2. Topics Introduction to Repetition Structures
The while Loop: a Condition-Controlled Loop
The for Loop: a Count-Controlled Loop
Calculating a Running Total

3. 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 code
Includes condition-controlled loops and count-controlled loops

4. The while Loop while loop: while condition is true, do something
Two parts:
Condition tested (just like in If/else)
Set of Statements repeated as long as condition is true
General format:
while condition:
statements
Notice
Indentation like If statement.
while statement ends when indentation ends
‘:’ after condition

5. The while Loop Flow Control

6. The while Loop
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 at start
Requires performing some steps prior to the loop

8. Commission Program
# This program calculates sales commissions. keep_going = 'y‘ # Create a variable to control the loop. # Calculate a series of commissions. while keep_going == 'y': sales = float(input('Enter the amount of sales: ')) comm_rate = float(input('Enter the commission rate: ')) commission = sales * comm_rate # Calculate the commission. # Display the commission. print('The commission is $', format(commission, ',.2f'), sep='') # See if the user wants to do another one. keep_going = input('Do you want to calculate another ' + \ 'commission (Enter y for yes): ')

9. 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 does not set the loop condition to be false

10. The for Loop
Count-Controlled loop: performs loop 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

12. Simple Loop With List of Numbers
print('I will display the numbers 1 through 5.') for num in [1, 2, 3, 4, 5]: print(num) Output I will display the numbers 1 through

13. Simple Loop With List of Odd Numbers
print('I will display the odd numbers 1 through 9.') for num in [1, 3, 5, 7, 9]: print(num) Output I will display the odd numbers 1 through

14. Simple Loop With List of Strings
for name in ['Winken', 'Blinken', 'Nod']: print(name) Output Winken Blinken Nod

15. Using range Function in for Loop
Simplifies the process of writing a for loop
range returns an iterable object
Iterable: contains a sequence of values that can be iterated over
For num in range (arg1, [arg2, [arg3]])
statements
range characteristics:
One argument: used as ending limit
Two arguments: starting value and ending limit
Three arguments: third argument is step value

16. Range Example For num in range (5): print(num) Same as
Output 1 2 3 4
Same as
For num in [0, 1, 2, 3, 4]:

17. Range Examples
For num in range (1,5): print(num) Output For num in range (1,10,2): 7 9

18. Using Target Variable Inside 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
Ending value will not be included in loop since that is when loop stops

19. Speed Converter Example
start_speed = 60 # Starting speed end_speed = 131 # Ending speed increment = 10 # Speed increment conversion_factor = # Km/hr to MPH conversion factor # Print the table headings. print('KPH\tMPH') print(' ') # Print the speeds. for kph in range(start_speed, end_speed, increment): mph = kph * conversion_factor print(kph, '\t', format(mph, '.1f'))

20. Speed Converter Output
KPH MPH

21. Letting User Control 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 range function in for clause using these variables
Be sure to consider the end cases: range does not include the ending limit

22. Find Squares Example
# Get the ending limit. print('This program displays a list of numbers') print('(starting at 1) and their squares.') end = int(input('How high should I go? ')) # Print the table headings. print() print('Number\tSquare') print(' ') # Print the numbers and their squares. for number in range(1, end + 1): # Add 1 since loop stops at end square = number**2 print(number, '\t', square)

23. Find Squares Output
This program displays a list of numbers (starting at 1) and their squares. How high should I go? 5 Number Square

24. 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)