Question

I’ve been attempting to port a program I’ve written in C++ to Python. Specifically, it is a program for generating heightmaps which can be exported to 3D models, and which ideally, when viewed, would look like realistic terrain.

The HeightMap class wraps a two-dimensional array of float values. For now I’ve kept it as integers because it prints nicer, and I haven’t had a chance to implement any other features. The glitch I’m having is that when I call set(self, x, y, value), which simply sets the value at x, y to value, with the code “self.rows[y][x] = value”, it seems to change the entire column, visiting each member of self.rows and setting the xth member of that array to value.

Here is my code. I am about 90% sure the mistake is somewhere in the 2-array’s initialization.

def filledArray(length, value) :
    result = []
    for i in range(1, length) :
        result.append(value)
    return result

def resizeArray(array, newLength, nullValue) :
    if newLength == len(array) :
        return array
    result = []
    for i in range(0, newLength) :
        if i < len(array) :
            result.append(array[i])
        else :
            result.append(nullValue)
    return result

class HeightMap:
    """A class that wraps a 2D array for generating height maps"""
    def __init__(self) :
        self.width = 0
        self.height = 0
        self.rows = []

    def __init__(self, initWidth, initHeight) :
        self.clear(initWidth, initHeight);

    def clear(self, initWidth, initHeight) :
        self.width = initWidth
        self.height = initHeight
        self.rows = filledArray(initHeight, filledArray(initWidth, 0))

    def setHeight(self, newHeight) :
        if self.height == newHeight :
            return
        self.rows = resizeArray(self.rows, newHeight, filledArray(self.width, 0))

    def setWidth(self, newWidth) :
        if self.width == newWidth :
            return
        for i in range(0, len(self.rows)) :
            self.rows[i] = resizeArray(self.rows[i], newWidth, 0);
        self.width = newWidth

    def get(self, x, y) :
        return self.rows[y][x]

    def set(self, x, y, value) :
        self.rows[y][x] = value

    def add(self, x, y, value) :
        self.rows[y][x] += value

    def multiply(self, x, y, value) :
        self.rows[y][x] *= value

Answer 1

Your problem is that when you call filledArray(initHeight, filledArray(initWidth, 0)) the second argument is passed by reference (this is always the case with python objects). Inside filledArray you need to make a copy of value for each. This will require adding a little bit of additional logic to your function, something along the lines of:

if type(value) == list:
   value = list(value)

Or you could do the slightly less pythonic:

type(value)(value)

to create a copy.