Python Help

[Doomkiller]

Anyone here good with python? (outside of Ra2)
I got this problem with this challenge:

Imagine you have a sheet of grid paper with the left and right sides   joined together to make a cylinder.  Each square on the grid is either a   period . or a star *.
Starting with some pattern in the top row of the grid, we determine the next row by applying a simple rule:
 

• for each position in the grid:
• • put a star in the grid if there is a star either to the left or the right in the row above, but not both.
  • otherwise put a period.

  There are eight different possible configurations of the left, middle   and right cell in the top row. The eight patterns are shown below with   the corresponding middle cell in the new row.  We have put vertical   lines in for clarity, your program should not put these in:
|*|*|*|   |*|*|.|   |*|.|*|   |*|.|.|    |.|       |*|       |.|       |*|    |.|*|*|   |.|*|.|   |.|.|*|   |.|.|.|    |*|       |.|       |*|       |.| 
Again, just to be clear, we only print a * in the middle cell in the row below when there is a * in either the left or the right cell in the row above, but not both.
This is a 1-dimensional cellular automaton.    Cellular automata are very simple computational systems inspired by   biology. The state of each cell is either on (a star in our example) or   off (a period), and the state of each cell in the future is governed by   simple rules about the current state of the cell and its neighbours.  In   our 1-dimensional automata, the future is the next row of output.
Your program will simulate the cellular automata described above. It should read a number and a string from raw_input.   The number will indicate the number of rows to generate, and the string   is the starting row. It should then print out the first row and all   subsequent rows until the number of rows printed is the number of rows   that were requested (the number from raw_input).
For example:
Number of lines: 8 
Start line: ........*......... 
........*......... 
.......*.*........ 
......*...*....... 
.....*.*.*.*...... 
....*.......*..... 
...*.*.....*.*....
..*...*...*...*...
.*.*.*.*.*.*.*.*..  This type of machine is called a cellular automata because it is inspired by the way bacterial cells grow. Perhaps the most famous of these is the two dimensional version called Conway's Game of Life.

Don't really want the solution, just want some help on to how to go about solving this. Did try the forum on the challenge website, but I have yet to recieve and answer :/

Also, im using python 2.6.5 in idle

[Pwnator]

Your best bet is to make a flowchart. This is what I usually do when I'm trying to solve complex but recursive problems.

Also, are there any restrictions to your program?

[Doomkiller]

Your best bet is to make a flowchart. This is what I usually do when I'm trying to solve complex but recursive problems.

Also, are there any restrictions to your program?

I have tried that, i know the general idea of how to do it, I just don't know how to get python to do it

Restrictions? by what do you mean?

[Pwnator]

Forbidding the use of certain functions, certain data types, etc...

Our instructor pisses us off that way. >.<

[Doomkiller]

Forbidding the use of certain functions, certain data types, etc...

Our instructor pisses us off that way. >.<

No restrictions as far as I know

btw, I has started a sortve code, that sortve works...

Code: [Select]

lines = int(raw_input("Number of lines: "))
start = raw_input("Start line: ")
firstline = start
nextline = []
print firstline
for i in range(lines-1):
    for j in range(len(start)-1):
        if (firstline[j-1] == "*" and firstline[j+1] == "."):
            nextline.append("*")
        elif (firstline[j-1] == "." and firstline[j+1] == "*"):
            nextline.append("*")
        else:
            nextline.append(".")
    firstline = "".join(nextline)
    print firstline
    nextline = []


It starts off good, going:
Number of lines: 8
Start line: ......*......
......*......
.....*.*....

but then:
Traceback (most recent call last):
  File "C:/Users/User/Documents/testingagain.py", line 10, in <module>
    elif (firstline[j-1] == "." and firstline[j+1] == "*"):
IndexError: string index out of range

[nightcracker]

It's not that hard, the problem is that the i-1 and i+1 go out of range. i-1 is already doing exactly what it should do for this program, access from the end. But if I have a list with 3 elements we want li[4] to go back to the beginning. I solve this by using the modulo operator.

Code: [Select]

def getnextrow(oldrow):
    size = len(oldrow)
    newrow = "." * size

    for i in range(0, size):
        char = "."
        if (oldrow[i - 1] == "*") != (oldrow[(i + 1) % size] == "*"): # modulo size so that the index wraps around
            char = "*"
        newrow[i] = char

    return newrow

firstrow = "..*.**.."
nextrow = getnextrow(firstrow)
print(nextrow)

[Doomkiller]

Im not too sure how you did it night, care to elaborate more?

Also, I solved it anyway :P

Code: [Select]

lines = int(raw_input("Number of lines: "))
start = raw_input("Start line: ")
firstline = start
nextline = []
print firstline
for i in range(lines-1):
    for j in range(len(start)):
        if (firstline[j-1] == "*" and firstline[j-len(start)+1] == "."):
            nextline.append("*")
        elif (firstline[j-1] == "." and firstline[j-len(start)+1] == "*"):
            nextline.append("*")
        else:
            nextline.append(".")
    firstline = "".join(nextline)
    print firstline
    nextline = []


[nightcracker]

Say we have a string row:

"..*.*"

Well basically, range(0, len(row) - 1) goes like this:

0 1 2 3

now, if we check their "neighbours" with -1 and +1 the range will turn into this:

-1 0 1 2 3 4

Because row[-1] equals row[len(row) - 1] it's fine. However, row[4] will give the IndexError: out of range. What do we want? We want 4 to go back to the beginning (row[0]). I did this with the modulo operator, you should read on it: http://en.wikipedia.org/wiki/Modulo_operation

[Doomkiller]

Oh, no I know what you mean by the modulo and how the neighbours worked, I meant when I looked at your code, i didn't quite understand what you were doing

[nightcracker]

Oh, no I know what you mean by the modulo and how the neighbours worked, I meant when I looked at your code, i didn't quite understand what you were doing

Quote my code and add comments to the parts you don't understand and I'll try to explain it.

[Doomkiller]

Code: [Select]

//what does def do?//
def getnextrow(oldrow):
    size = len(oldrow)
    newrow = "." * size

    for i in range(0, size):
        char = "."
          //and what happens here?//
        if (oldrow[i - 1] == "*") != (oldrow[(i + 1) % size] == "*"): # modulo size so that the index wraps around
            char = "*"
        newrow[i] = char

    return newrow

firstrow = "..*.**.."
nextrow = getnextrow(firstrow)
print(nextrow)



[nightcracker]

Comments in python are preceded with #, not //.

Code: [Select]

def function_name(argument1, argument2):
    # code here
creates a new function that can be called and can return things. Like this:

Code: [Select]

def add_one(number):
    return number + 1

print(add_one(4)) # this will print out 5
What happens here is a bit more complicated:

Code: [Select]

if (oldrow[i - 1] == "*") != (oldrow[(i + 1) % size] == "*"):
Basically I'm first asking two questions (the two parts in the brackets): is my topleft neighbour a star? and is my topright neighbour a star? Then I check with != if the answer to those two questions is not the same.

[Doomkiller]

Ohh ok thanks

Also, yeah im used to actionscript atm, hence the // // comments :P