Why is my loop still running? (Python Zelle Graphics)

Question

I'm trying to figure out why the while loop in one of my functions is still running even after the points in my graphics are equal, which is when I set it to stop. Is there anything I'm doing wrong? I've tried to switch other things around to get it to work but no luck. It's for a game--when the character reaches the endbox the loop needs to break, but it isn't doing that after I explicitly coded it to. It's in the second function I have:

from graphics import *

def field():
    #creating the window
    win = GraphWin('The Field',400,400)
    win.setBackground('white')
    #drawing the grid
    boxlist = []
    for i in range(0,400,40):
        for j in range(0,400,40):
            box = Rectangle(Point(i,j),Point(i+40,j+40))
            box.setOutline('light gray')
            box.draw(win)
            boxlist.append(box)
    #creating other boxes
    startbox = Rectangle(Point(0,0),Point(40,40))
    startbox.setFill('lime')
    startbox.setOutline('light gray')
    startbox.draw(win)
    endbox = Rectangle(Point(360,360),Point(400,400))
    endbox.setFill('red')
    endbox.setOutline('light gray')
    endbox.draw(win)
    boxlist.append(startbox)
    boxlist.append(endbox)
    #creating Pete
    pete = Rectangle(Point(2,2),Point(38,38))
    pete.setFill('gold')
    pete.draw(win)
    return win,boxlist,pete

def move(win2,boxlist,pete,endbox):
    peteloc = pete.getCenter()
    #creating loop to move pete
    while peteloc != endbox.getCenter():
        click = win2.getMouse()
        x = click.getX()
        y = click.getY()
        peteloc = pete.getCenter()
        petex = peteloc.getX()
        petey = peteloc.getY()
        #moving pete
        if x>=petex+20 and y<=petey+20 and y>=petey-20:
            pete.move(40,0)
        elif x<=petex-20 and y<=petey+20 and y>=petey-20:
            pete.move(-40,0)
        elif y>=petey+20 and x<=petex+20 and x>=petex-20:
            pete.move(0,40)
        elif y<=petey-20 and x<=petex+20 and x>=petex-20:
            pete.move(0,-40)
        peteloc = pete.getCenter()

# The main function
def main():
    win2,boxlist,pete = field()
    endbox = boxlist[len(boxlist)-1]
    move(win2,boxlist,pete,endbox)

main()

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recnac · Answer 1 · 2019-03-29T07:37:11.167

I think maybe it is caused by precision of float. I guess pete.getCenter() and endbox.getCenter() are something like [float, float], you should avoid using != between float, such as 1.0000001 is not equal to 1.

So even if the character reaches the endbox, the position will still get a little float bias.

So you can change a != b to abs(a - b) > acceptable_error when the error is acceptable. Sample code is like:

# while peteloc != endbox.getCenter():
while abs(peteloc.getX() - endbox.getCenter().getX()) > 0.01 and abs(peteloc.getY() - endbox.getCenter().getY()) > 0.01:

Hope that will help you.

score 1 · Answer 2 · answered Mar 31 '19 at 18:15

Zelle graphics Point objects don't appear to ever compare as equal:

>>> from graphics import *
>>> a = Point(100, 100)
>>> b = Point(100, 100)
>>> a == b
False
>>>

We have to extract coordinates and do our own comparison. Although @recnac provides a workable solution (+1), I'm going to suggest a more general one. We'll create a distance() method that's valid for any object that inherits from _BBox, which includes Rectangle, Oval, Circle and Line:

def distance(bbox1, bbox2):
    c1 = bbox1.getCenter()
    c2 = bbox2.getCenter()

    return ((c2.getX() - c1.getX()) ** 2 + (c2.getY() - c1.getY()) ** 2) ** 0.5

We can now measure the distance between objects, horizontally, vertically and diagonally. Since your boxes are moving twenty pixels at a time, we can assume that if they are withing 1 pixel of each other, they are in the same location. Your code rewritten to use the distance() method and other tweaks:

from graphics import *

def field(win):
    # drawing the grid
    boxlist = []

    for i in range(0, 400, 40):
        for j in range(0, 400, 40):
            box = Rectangle(Point(i, j), Point(i + 40, j + 40))
            box.setOutline('light gray')
            box.draw(win)
            boxlist.append(box)

    # creating other boxes
    startbox = Rectangle(Point(0, 0), Point(40, 40))
    startbox.setFill('lime')
    startbox.setOutline('light gray')
    startbox.draw(win)
    boxlist.append(startbox)

    endbox = Rectangle(Point(360, 360), Point(400, 400))
    endbox.setFill('red')
    endbox.setOutline('light gray')
    endbox.draw(win)
    boxlist.append(endbox)

    # creating Pete
    pete = Rectangle(Point(2, 2), Point(38, 38))
    pete.setFill('gold')
    pete.draw(win)

    return boxlist, pete

def distance(bbox1, bbox2):
    c1 = bbox1.getCenter()
    c2 = bbox2.getCenter()

    return ((c2.getX() - c1.getX()) ** 2 + (c2.getY() - c1.getY()) ** 2) ** 0.5

def move(win, pete, endbox):
    # creating loop to move pete

    while distance(pete, endbox) > 1:

        click = win.getMouse()
        x, y = click.getX(), click.getY()

        peteloc = pete.getCenter()
        petex, petey = peteloc.getX(), peteloc.getY()

        # moving pete
        if x >= petex + 20 and petey - 20 <= y <= petey + 20:
            pete.move(40, 0)
        elif x <= petex - 20 and petey - 20 <= y <= petey + 20:
            pete.move(-40, 0)
        elif y >= petey + 20 and petex - 20 <= x <= petex + 20:
            pete.move(0, 40)
        elif y <= petey - 20 and petex - 20 <= x <= petex + 20:
            pete.move(0, -40)

# The main function
def main():
    # creating the window
    win = GraphWin('The Field', 400, 400)
    win.setBackground('white')

    boxlist, pete = field(win)
    endbox = boxlist[-1]

    move(win, pete, endbox)

main()

very clear and elaborate answer, some little suggestion: to get a better performance, you can use `sqr_distance` instead of `distance`, to avoid the square root operation. That is what our game do in game develop. — recnac, Apr 01 '19 at 02:08

Why is my loop still running? (Python Zelle Graphics)

2 Answers2