a couple of things I don't understand wrt lists
hello, I am a beginner programmer. I started learning programming about a year and a half ago, using C. I picked up python a few months ago, but only wrote very few scripts. I am currently trying to learn more about the python way of doing things by writing a script that generates png images using a 1D cellular automaton. While writing preliminary code for that project, I ran into a behaviour that I don't understand. I am using python 2.7 on a linux system. I represent the CA's rule with a list of integers, of value 1 or 0. Here is the function I use to generate the list: def get_rule(rulenum): rule = [] while rulenum > 0: rule.append(rulenume % 2) rulenum /= 2 while len(rule) < 8: rule.append(0) rule.reverse() return rule if i call it by writing: rule = getrule(int(8)) and then call: print rule the output is good: [0, 0, 0, 0, 1, 0, 0, 0] I then tried to print each item of the list using a for loop: for i in range(rule): print rule[i] the output is, as expected: 0 0 0 0 1 0 0 0 but when I do: for i in rule: print rule[i] I get the "complement": 1 1 1 1 0 1 1 1 There must be something I didn't understand correctly in the for statement, but I really can't think of a reason why the output is what it is. I tried this using the interactive console, and the results are the same, whatever the length of the list, i always get the complement of my bit pattern. Any pointers to help me understand this would be appreciated. I am also running into an other issue with this script, but I'd rather understand this before asking further questions. Thanks, and sorry for the rather long post. -- http://mail.python.org/mailman/listinfo/python-list
Re: a couple of things I don't understand wrt lists
Hello, Thanks for all your replies, things are getting clearer. - copy/paste vs retyping: Several people have remarked that I had retyped instead of copy/pasting. This is exactly what happened, the fact is I haven't figured out yet how to enable copy/pasting from urxvt to vim. I'll try to get this done before posting output from an interactive session. - iterating a list: If I understand what you've told me, bitpattern = [1, 0, 0, 1, 1, 0, 1] for bit in bitpattern: print bit and bitpattern = [1, 0, 0, 1, 1, 0, 1] for i in range(len(bitpattern)): print bitpattern[i] are equivalent, the former being more "python" and the latter being something like C written using python syntax. - the "complement" thing: I haven't yet tried to reproduce this, but I will, and I will post back if I see this happening again, this time with a real log of python's interactive console, or a complete script which people can use. - list comprehension: I wasn't at all aware of this, thanks a lot for pointing it out. I had an other function that generated a list of cells for the CA, each cell being randomly alive or dead. I have rewritten it using list comprehension, def populate(n): random.seed() return [random.randint(0,1) for i in range(n)] which works the same as my previous, C-like, version but feels much better. I might come back to you all but you've already given me a good push forward, so I guess I'll be trying to use it as much as I can before asking for an other. -- http://mail.python.org/mailman/listinfo/python-list
Re: a couple of things I don't understand wrt lists
Hello, I am still in the process of writing preliminary code for my CA project. I am now running into a behavior that I can't explain. Here is a script which, at least on my system, shows the issue (python2.7 on a linux system). The final project will be wrapping these functions (and others) into a class which will be in a module which can be imported into a script. #!/usr/bin/python2 import sys import random def get_rule(rulenum): bitpattern = bin(rulenum)[2:] return [0]*(8-len(bitpattern)) + [int(bit) for bit in bitpattern] def populate(n): random.seed() return [random.randint(0,1) for i in range(n)] def get_index(thisgen, i): n = len(thisgen)-1 cell = thisgen[i] if i is 0: print "i==0" prev, next = thisgen[n], thisgen[i+1] elif i is n: print "i==%d" % n prev, next = thisgen[i-1], thisgen[0] else: prev, next = thisgen[i-1], thisgen[i+1] return prev*4 + cell*2 + next def get_nextgen(thisgen, rule): return [rule[get_index(thisgen, i)] for i in range(len(thisgen))] if len(sys.argv) == 2: n = int(sys.argv[1]) else: n = 257 rule = get_rule(145) thisgen = populate(n) nextgen = get_nextgen(thisgen, rule) print "done for n == 257" n = 258 thisgen = populate(n) nextgen = get_nextgen(thisgen, rule) My issue is that when n == 257, the script runs as expected, but if n >= 258, I get an "IndexError: list index out of range". The script is also attached to the email in case someone would like to try it on their machine. The print statements in the get_index() function's branching show me that, when I get the "out of range" error, the program doesn't enter the "elif i is n" condition, although it does for lower values of n. I'm sorry, but I still haven't found a way to copy/paste from the terminal to vim. This behaviour occurred previously and had stopped when I used an other version of the get_index() function. At that time, get_index() wasn't a function and was just conditional branching inside get_nextgen, I wanted to use the list comprehension syntax to build nextgen, and this got me into seperating the get_index() routine. #!/usr/bin/python2 import sys import random def get_rule(rulenum): bitpattern = bin(rulenum)[2:] return [0]*(8-len(bitpattern)) + [int(bit) for bit in bitpattern] def populate(n): random.seed() return [random.randint(0,1) for i in range(n)] def get_index(thisgen, i): n = len(thisgen)-1 cell = thisgen[i] if i is 0: print "i==0" prev, next = thisgen[n], thisgen[i+1] elif i is n: print "i==%d" % n prev, next = thisgen[i-1], thisgen[0] else: prev, next = thisgen[i-1], thisgen[i+1] return prev*4 + cell*2 + next def get_nextgen(thisgen, rule): return [rule[get_index(thisgen, i)] for i in range(len(thisgen))] if len(sys.argv) == 2: n = int(sys.argv[1]) else: n = 257 rule = get_rule(145) thisgen = populate(n) nextgen = get_nextgen(thisgen, rule) print "done for n == 257" n = 258 thisgen = populate(n) nextgen = get_nextgen(thisgen, rule) -- http://mail.python.org/mailman/listinfo/python-list
Re: a couple of things I don't understand wrt lists
> The second guess, more likely, is that you're using "is" to compare > numbers, and that's never a safe idea. It might happen to work for > small numbers, but you should be using ==. The second guess was right, changing "is" for "==" solved it, thanks. I still have a lot to learn about python semantics. The 8 bit pattern is used to store the rule, not the cells. To compute the next cell, I index the rule list, based on the values of that cell and it's two immediate neighbours in the previous generation. This probably isn't the simplest way. Since I am using only 3 cells to compute 1 cell, the biggest index would be 2**3 - 1, hence the 8bit pattern for the rule. -- http://mail.python.org/mailman/listinfo/python-list
