1. Eamonn Keogh eamonn@cs.ucr.edu
CS005 Introduction to Programming: Matlab
Eamonn Keogh

3. Review: Incrementing a Variable
Adding one to a variable is called incrementing
It is an important trick we use a lot with loops
We do it by…
>> HisAge = 25; % Give some value
>> HisAge = HisAge + 1; % Increment HisAge
Before incrementing
HisAge 3
inf 25 18 4
After incrementing
HisAge 3
inf 26 18 4

4. Review: Incrementing a Variable
Subtracting one from a variable is called decrementing
We do it by…
>> HisAge = 25; % Give some value
>> HisAge = HisAge - 1; % Increment HisAge
We can increment by more than one, for example, increment by five
>> HisAge = HisAge + 5; % Increment HisAge
But if we say just increment, we mean one

5. Review: We have seen how to use the for loop
We can repeat a code block multiple times with the for statement
for index = values
do these statements
end
Review: We have seen how to use the for loop

6. Review: We have seen how to use the for loop
Start with i at this value
Stop when i hits this value
for i = 1 : 10
do these statements
end
Make i one larger when you get here
Review: We have seen how to use the for loop

7. We can repeat a code block multiple times with the while statement
while expression
do these statements
end
What is the difference between a for loop and a while loop?
Informally
If we know just before we get to the loop, how often it will run, we want a for loop
If we don’t know just before we get to the loop, how often it will run, we want a while loop

8. % This is a script called DemoWhileEamonn i = 1; % Initialize i
while i <= 10
disp(i); % Display i
i = i + 1; % Increment i
end
EDU>> DemoWhileEamonn 1 2 3 4 5 6 7 8 9 10

9. i = 1; % Initialize i while i <= 10 disp(i);
disp(' times five is ');
disp(i * 5);
disp(' ');
i = i + 1; % Increment i
end
EDU>> DemoWhileEamonn 1
times five is 5 2 10
Anything we did with a for loop, we could do with a while loop. However, we are responsible for incrementing the index variable

10. % I forgot it increment the index variable!!! end
i = 1; % Initialize i
while i <= 10
disp(i);
disp(' times five is ');
disp(i * 5);
disp(' ');
% I forgot it increment the index variable!!!
end
EDU>> DemoWhileEamonn 1
times five is 5
(a few hundred years from now…)
e+017
e+018
We are responsible for incrementing the index variable.
Failing to do so, will produce an infinite loop

11. EDU>> TaxPayersAge = GetUsersAge(); Enter your age : 34
EDU>> TaxOwed = 1000/ TaxPayersAge
TaxOwed =
Let us imagine that a tax law requires every adult to pay a charge equal to your $1000 divided by your age.
The code above shows what the code we write might look like…
We just need to write the GetUsersAge function…

12. function UsersAge = GetUsersAge()
UsersAge = input('Enter your age : ');
end
This function works, but it is brittle.
It allows the user to enter values that are impossible, and these strange values can trickle up the rest of the code and produce strange results…
EDU>> TaxPayersAge = GetUsersAge();
Enter your age : 0
EDU>> TaxOwed = 1000/ TaxPayersAge
TaxOwed =
Inf
EDU>> TaxPayersAge = GetUsersAge();
Enter your age :
EDU>> TaxOwed = 1000/ TaxPayersAge
TaxOwed =
-1000

13. The solution is to write a robust function that will get a reasonable value.
while loops are perfect for this….
The basic idea is to give the user an initial chance to enter his/her age, and while the value is unacceptable, ask again….

14. function UsersAge = RoubustGetUsersAge()
UsersAge = input('Enter your age : ');
while UsersAge < 18
UsersAge = input('You made a mistake, Enter your age : ');
end
EDU>> TaxPayersAge = RoubustGetUsersAge()
Enter your age : 55
TaxPayersAge = 55
EDU>> TaxPayersAge = RoubustGetUsersAge()
Enter your age : 7
You made a mistake, Enter your age : 9
You made a mistake, Enter your age : 22
TaxPayersAge = 22

15. function UsersAge = RoubustGetUsersAge() % Do NOT use
UsersAge = input('Enter your age : ');
while UsersAge < 18
Age = input('You made a mistake, Enter your age : ');
end
This function has a bug.
We must always make sure there is a way out of the while loop
EDU>> TaxPayersAge = RoubustGetUsersAge()
Enter your age : 2
You made a mistake, Enter your age : 21
You made a mistake, Enter your age : 22
You made a mistake, Enter your age : 222
You made a mistake, Enter your age : 21112
You made a mistake, Enter your age : stop!!

16. Let us write a script that converts numbers from inches to millimeters, while the user wants to continue..
We need a way for the user to signal that he/she is finished, let us use a negative number to signal this…
EDU>> inches2mm
I will convert inches to mm for you.
Enter number to convert or negative number to quit : 2 2
inches is mm
Enter number to convert or negative number to quit : -1
Thank you for using my program
Sidenote
Matlab functions/scripts that convert one thing to another are usually called something2somethingelse
str2num
hex2num
cart2compass

17. Enter number to convert : 2 2 inches is 50.8000 mm
NumToConvert = input('Enter number to convert :');
disp(NumToConvert);
disp(' inches is ');
disp(NumToConvert * 25.4)
disp(' mm')
EDU>> inches2mm
Enter number to convert : 2 2
inches is mm
We will slowly build up to our full script

18. The last time through the loop it converts the exit value
NumToConvert = 0; % Initialize to make sure loop happens at least once
disp('I will convert inches to mm for you.')
while NumToConvert >= 0
NumToConvert = input('Enter number to convert or negative number to quit : ');
disp(NumToConvert);
disp(' inches is ');
disp(NumToConvert * 25.4)
disp(' mm')
end
disp('Thank you for using my program')
EDU>> inches2mm
I will convert inches to mm for you.
Enter number to convert or negative number to quit : 2 2
inches is mm
Enter number to convert or negative number to quit : -1 -1
Thank you for using my program
This function has a bug.
The last time through the loop it converts the exit value

19. NumToConvert = 0; % Initialize to make sure loop happens at least once
disp('I will convert inches to mm for you.')
while NumToConvert >= 0
NumToConvert = input('Enter number to convert or negative number to quit : ');
if NumToConvert >= 0 % User wants to convert this value
disp(NumToConvert);
disp(' inches is ');
disp(NumToConvert * 25.4)
disp(' mm')
end
disp('Thank you for using my program')
This fixes it…
EDU>> inches2mm
I will convert inches to mm for you.
Enter number to convert or negative number to quit : 2 2
inches is mm
Enter number to convert or negative number to quit : -1
Thank you for using my program

20. This code is slightly more elegant and readable
NumToConvert = 0; % Intialize to make sure loop happens at least once
disp('I will convert inches to mm for you.')
while NumToConvert >= 0
NumToConvert = input('Enter number to convert or negative number to quit : ');
if NumToConvert >= % User wants to convert this value
disp(NumToConvert);
disp(' inches is ');
disp(NumToConvert * 25.4)
disp(' mm')
else % User wants to quit
disp('Thank you for using my program')
end
EDU>> inches2mm
I will convert inches to mm for you.
Enter number to convert or negative number to quit : 2 2
inches is mm
Enter number to convert or negative number to quit : -1
Thank you for using my program
This code is slightly more elegant and readable

21. GetAgesOfTeamPlayers
Let us write a function that gets the ages of players in a team.
We have done this before, but we assumed we knew how many players there are ahead of time. However, this time we assume we don’t know. So we will keep adding ages while there are more players
GetAgesOfTeamPlayers
[ ]

22. We will slowly build up to our full function.
function ArrayOfTeamAges = GetAgesOfTeamPlayers()
temp = input('How old? Type -1 to finish : '); % Prompt the user
if temp > % He wants to add a player
disp('this means process age ')
else
disp('this means quit') % User has signaled to quit
end
EDU>> GetAgesOfTeamPlayers()
How old? Type -1 to finish : 4
this means process age
How old? Type -1 to finish : -1
this is quit
We will slowly build up to our full function.

23. We will slowly build up to our full function.
function ArrayOfTeamAges = GetAgesOfTeamPlayers()
temp = input('How old? Type -1 to finish : '); % Prompt the user
if temp > % He wants to add a player
disp('this means process age ')
ArrayOfTeamAges(1) = temp; % Add the age to array
else
disp('this means quit') % User has signaled to quit
end
EDU>> GetAgesOfTeamPlayers()
How old? Type -1 to finish : 56
this means process age
ans = 56
We will slowly build up to our full function.

24. function ArrayOfTeamAges = GetAgesOfTeamPlayers()
PlayerID = 1; % Start with player 1
while PlayerID > % Check if there is another player to add
temp = input('How old? Type -1 to finish : '); % Prompt the user
if temp > % He wants to add a player
ArrayOfTeamAges(PlayerID) = temp; % Add the age to array
PlayerID = PlayerID + 1; % Increment to next player
else
PlayerID = ; % User has signaled to quit
end
EDU>> GetAgesOfTeamPlayers()
How old? Type -1 to finish : 34
How old? Type -1 to finish : 51
How old? Type -1 to finish : -1
ans =
EDU>>