1. Week 8 - Programming II Today – more features: Loop control
Extending if/else
Nesting of loops
Debugging tools
Textbook chapter 7, pages
(sections 7.2.3, 7.5, 7.6 )

2. Loop Controls
Loops contain sets of commands that you want to do repeatedly.
You might want to:
Skip commands in the current iteration
Stop the loop itself
Why continue once you’ve found what you’re looking for !!

3. Skipping Ahead: Continue
Continue – jumps to next loop iteration:
for k = 1:25000
if x(k) > 0
continue
end
{ more commands }
skip ahead

4. Early Termination: Break
Break – ends the loop:
for variable = {array of length n}
…..
break
end
go to commands beyond end

5. Example – calculating interest until the amount doubles using a for loop:
will calculate up to 1000
years, if necessary
if condition decides
when to terminate loop

6. only needed 10 years

7. Example – accept input, appending it to vector, until a negative number is entered:
allow up to 1000
values, if necessary

8. negative value stops the input

9. provide hi/lo feedback
Example – Hi-Lo: a guessing game with feedback
select hidden number
input guess
correct?
yes no
provide hi/lo feedback
5 tries?
win
lose

11. Extensions of if/else if expression
As introduced, if/else allows for two choices:
if expression
{commands if expression is true }
else
{commands if false }
end

12. What if there are more than 2 situations?
find the largest of 3 variables a, b, c
a ≥ b ≥ c a ≥ c ≥ b
b ≥ a ≥ c b ≥ c ≥ a
c ≥ b ≥ a c ≥ a ≥ b
4 situations:
convert a compass angle to a direction:
0º  east
90º  north
180º  west
270º  south

13. Could use “nested” if/else commands

14. or

15. if expression1 The “elseif” command {commands if expression1 is true }
elseif expression2
{commands if expression2 is true }
else
{commands if both expressions are false }
end

16. Examples:
Note – many elseifs
are allowed,
but only 1 “else”

17. Loops within Loops – Nesting
for index1 = array1
{outer loop commands}
for index2 = array2
{inner loop commands}
end
{more outer loop commands}
can be more than 2 levels deep

18. Variable values by example
index1
index2
Variable values by example

19. Example – computing a table of z = x2+y2 for x and y equal to the integers 1, 2,…6:

20. Example – matching of people’s skills and tasks:
Job 1
Job 2
Job 3
Job 4
Joe 7 4 2
Sue 6 8 5
Bob 1 3
Liz
Situation:
4 tasks
4 people with
different skills to do
them
Skill table as shown
Goal – assign tasks to
maximize the sum
Example solution of 20

21. Solution – use nested loops to try all combinations, skipping repeats:
First, let’s initialize variables:
Job 1 2 3 4
Joe 7
Sue 6 8 5
Bob
Liz

22. Next, start nested loops:
Check for repeats and skip

23. Test a valid assignment for quality:
And then terminate the 4 for loops:

24. Job 1
Job 2
Job 3
Job 4
Joe 7 4 2
Sue 6 8 5
Bob 1 3
Liz
The result:

25. Debugging Tools
so far, Run

26. Debugging ≡ finding and correcting errors (bugs) in programs
Useful debugging tools:
Ability to stop a program in the middle of its execution (at a breakpoint)
Ability to examine variable values at that point
Ability to modify variable values at that point

27. controls for
creating and
removing
breakpoints

28. indicator of breakpoint location (can have multiple breakpoints)

29. What shows up at the breakpoint
Command window:
Editor window:
different
prompt
location indicator

30. Can single step (F10) or continue (F5) to the next breakpoint (or end)