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Eamonn Keogh eamonn@cs.ucr

CS005 Introduction to Programming: Matlab. Eamonn Keogh eamonn@cs.ucr.edu. Swapping two numbers. EDU>> HisAge = 25; EDU>> HerAge = 48;. Let us say we have assigned two variables, HisAge and HerAge …

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Eamonn Keogh eamonn@cs.ucr

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

  2. Swapping two numbers EDU>> HisAge = 25; EDU>> HerAge = 48; Let us say we have assigned two variables, HisAge and HerAge… However we realize that we have made a mistake, we gave the wrong ages to the wrong person…

  3. Swapping two numbers EDU>> HisAge = 25; EDU>> HerAge = 48; EDU>> HerAge = HisAge; EDU>> HisAge = HerAge; We can just swap them….

  4. Swapping two numbers EDU>> HisAge = 25; EDU>> HerAge = 48; EDU>> HerAge = HisAge; EDU>> HisAge = HerAge; EDU>> HerAge HerAge = 25 EDU>> HisAge HisAge = 25 We can just swap them…. NO! Look at the values…

  5. Swapping two numbers EDU>> HisAge = 25; EDU>> HerAge = 48; EDU>> Temp = HisAge; EDU>> HisAge = HerAge; EDU>> HerAge = Temp; EDU>> EDU>> HisAge HisAge = 48 EDU>> HerAge HerAge = 25 We need a temp variable

  6. DeBlanking a String EDU>> MyPass = 'do geese see god' MyPass = do geese see god EDU>> DeBlank(MyPass) ans = dogeeseseegod EDU>> DeBlank('do geese see god') ans = dogeeseseegod Sometimes it is useful to remove blank spaces from strings… 'do geese see god' DeBlank 'dogeeseseegod'

  7. functionDeBlankedString = DeBlank(S) fori = 1 : length(S) disp(S(i)) end EDU>> DeBlank('do geese see god') d o g e e s e s e e g o d

  8. functionDeBlankedString = DeBlank(S) DeBlankedString = []; fori = 1 : length(S) if S(i) ~= ' ' disp(S(i)); end end EDU>> DeBlank('do geese see god') d o g e e s e s e e g o d

  9. functionDeBlankedString = DeBlank(S) DeBlankedString = []; fori = 1 : length(S) if S(i) ~= ' ' DeBlankedString = [DeBlankedString S(i)]; end end EDU>> DeBlank('do geese see god') dogeeseseegod EDU>> DeBlank('eamonn john keogh') ans = eamonnjohnkeogh

  10. Reversing an Array EDU>> ReverseMe([ 4, 2, 9, 7]) ans = 7 9 2 4 Let us write a function that takes in an array, and returns it reversed [ 4, 2, 9, 7] ReverseMe [ 7, 9, 2, 4]

  11. function Reversed = ReverseMe(InputArray) fori = 1 : length(InputArray) disp( InputArray(i) ); end EDU>> ReverseMe([ 4, 2, 9, 7]) 4 2 9 7

  12. function Reversed = ReverseMe(InputArray) Len = length(InputArray); fori = 1 : Len disp( InputArray(i) ); end EDU>> ReverseMe([ 4, 2, 9, 7]) 4 2 9 7

  13. function Reversed = ReverseMe(InputArray) Len = length(InputArray); fori = 1 : Len disp( InputArray( Len + 1 - i ) ); end EDU>> ReverseMe([ 4, 2, 9, 7]) 7 9 2 4

  14. function Reversed = ReverseMe(InputArray) Len = length(InputArray); fori = 1 : Len Reversed(i) = InputArray( Len + 1 - i ); end EDU>> ReverseMe([ 4, 2, 9, 7]) ans = 7 9 2 4

  15. EDU>> ReverseMe([ 4, 2, 9, 7]) ans = 7 9 2 4 EDU>> ReverseMe([ 4 ]) ans = 4 EDU>> ReverseMe([ 4 4 inf]) ans = Inf 4 4

  16. Testing for Palindromes Let us write a function that takes in string, and returns true if it is an palindrome, otherwise it returns false EDU>> Palindrome('radar') ans = 1 EDU>> Palindrome(‘eamonn') ans = 0 'radar' Palindrome 1

  17. Testing for Palindromes

  18. functionIsPalindrome = Palindrome(S) SLen = length(S); fori = 1 : SLen disp( S(i) ) end EDU>> Palindrome(‘radar') r a d a r EDU>> Palindrome('eamonn') e a m o n n

  19. functionIsPalindrome = Palindrome(S) SLen = length(S); fori = 1 : SLen disp( S(SLen + 1 - i) ) end EDU>> Palindrome(‘radar') r a d a r EDU>> Palindrome('eamonn') n n o m a e

  20. functionIsPalindrome = Palindrome(S) SLen = length(S); fori = 1 : SLen disp( S(i) ) disp( S(SLen + 1 - i) ) disp('****************') end EDU>> Palindrome(‘radar') r r **************** a a **************** d d **************** a a **************** r r **************** EDU>> Palindrome('eamonn') e n **************** a n **************** ...(deleted for clarity)

  21. functionIsPalindrome = Palindrome(S) SLen = length(S); IsPalindrome = 1; fori = 1 : SLen if S(i) ~= S(SLen + 1 - i) IsPalindrome = 0; end end EDU>> Palindrome('radar') ans = 1 EDU>> Palindrome('eamonn') ans = 0

  22. Our palindrome function works on single word palindromes: radar, level, rotor, kayak, redder, madam • It works on some multiword palindromes: step on no pets • It does not work on most multiword palindromes: a man, a plan, a canal Panama,  Madam, Im Adam, never odd or even , rise to vote sir • We could fix that, with DeBlank()

  23. Last times slides are below for ref

  24. Searching Arrays • Many problems involve searching an array (including a string) for special values. • Examples include • Searching an array for the largest value (which player in my team makes the most money?) • Searching an array for a particular value (do I have a student with the ID 8932328?) • Searching a string for a particular value(s) ( Does this string “control” have vowels? CNTRL)

  25. Searching Arrays • Many problems involve searching an array (including a string) for special values. • These problems are so important that Matlab has special build-in functions to solve some of them. • We have seen: EDU>> max([ 2 3 5 3 1 6 4 3 8]) ans = 8 • However, let us write some of these function from scratch to build our skill set…

  26. Finding Existence of a Given Number EDU>> TeamAges = [ 5 , 6, 11, 9 , 8]; EDU>> AgeToFind = 9; EDU>> EamonnsFindNumber(TeamAges,AgeToFind) ans = 1 EDU>> EamonnsFindNumber(TeamAges,77) ans = 0 EDU>> EamonnsFindNumber([66, 77, 45],77) ans = 1 Let us write a function that takes in an array, a number to find in that array, and returns true/false if the number is in the array [ 4, 2, 9] 2 EamonnsFindNumber 1

  27. functionTargetNumberExists = EamonnsFindNumber(ArrayToSearch,TargetNum) TargetNumberExists = 0; % Assume the number does not exist for now fori = 1 : length(ArrayToSearch) ifArrayToSearch(i) == TargetNum TargetNumberExists = 1; % Change our assumption end end ArrayToSearch EDU>> TeamAges = [ 5 , 6, 11, 9 , 8]; EDU>> AgeToFind = 9;

  28. functionTargetNumberExists = EamonnsFindNumber(ArrayToSearch,TargetNum) TargetNumberExists = 0; % Assume the number does not exist for now fori = 1 : length(ArrayToSearch) ifArrayToSearch(i) == TargetNum disp(['the if statement was triggered with i equals ', num2str(i)]) TargetNumberExists = 1; % Change our assumption end end Useful hint: It is good idea to add temporary code to echo what is happening in the code. This is especially true as we consider more complex code EDU>> EamonnsFindNumber([66, 77, 45],77) the if statement was triggered with i equals 2 ans = 1

  29. Finding Location of Given Number EDU>> TeamAges = [ 5 , 6, 11, 9 , 8]; EDU>> AgeToFind = 9; EDU>> FindNumberLocation(TeamAges,AgeToFind) ans = 4 EDU>> FindNumberLocation(TeamAges,77) ans = NaN [ 4, 2, 9] 9 Let us write a function that takes in an array, a number to find in that array, and returns the location of that number in the array. If the number is not in the array, it return NaN FindNumberLocation 3

  30. Finding Location of Given Number • Important note: • The previous slide did not specify what to do if the target number appears more than once. • It is our job to clarify these things • We could: • Report the first occurrence • Report the second occurrence • Do something else [ 9, 2, 9] 9 Let us write a function that takes in an array, a number to find in that array, and returns the location of that number in the array. If the number is not in the array, it return NaN FindNumberLocation 3

  31. functionNumLocation = FindNumberLocation(ArrayToSearch,TargetNum) NumLocation = NaN; % Assume the number does not exist for now fori = 1 : length(ArrayToSearch) ifArrayToSearch(i) == TargetNum NumLocation = i; % Assumption was wrong, record the location end end EDU>> TeamAges = [ 5 , 6, 5, 9 , 7]; EDU>> FindNumberLocation(TeamAges,5) ans = 3

  32. functionNumLocation = FindNumberLocation(ArrayToSearch,TargetNum) NumLocation = NaN; % Assume the number does not exist for now fori = 1 : length(ArrayToSearch) ifArrayToSearch(i) == TargetNum NumLocation = i; % Assumption was wrong, record the location end end Does our code find the first or last occurrence of a number? It finds the last occurrence In the next slide we will see how to find the first instead EDU>> TeamAges = [ 5 , 6, 5, 9 , 7]; EDU>> FindNumberLocation(TeamAges,5) ans = 3

  33. functionNumLocation = FindNumberLocation(ArrayToSearch,TargetNum) NumLocation = NaN; % Assume the number does not exist for now fori = 1 : length(ArrayToSearch) ifArrayToSearch(i) == TargetNum NumLocation = i; % Assumption was wrong, record the location break; end end The matlab command break terminates a for or while loop. The moment that Matlab encounters a break it jumps to the end of the loop and continues with the rest of the program… EDU>> TeamAges = [ 5 , 6, 5, 9 , 7]; EDU>> FindNumberLocation(TeamAges,5) ans = 1

  34. Finding Vowel Count Let us write a function that takes in a string, and returns a count of how many vowels it has. Let us use our strategy of solving a simpler problem and working our way upto the final solution… ‘eamonn’ Let us use our strategy of echoing to the screen so we can debug the code.. FindVowels 3

  35. functionVowelCount = FindVowels(StringToSearch) VowelCount = 0; % Assume there are no vowels fori = 1 : length(StringToSearch) ifStringToSearch(i) == 'a' VowelCount = VowelCount + 1; disp(['An a was found when i was ', num2str(i)]) end end EDU>> FindVowels('eamonn') An a was found when i was 2 ans = 1 EDU>> FindVowels('abba') An a was found when i was 1 An a was found when i was 4 ans = 2

  36. functionVowelCount = FindVowels(StringToSearch) VowelCount = 0; % Assume there are no vowels fori = 1 : length(StringToSearch) ifStringToSearch(i) == 'a' VowelCount = VowelCount + 1; disp(['An a was found when i was ', num2str(i)]) end end There is a bug! We are not finding uppercase ‘A’ EDU>> FindVowels('ABBA') ans = 0

  37. function VowelCount = FindVowels(StringToSearch) VowelCount = 0; % Assume there are no vowels for i = 1 : length(StringToSearch) if lower(StringToSearch(i)) == 'a' VowelCount = VowelCount + 1; disp(['An a was found when i was ', num2str(i)]) end end There is a simple fix EDU>> FindVowels('ABBA') An a was found when i was 1 An a was found when i was 4 ans = 2

  38. functionVowelCount = FindVowels(StringToSearch) VowelCount = 0; % Assume there are no vowels fori = 1 : length(StringToSearch) if lower(StringToSearch(i)) == 'a' VowelCount = VowelCount + 1; disp(['An a was found when i was ', num2str(i)]) end if lower(StringToSearch(i)) == 'e' VowelCount = VowelCount + 1; disp(['An e was found when i was ', num2str(i)]) end end EDU>> FindVowels('eamonn') An e was found when i was 1 An a was found when i was 2 ans = 2 We can begin to add more code..

  39. Finding The Largest Value Let us write a function that finds the largest number in an array. As it happens, the built in max function does this, but we need to sharpen our skills… [ 1 3 4 5] EamonnsMax 5

  40. functionLargestNumber = EamonnsMax(InputArray) LargestNumber = -Inf; % Assume this is the largest number fori = 1 : length(InputArray) ifInputArray(i) > LargestNumber disp('I need to update the largest number') LargestNumber = InputArray(i); end end EDU>> EamonnsMax([1 4 2 8 4]) I need to update the largest number I need to update the largest number I need to update the largest number ans = 8 EDU>> EamonnsMax([4 3 2 1]) I need to update the largest number ans = 4

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