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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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