Basic Problem Solving with Programming – Iteration (or loops)

1. Basic Problem Solving with Programming – Iteration (or loops)

2. What is a loop? • A loop is a repetition control structure • It causes a single statement or block to be executed repeatedly while an expression is true

3. When use a Loop? Stated simply, one should use a loop at any point where you wish to repeat a process, idea, or function. Example: Write a small program that will display the numbers 1 – 100 and their squares.

4. Loop Usage A. Solve the equation 2x2 + x + 5 for all integer x between 5 and 10 B. Sum inputted integers until the user enters -1 C. The user enters in the current year and then his/her birth year. Your program computes the users age. Perform this task again if he or she wishes.

5. Two types of loops count-controlled loops repeat a specified number of times event-controlled loops something happens inside the loop body that causes the repetition to stop after the current iteration completes

6. Count-controlled • The problem describes how many times the iteration is to executeOR • The problem indicates a starting value, ending value and increment for the loop control value. • Sometime the loop control value is needed for some calculation within the block of the loop, sometimes not.

7. Event-controlled • Sentinel controlled - Keep processing data until a special value which is not a possible data value is entered to indicate that processing should stop • End-of-file controlled - Keep processing data as long as there is more data in the file • Flag controlled - Keep processing data until the value of a flag changes in the loop body

8. Sentinel-controlled loop • Requires a “priming read” • “Priming read” means you read one data value (or set of data values) before entering the while loop • Process data value(s) and then read next value(s) at end of loop

9. Flag-controlled loops • Use meaningful name for the flag • Initialize flag (to true or false) • Test the flag in the loop test expression • Change the value of the flag in loop body when the appropriate condition occurs

10. Nested loops • Some problems require iteration structures to be nested • Examples: • Process data in rows and columns of a table, outer loop is for one row at a time, inner loop is for each column in the current row • Outer loop is to allow the user to “play again” or “re-enter data”, inner loop is to do some iterative processing for each outer

11. Loop Testing & Debugging • Test data should test all sections of the program • Beware of infinite loops - the program doesn’t stop • Check loop termination condition, and watch for an OBOB (off-by-1 bug) • Use algorithm walk-through to verify that appropriate conditions occur in the right places • Trace execution of loop by hand with code walk-through • Use a debugger to run program in “slow motion” or use debug output statements

12. Scratch iteration control structures • forever • forever if (condition) • repeat until (condition) • repeat (number of times)