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

Start a variable called distance at 0.
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

Start a variable called length at 0.
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

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

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

`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 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(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?

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

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

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

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)

Print each character in a string

Convert this code to use a for loop:

idx = 0
while idx < len(a_str):
    print(a_str[idx])
    idx += 1

The Accumulator Pattern

Accumulator: A variable that is incrementally updated to compute a final result.
Initialize step: Set the accumulator to an initial value: what the result should be if there are no items to process.
Update step: Change the accumulator based on each item in a sequence.
Accumulator pattern:

accumulator = initial_value  # Initialize
for item in sequence: # or while loop
    if condition:  # optional
        accumulator = new_value  # Update

Example: Sum of numbers 1 through 5

Accumulator: total
Initialize: total = 0
Update: total = total + i for each number i from 1 to 5

total = 0  # Initialize
for i in range(1, 6):  # 1, 2, 3, 4, 5
    total = total + i  # Update
print(total)  # 15

Count how many `a` chars in string `s`

Accumulator: count
Initialize: count = 0
Update: count = count + 1 if character is ‘a’

count = 0  # Initialize
for c in s:
    if c == 'a':  # Condition
        count = count + 1  # Update
print(count)

Find largest number in a list `numbers`

Accumulator: largest
Initialize: largest = None (no largest yet)
Update: if current number n is larger than largest, set largest = n

largest = None  # Initialize
for n in numbers:
    if largest is None or n > largest:  # Condition
        largest = n  # Update
print(largest)

Summary

Two different kinds of loops:

while loop: repeat while a condition is true
for loop: repeat for each item in a sequence

for loops can iterate over two different kinds of sequences:

str, list, tuple, set, dict: containers
range: numbers

Accumulator pattern: initialize an accumulator variable, then update it in a loop.

Looping (Iteration)

Graphical Examples

Exercise 1: Star

Exercise 2: Colored Star

Exercise 3: Starfield

Exercise 4: Gradually moving outward

Exercise 5: Gradually coloring outward

Exercise 6: Rotated Squares

while loops

while loops

Example

Exercise

A Very Welcoming Program

Which number will stop the loop?

Terminology

Mid-loop test

Converting to a mid-loop test

Another example

for loops

for loops

What is a good description?

What is a good description?

range function

range function

What values will idx take?

Which will add numbers between 1 and 4?

Iterating by index vs value

Print each character in a string

The Accumulator Pattern

The Accumulator Pattern

Example: Sum of numbers 1 through 5

Count how many a chars in string s

Find largest number in a list numbers

Summary

`while` loops

`while` loops

`for` loops

`for` loops

`range` function

`range` function

What values will `idx` take?

Count how many `a` chars in string `s`

Find largest number in a list `numbers`