Introduction to Repetition Structures and Looping in Programming

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 5. 1 2 3 4 5

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 9. 1 3 5 7 9

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) Output 0 1 2 3 4 Same as for num in [0, 1, 2, 3, 4]: print(num)

17. Range Examples for num in range (1,5): print(num) Output 1 2 3 4 for num in range (1,10,2): print(num) Output 1 3 5 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 = 0.6214 # 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 -------------- 60 37.3 70 43.5 80 49.7 90 55.9 100 62.1 110 68.4 120 74.6 130 80.8

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 ------------------- 1 1 2 4 3 9 4 16 5 25

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)