [oats, peas, barley] And you're not adding *variables* to a list, you're adding *values* s/values/references to objects/, actually (snip) -- http://mail.python.org/mailman/listinfo/python-list
Re: indirectly addressing vars in Python
On Wed, 01 Oct 2008 10:53:08 -0400, Ross wrote: Forgive my newbieness - I want to refer to some variables and indirectly alter them. Not sure if this is as easy in Python as it is in C. Say I have three vars: oats, corn, barley I add them to a list: myList[{oats}, {peas}, {barley}] Then I want to past that list around and alter one of those values. That is I want to increment the value of corn: myList[1] = myList[1] + 1 Is there some means to do that?. Here's my little session trying to figure this out: oats = 1 peas = 6 myList=[] myList [] myList.append(oats) myList [1] myList.append(peas) myList [1, 6] myList[1]= myList[1]+1 myList [1, 7] peas 6 So I don't seem to change the value of peas as I wished. I'm passing the values of the vars into the list, not the vars themselves, as I would like. Your guidance appreciated... Ross. Short answer: Python is not C. Long answer: In python, integers are immutable (they do not change), i.e. if you do: a = 1 b = 2 c = a + b a + b creates a new integer object that has the value 3 and assign it to c Other examples of immutable values are: number types, string, tuple, etc. In python, variable is usually called name. The concept of variable and name is slightly different. In C, a variable contains an object. In python, a name contains a pointer to an object. (the term pointer is used a bit loosely here) In python, a list is a list of pointers to objects. i.e.: |-- Hello World myList |-- 1 |-- 6 when you do, for example, myList[2] + 1 what you're doing is: 1. fetch the value of myList[2] (i.e. 6) 2. do an arithmetic addition of 6 + 1 3. create a new integer object with value 7 4. bind myList[2] to that new integer object (_new integer object_ since integer is immutable, you cannot change its value[1]) in short, pea is left intact since pea points to integer object 6. beware though, that things are different if myList[2] is a mutable value instead and the operation is an in-place operation, e.g. list.sort a = [1, 2] b = [3, 2] myList.append(a) myList.append(b) myList[1].sort() myList [[1, 2], [2, 3]] b [2, 3] in this case a, b is mutable object, so: myList.append(x) binds myList to the list object pointed by x myList[1].sort() is an in-place sort to the list object [3, 2] (i.e. it mutates the list [3, 2] instead of creating a new list[1]). in this case: myList |-- [1, 2] |-- [3, 2] myList[1].sort(), being an in-place sort, modifies the list object [3, 2] in-place. Since b points to the same list object, b's list is changed too. To summarize, you don't usually use that line of thinking in python. If you really insists, though is not pythonic (and is really absurd anyway), you may use this: a = [1] b = [2] myList.append(a) myList.append(b) myList[1][0] = myList[1][0] + 1 myList [[1], [2]] b [3] [1] Well, not always though, in CPython (the standard python reference implementation), small immutable values are cached as speed optimization. This is a safe operation, except for identity testing. But that doesn't matter much, since identity testing of immutable is moot. [2] If you want a sorting operation that returns a new list, use sorted (list) -- http://mail.python.org/mailman/listinfo/python-list