Wrap Around the Edge

Part of the Python in a Game Window section of Coddy's Pygame journey. Lesson 10 of 32.

An if runs its lines only when a condition is true:

if comet_x > 640:
    comet_x = 0

The condition ends with a colon. The lines under it are indented 4 spaces: they belong to the if. Python tests the condition on every frame. While the comet is in the window it is false and nothing happens. Once the comet passes the right edge it is true, and the comet goes back to the left.

A comparison is either True or False. > means greater than, < less than, and == asks whether two values are equal. One = stores; two compare.

Test a range such as > 640, not one exact value. The comet starts at 50 and moves 3 pixels a frame: 635, 638, 641. It never lands on 640, so comet_x == 640 would never be true.

Challenge

Easy

After the line that moves the comet, add an if: when comet_x is greater than 640, set comet_x to 0. The comet then flies across the window again and again.

Try it yourself

import pygame

pygame.init()
screen = pygame.display.set_mode((640, 480))
pygame.display.set_caption("Night Sky")
clock = pygame.time.Clock()

# Set things up here, before the game loop.
moon_x = 160
moon_y = 120
comet_x = 50
comet_y = 200

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # Change things here, once a frame.
    comet_x = comet_x + 3

    screen.fill((20, 24, 48))
    # Draw here, after the fill.
    pygame.draw.circle(screen, (240, 230, 160), (moon_x, moon_y), 50)
    pygame.draw.circle(screen, (120, 200, 255), (comet_x, comet_y), 8)

    pygame.display.flip()
    clock.tick(60)

pygame.quit()