Looping (Iteration)

CS 106

Graphical Examples

Exercise 1: Star

Write a turtle program that draws a 5-pointed star. Use a loop.

Turtle template code:

import random
import turtle

window = turtle.Screen()
window.bgcolor('black')
pen = turtle.Turtle()
pen.color('white')

Exercise 2: Colored Star

Write a turtle program that draws a 5-pointed star. Loop over the following list of colors for the sides: ['red', 'green', 'purple', 'blue', 'yellow']

Tip

Use a wider pen so it’s easier to see:

pen.width(10)

Exercise 3: Starfield

Write a turtle program that draws 50 5-pointed stars at random positions on the screen.

Use the following code to get positions:

x = random.uniform(-200, 200)
y = random.uniform(-150, 150)
pen.goto(x, y)

Tip

Add pen.speed('fastest') before your loop to speed up the drawing.

Exercise 4: Gradually moving outward

  1. Start a variable called distance at 0.
  2. Do the following 500 times:
    1. move forward by distance
    2. turn right 90 degrees
    3. increase distance by 2

Refinement: Instead of “Do the following 500 times”, do the following as long as pen.distance(0, 0) is less than 500.

Exercise 5: Gradually coloring outward

  1. Start a variable called length at 0.
  2. Do the following as long as pen.distance(0, 0) is less than 500:
    1. for each color in a list of colors:
      1. set the pen color to that color
      2. move forward by length
      3. turn right some amount
      4. increase length by 1

Exercise 6: Rotated Squares

  1. Draw n=6 rotated squares. (turn 360/n degrees between each.)
  2. Now try n=50.
  3. Same, but use a different color for each square. Cycle the colors.

Example: colors = [‘red’, ‘black’, ‘blue’]

while loops

while loops

A while loop executes a statement based on a boolean condition.

# Before
while condition:
    statements
# After

graph TD
    A[Before] --> B{Condition}
    B -- True --> C[Statements]
    C --> B
    B -- False --> D[After]

Analogy: while there are dishes in the sink, wash another dish.

Example

x = int(input("Enter a positive number: "))
while x <= 0:
    print("Try again")
    x = int(input("Enter a positive number: "))

print("Good job!")

Exercise

What will this output?

x = 6
while x > 4:
      print(x, end=' ')
      x = x - 1
  • 6 5
  • 6 5 4
  • 6 5 4 3
  • 5 4 3

A Very Welcoming Program

name = input("What is your name? ")
while True:
    print("Hello, " + name + "!")

To get out of this infinite loop, press Ctrl+C in the Shell, or press the stop button in the toolbar.

Which number will stop the loop?

valid = False
while not valid: 
    x = int(input("Enter a number: "))
    valid = (x%2 == 1 and x%3 == 0)
  • 2
  • 6
  • 9
  • None of the above

Terminology

  • Loop body: The statements that are executed each time through the loop.
  • Loop condition: The boolean expression that determines whether the loop body should be executed again.
  • Loop variable: A variable that changes each time through the loop.
    • Often used to control the loop condition.

Mid-loop test

In an otherwise infinite loop, use break to exit:

while True:
    statemets
    if condition:
        break
    more_statements

Either of the statements blocks may be empty.

Converting to a mid-loop test

Before:

valid = False
while not valid: 
    x = int(input("Enter a number: "))
    valid = (x%2 == 1 and x%3 == 0)

After:

while True:
    x = int(input("Enter a number: "))
    if x%2 == 1 and x%3 == 0:
        break

Another example

How would you convert this to a mid-loop test?

x = int(input("Enter a positive number: "))
while x <= 0:
    print("Try again")
    x = int(input("Enter a positive number: "))

print("Good job!")

Solution:

while True:
    x = int(input("Enter a positive number: "))
    if x > 0:
        break
    print("Try again")

print("Good job!")

We don’t have to repeat the input statement.

for loops

for loops

for variable in sequence:
    statements

A Python for loop iterates over a sequence of values:

  • str: each character
  • list, tuple, set: each element
  • range: each number
  • dict: each key

What is a good description?

s = input("Please enter a string:")
new_s = ''
for c in s:
    new_s = c + new_s
  • new_s is a copy of s
  • new_s is the reverse of s
  • new_s is the final character of s
  • new_s is the first character of s

What is a good description?

s = input("Please enter a string: ")
new_s = ''
for char in s:
    if char != ' ':
        new_s = new_s + char
print(new_s)
  • new_s is a copy of s
  • new_s is the spaces of s
  • new_s is s with no spaces
  • new_s is s with spaces between every character

range function

range function

range(start, stop, step)
range(start, stop) # step=1
range(stop) # start=0, step=1
  • start: first number
  • stop: up to but not including (“stop before”)
  • step: increment

Often used with for loops.

What values will idx take?

  • for idx in range(7):
  • for idx in range(1, 7):
  • for idx in range(0, 7, 2):
  • for idx in range(13, 10, -1):

Which will add numbers between 1 and 4?

A:

for i in range(1,5):
    sum = 0
    sum = sum + i

B:

sum = 0
for i in range(1,5):
    sum = sum + sum

C:

sum = 0
for i in range(1,5):
    sum = sum + 1

D:

sum = 0
for i in range(1,5):
    sum = sum + i

Iterating by index vs value

By value:

s = 'Hi!'
for c in s:
    print(c)

By index:

s = 'Hi!'
length = len(s)
for i in range(length):
    c = s[i]
    print(c)