In order to draw an octagon with turtle graphics, you would need a loop that iterates eight times.

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C H A P T E R 4

Repetition Structures

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Topics

Introduction to Repetition Structures
The while Loop: a Condition-Controlled Loop
The for Loop: a Count-Controlled Loop
Calculating a Running Total
Sentinels
Input Validation Loops
Nested Loops
Turtle Graphics: Using Loops to Draw Designs
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Introduction to Repetition Structures

Often have to write code that performs the same task multiple times
Disadvantages to duplicating code

Makes program large

Time consuming

May need to be corrected in many places

Repetition structure: makes computer repeat included code as necessary
Includes condition-controlled loops and count-controlled loops

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The while Loop: a Condition-Controlled Loop

while loop: while condition is true, do something
Two parts:

Condition tested for true or false value

Statements repeated as long as condition is true

In flow chart, line goes back to previous part

General format:

while condition:

statements

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The while Loop: a Condition-Controlled Loop (cont’d.)

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The while Loop: a Condition-Controlled Loop (cont’d.)

In order for a loop to stop executing, something has to happen inside the loop to make the condition false
Iteration: one execution of the body of a loop
while loop is known as a pretest loop
Tests condition before performing an iteration

Will never execute if condition is false to start with

Requires performing some steps prior to the loop

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Copyright © 2018 Pearson Education, Inc.

Infinite Loops

Loops must contain within themselves a way to terminate
Something inside a while loop must eventually make the condition false

Infinite loop: loop that does not have a way of stopping
Repeats until program is interrupted

Occurs when programmer forgets to include stopping code in the loop

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The for Loop: a Count-Controlled Loop

Count-Controlled loop: iterates a specific number of times
Use a for statement to write count-controlled loop

Designed to work with sequence of data items

Iterates once for each item in the sequence

General format:

for variable in [val1, val2, etc]:

statements

Target variable: the variable which is the target of the assignment at the beginning of each iteration

Copyright © 2018 Pearson Education, Inc.

Copyright © 2018 Pearson Education, Inc.

Using the range Function with the for Loop

The range function simplifies the process of writing a for loop
range returns an iterable object

Iterable: contains a sequence of values that can be iterated over

range characteristics:
One argument: used as ending limit

Two arguments: starting value and ending limit

Three arguments: third argument is step value

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Using the Target Variable Inside the Loop

Purpose of target variable is to reference each item in a sequence as the loop iterates
Target variable can be used in calculations or tasks in the body of the loop
Example: calculate square root of each number in a range

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Letting the User Control the Loop Iterations

Sometimes the programmer does not know exactly how many times the loop will execute
Can receive range inputs from the user, place them in variables, and call the range function in the for clause using these variables
Be sure to consider the end cases: range does not include the ending limit

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Generating an Iterable Sequence that Ranges from Highest to Lowest

The range function can be used to generate a sequence with numbers in descending order
Make sure starting number is larger than end limit, and step value is negative

Example: range (10, 0, -1)

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Calculating a Running Total

Programs often need to calculate a total of a series of numbers
Typically include two elements:

A loop that reads each number in series

An accumulator variable

Known as program that keeps a running total: accumulates total and reads in series

At end of loop, accumulator will reference the total

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Calculating a Running Total (cont’d.)

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The Augmented Assignment Operators

In many assignment statements, the variable on the left side of the = operator also appears on the right side of the = operator
Augmented assignment operators: special set of operators designed for this type of job
Shorthand operators

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The Augmented Assignment Operators (cont’d.)

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Sentinels

Sentinel: special value that marks the end of a sequence of items
When program reaches a sentinel, it knows that the end of the sequence of items was reached, and the loop terminates

Must be distinctive enough so as not to be mistaken for a regular value in the sequence

Example: when reading an input file, empty line can be used as a sentinel

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Input Validation Loops

Computer cannot tell the difference between good data and bad data
If user provides bad input, program will produce bad output

GIGO: garbage in, garbage out

It is important to design program such that bad input is never accepted

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Input Validation Loops (cont’d.)

Input validation: inspecting input before it is processed by the program
If input is invalid, prompt user to enter correct data

Commonly accomplished using a while loop which repeats as long as the input is bad

If input is bad, display error message and receive another set of data

If input is good, continue to process the input

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Input Validation Loops (cont’d.)

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Nested Loops

Nested loop: loop that is contained inside another loop
Example: analog clock works like a nested loop

Hours hand moves once for every twelve movements of the minutes hand: for each iteration of the “hours,” do twelve iterations of “minutes”

Seconds hand moves 60 times for each movement of the minutes hand: for each iteration of “minutes,” do 60 iterations of “seconds”

Copyright © 2018 Pearson Education, Inc.

Copyright © 2018 Pearson Education, Inc.

Nested Loops (cont’d.)

Key points about nested loops:
Inner loop goes through all of its iterations for each iteration of outer loop

Inner loops complete their iterations faster than outer loops

Total number of iterations in nested loop: number_iterations_inner x

number_iterations_outer

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Turtle Graphics: Using Loops to Draw Designs

You can use loops with the turtle to draw both simple shapes and elaborate designs. For example, the following for loop iterates four times to draw a square that is 100 pixels wide:
for x in range(4):

turtle.forward(100)

turtle.right(90)

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Turtle Graphics: Using Loops to Draw Designs

This for loop iterates eight times to draw the octagon:
for x in range(8):

turtle.forward(100)

turtle.right(45)

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Turtle Graphics: Using Loops to Draw Designs

You can create interesting designs by repeatedly drawing a simple shape, with the turtle tilted at a slightly different angle each time it draws the shape.
NUM_CIRCLES = 36 # Number of circles to draw

RADIUS = 100 # Radius of each circle

ANGLE = 10 # Angle to turn

for x in range(NUM_CIRCLES):

turtle.circle(RADIUS)

turtle.left(ANGLE)

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Turtle Graphics: Using Loops to Draw Designs

This code draws a sequence of 36 straight lines to make a "starburst" design.
START_X = -200 # Starting X coordinate

START_Y = 0 # Starting Y coordinate

NUM_LINES = 36 # Number of lines to draw

LINE_LENGTH = 400 # Length of each line

ANGLE = 170 # Angle to turn

turtle.hideturtle()

turtle.penup()

turtle.goto(START_X, START_Y)

turtle.pendown()

for x in range(NUM_LINES):

turtle.forward(LINE_LENGTH)

turtle.left(ANGLE)

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Summary

This chapter covered:
Repetition structures, including:

Condition-controlled loops

Count-controlled loops

Nested loops

Infinite loops and how they can be avoided

range function as used in for loops

Calculating a running total and augmented assignment operators

Use of sentinels to terminate loops

Using loops to draw turtle graphic designs