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A Concise Introduction to Programming in Python by Mark J. Johnson

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Chapter 6

Selection: If Statements

At both high levels and at the machine level, programs execute statements one after the other. Selection statements allow a program to execute different code depending on what happens as the program runs. This flexibility is another key to the power of computation.

Listing 6.1: Centipede

 1 # centipede.py
 2
 3 from turtle import *
 4
 5 def centipede(length, step, life):
 6 penup()
 7 theta = 0
 8 dtheta = 1
 9 for i in range(life):
10  forward(step)
11  left(theta)
12  theta += dtheta
13  stamp()
14  if i > length:
15   clearstamps(1)
16  if theta > 10 or theta < −10:
17   dtheta = −dtheta
18  if ycor() > 350:
19   left(30)
20
21 def main():
22 setworldcoordinates(−400, −400, 400, 400)
23 centipede(14, 10, 200)
24 exitonclick() ...

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