Designing While Loops

1. Designing While Loops CSIS 1595: Fundamentals of Programming and Problem Solving 1

2. Validation Loops • Prompt user for value while value invalid • Basic structure: prompt for initial value of variable while condition which is true if not valid explain error and prompt again code using that variable

3. Validation in Tax Program • Use loops to insure income, dependents non-negative • Note no longer separate branch for valid inputs

4. Sentinel Loops • Goal: Continue some action until user wants to quit • Example: Converting Fahrenheit to Celsius until user wants to quit

5. Sentinel Loops • Sentinel = variable that stores “quit”/”continue” value • Basic structure: set initial sentinel value to “continue” while sentinel != “continue” value do things inside loop prompt user whether to do loop again set sentinel to “quit” if they don’t

6. Sentinel Example • Sentinel: “yes” or “no” value entered by user • Prompt at end of loop: “Do you want to convert another temperature (yes or no)?” • Always tell user what the expected values are! • Condition: while sentinel == “yes” • Initialization: set sentinel to “yes” before loop • Assumption: user will want to convert at least one temperature

7. Sentinel Example

8. Menu Example • Often use sentinels in menu driven loops • User repeatedly chooses option from menu • One option is quit • Idea: Use menu choice as sentinel • Algorithm: initialize choice to something besides “quit” while choice != quit prompt for choice if/elif’s for non-quit choices else error message if not quit

9. Menu Example

10. Counting Inside Loop • May need to keep track of how many times loop runs • Example: number of guesses in guessing game • Use counter variable to keep track of times • Increment variable each time through loop • counter_variable+= 1 • Initialize before start of loop • Use counter variable inside/after loop as needed

11. Guessing Game Example • Use counter variable to keep track of times • Call it guesses • Increment variable each time through loop • guesses+= 1 • Initialize before start of loop • Set to 1 since that will be the number of guesses if the user enters the correct number the first time • Use counter variable inside/after loop as needed • Print the number of guesses after the loop

12. Guessing Game Example

13. Loops for Searching • Example: Finding which two integers a square root is between • Algorithm: • Prompt user for number and keep squaring increasingly higher integers until find one greater than that number • Example: number = 11 • 12 < 11 • 22< 11 • 32< 11 • 42 >= 17 • “Square root of 11 is between 3 and 4

14. Search Loop Example

15. Search Loop Ideas • Decide what you are searching for, and how you will know you have found it • Use a loop to successively narrow the range where the value can be found • Example: How would you guess a number between 1 and 100? • Where would the search start? • How would information that guess too high/too low change the range of the search?