Using an array in Python Pygame to generate lines

Question

I am trying to draw lines in Pygame by calling point coordinates from an array index. However, this returns an error:

Traceback (most recent call last): File "C:/Python33/Games/lineTest.py", line 33, in pygame.draw.line(windowSurface,BLACK,(0,0), (0, list[j]), 3) IndexError: list index out of range

Here is my code:

import pygame, sys, random, time
from pygame.locals import *

# sets up pygame
pygame.init()

# sets up the window
windowSurface = pygame.display.set_mode((500, 500), 0, 32)
pygame.display.set_caption('Line Test')

# sets up the colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

# draw white background
windowSurface.fill(WHITE)

print('Please enter a number:')
number = input()

# generate random numbers for array
i = 0
list = []
while int(number) > i:
    i = i+1
    x = random.randint(1, 500)
    list.append(x)

# draw lines
j = 0
while int(number) > j:
    j = j+1
    pygame.draw.line(windowSurface,BLACK,(0,0), (0, list[j]), 3)

# Draw the window to the screen
pygame.display.update()

I was wondering if anyone might have a solution for getting past this error?

Swap j = j+1 and pygame.draw.line(windowSurface,BLACK,(0,0), (0, list[j]), 3) Also look at docs.python.org/2/tutorial/controlflow.html#the-range-function instead of while loops. — Jonathan
– Jonathan, Commented Feb 13, 2014 at 21:47

jonrsharpe · Accepted Answer · 2014-02-13 21:49:11Z

5

You add one to your counters i and j before using them, so you try to access an item one index beyond the end of the list.

Also:

Don't call your own variable list; and
Use for loops, it's what they're there for.

Example:

lst = []
for n in range(number):
     lst.append(...) # or google "python list comprehension"

for item in lst:
    # use item

answered Feb 13, 2014 at 21:49

jonrsharpe

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1 Comment

stewSquared Over a year ago

+1 for using for loops. No one should have to deal with off-by-one errors in 2014.

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