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

2. Review: Arrays (and Matrices)
Recall our view of variables as locations in memory..
If we typed…
>> HisAge = 25
HisAge = 25
HisAge 3
inf 25 23 48
Nan 2 -2

3. Review: Arrays (and Matrices)
We can have arrays (vectors) of variables instead
If we type…
EDU>> KidsAge = [15, 19, 4 11]
KidsAge =
KidsAge 3
inf 15 19 4 11 2 -2 23

4. 3 inf 15 19 4 11 2 -2 23 KidsAge(1) KidsAge
Review:
In order to tell matlab which of the four values we are interested in, we must index the variables
EDU>> KidsAge(1)
ans = 15
EDU>> KidsAge(4) 11
EDU>> KidsAge(2) 19
KidsAge(1)
This is the index, or the subscript of the variable KidsAge. We can pronounce it as:
KidsAge at 1 Or
KidsAge sub 1
KidsAge 3
inf 15 19 4 11 2 -2 23

5. Review:
We can use an index to change values. Suppose we realized we had made a mistake, and the second child is really 18…
EDU>> KidsAge(2) = 18
KidsAge =
KidsAge 3
inf 15 18 4 11 2 -2 23

6. Review: Indexed Variables and Looping
The real power of indexed variables comes from use them with loops (and vice versa)
For the rest of the quarter, virtually everything we do will involve these two things.

7. Let us write a function to test if an Array has at least one odd number12 18 4 11
AtLeastOneOdd 1

8. function HasOdd = AtLeastOneOdd(InArray) for i = 1 : length(InArray) i
end
EDU>> testme = [ ]
testme =
EDU>> AtLeastOneOdd(testme)
i = 1 2 3 4
As always, we will build up to the task, starting with the simplest subset of the problem.
Let us just count from one to the size of the array…

9. function HasOdd = AtLeastOneOdd(InArray) for i = 1 : length(InArray)
if IsOdd( InArray(i) )
disp('found an odd, when i was ')
disp(i)
end
Let us display to the screen if we find an odd…
Works, but we want to return true/false, not display anything (next slide)
EDU>> testme = [ ]
testme =
EDU>> AtLeastOneOdd(testme)
found an odd, when i was 3

10. function HasOdd = AtLeastOneOdd(InArray)
HasOdd = 0; % Assume there is no odd
for i = 1 : length(InArray)
if IsOdd( InArray(i) )
HasOdd = 1; % Change assumption
end
EDU>> AtLeastOneOdd(testme)
ans = 1
EDU>> AtLeastOneOdd([ ])
EDU>> AtLeastOneOdd([ ])
We make an assumption, and give the for loop every chance to change it

11. function HasOdd = AtLeastOneOdd(InArray) % This code is wrong
for i = 1 : length(InArray)
if IsOdd( InArray(i) )
HasOdd = 1;
elseif
HasOdd = 0;
end
This code is a common mistake, it will return true or false based only on the LAST element in the array.

12. Let us write a function to count the number of odd numbers in an Array12 7 4 11
NumberOfOdds 2

13. function HasOdd = AtLeastOneOdd(InArray)
HasOdd = 0; % Assume there is no odd
for i = 1 : length(InArray)
if IsOdd( InArray(i) )
HasOdd = 1; % Change assumption
end
Start with working related code.
function OddCount = NumberOfOdds(InArray)
HasOdd = 0; % Assume there is no odd
for i = 1 : length(InArray)
if IsOdd( InArray(i) )
HasOdd = 1; % Change assumption
end
Begin by renaming…

14. function OddCount = NumberOfOdds(InArray)
OddCount = 0; % Assume there is no odd, so count is 0
for i = 1 : length(InArray)
if IsOdd( InArray(i) )
OddCount = OddCount + 1; % Change assumption
end
EDU>> NumberOfOdds([ ])
ans = 1
EDU>> NumberOfOdds([ ]) 2
EDU>> NumberOfOdds([ ]) 5

15. Let us write a function to reverse an Array
We have…
KidsAge 3
inf 15 18 4 11 2 -2 23
We want…
KidsAge 3
inf 11 4 18 15 2 -2 23

16. 15 18 4 11 ReverseArray 11 4 18 15 EDU>> KidsAge KidsAge =
EDU>> ReverseArray(KidsAge)
ans =

17. 15 18 4 11
OutArray 15 18 4 11 15
OutArray 15 18 4 11 18 15
OutArray 15 18 4 11 4 18 15
OutArray

18. 15 18 4 11 15
OutArray
The length of the array is 4 1 2 3 4
4-4+1
4-3+1
4-2+1
4-1+1

19. function OutArray = ReverseArray(InArray) for i = 1 : length(InArray)
end
EDU>> ReverseArray(KidsAge)
i = 1 2 3 4
As always, we will build up to the task, starting with the simplest subset of the problem.
Let us just count from one to the size of the array…

20. function OutArray = ReverseArray(InArray) for i = 1 : length(InArray)
InArray(i)
end
EDU>> ReverseArray(KidsAge)
ans = 15 18 4 11
Let us list the items in the array…

21. function OutArray = ReverseArray(InArray) for i = 1 : length(InArray)
InArray(i)
length(InArray)-i + 1
end
EDU>> ReverseArray(KidsAge)
ans = 15 4 18 3 2 11 1
First time thru the loop
Let us list the items in the array…
And count backwards from the size of the array
Second time thru the loop
Third time thru the loop

22. function OutArray = ReverseArray(InArray) for i = 1 : length(InArray)
OutArray( length(InArray)-i + 1) = InArray(i);
end
EDU>> KidsAge
KidsAge =
EDU>> ReverseArray(KidsAge)
ans =
It works!
Here we have not changed KidsAge, to do that (next slide)

23. EDU>> KidsAge
KidsAge =
EDU>> KidsAge = ReverseArray(KidsAge)
To change KidsAge we need to assign the reversed array to KidsAge