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COMPSCI 101 S1 2014 Principles of Programming

COMPSCI 101 S1 2014 Principles of Programming. 11 Conditions within Loops. Learning outcomes. At the end of this lecture, students should be able to: Write a program that combines a for loop and an if statement. Examples and Exercises Exercise 1: Printing Even Numbers

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COMPSCI 101 S1 2014 Principles of Programming

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  1. COMPSCI 101 S1 2014Principles of Programming 11 Conditions within Loops

  2. Learning outcomes • At the end of this lecture, students should be able to: • Write a program that combines a for loop and an if statement. • Examples and Exercises • Exercise 1: Printing Even Numbers • Exercise 2: Counting 3-letter Words • Case Study 1: Counting Alphabetic Letters • Case Study 2: Counting Vowel Letters • Case Study 3: Comparing Strings COMPSCI101

  3. 11.1 ReviewConditions • Simple Conditions • The statements involve tests or conditions • Simple if Statements • A one-way if statement executes the statements if the condition is true • The general Python syntax for a simple if statement is x == 11 x > 10 if condition : indented_Code_Block if weight > 50: print("There is a $25 charge for luggage that heavy.") print("Thank you for your business.") COMPSCI101

  4. 11.1 ReviewConditions • if-else Statements • A two-way if-else statement decides which statements to execute based on whether the condition is true or false. • The general Python if-else syntax is • Example: if condition : indented_Code_Block_For_True_Condition else: indented_Code_Block_For_False_Condition if temperature > 70: print('Wear shorts.') else: print('Wear long pants.') print('Get some exercise outside.') COMPSCI101

  5. 11.1 Review Grade Test Demo 02 • Using nested if-else statements to convert a numerical mark to a letter grade defscore_to_grade(score): if score >= 90: letter = 'A' else: # grade must be B, C, D or F if score >= 80: letter = 'B' else: # grade must be C, D or F if score >= 70: letter = 'C' else: # grade must D or F if score >= 60: letter = 'D' else: letter = 'F' return letter COMPSCI101

  6. 11.1 Review Grade Test V2 Demo 03 • Using if-elif statements to convert a numerical mark to a letter grade • Exactly one of the indented blocks is executed! defscore_to_grade_v2 (score): if score >= 90: letter = 'A' elif score >= 80: letter = 'B' elif score >= 70: letter = 'C' elif score >= 60: letter = 'D' else: letter = 'F' return letter COMPSCI101

  7. 11.1 ReviewConditions (con’t) • Nested if and Multi-way if-elif-else Statements • The general Python if-elif-else syntax is if condition1 : indented_Code_Block_For_True_Condition_1 elif condition2 : indented_Code_Block_For_True_Condition_2 elif condition3 : indented_Code_Block_For_True_Condition_3 else: indented_Code_Block_ForEachCondition_False COMPSCI101

  8. 11.1 ReviewCommon Errors SyntaxError: invalid syntax IndentationError: expected an indented block • What is wrong? if x = 0: print (x) if weight > 50: print("There is a $25 charge for luggage that heavy.") D if score >= 60: letter = 'D' elif score >= 70: letter = 'C' elif score >= 80: letter = 'B' elif score >= 90: letter = 'A' else: letter = 'F' D D COMPSCI101

  9. 11.1 Review The for loop Demo 01 • A Python for loop iterates through each value in a sequence. • The general Python syntax is • Example: for item in sequence: indented_Code_Block for count in [1, 2, 3]: print(count) print('Done counting.') 1 2 3 Done counting. red blue green for colour in ['red', 'blue', 'green']: print(colour) COMPSCI101

  10. 11.1 ReviewCommon Errors • What is wrong? IndentationError: expected an indented block for count in [1, 2, 3]: print (count) ‘red’ ‘red’ ‘red’ for colourin ['red', 'blue', 'green']: print('red') COMPSCI101

  11. 11.2 Conditions within LoopsConditions within Loops • for and if statements can be nested inside each other’s indented blocks • The general Python if-elif-else syntax is If no more items in sequence Item from sequence for item in sequence: if condition : indented_Code_Block Non_indented_statement Next item Condition? True Execute statements False COMPSCI101

  12. 11.2 Conditions within Loops Printing Positive Numbers Demo 04 • Task: • Complete the print_positive() function which prints positive numbers in a list • Arguments: a list of integers print_positive([-1, 2, 3]) 2 3 Item from sequence for number in number_list: If no more items in sequence Next item if number > 0 : >0 ? Print item print (number) COMPSCI101

  13. Exercise1: Printing Even Numbers • Task: • Complete the print_evens() function which prints even numbers in a list. • Arguments: a list of Integers • Information produced: prints all even numbers in the list COMPSCI101

  14. Case Study 1:Counting Alphabetic Letters • Task: • Complete the count_alphas() function which counts the number of alphabetic letters in a string • Arguments: a string • Returns: the number of alphabetic letters in a string • Cases: A, B…Z a, b, … z 0 count_alphas('123') 10 count_alphas('Hello world') 11 len('Hello world') COMPSCI101

  15. Try Demo 5 Case Study 1: Counting Alphabetic Letters • Algorithm: for letter in word: if letter.isalpha(): COMPSCI101

  16. Case Study 2:Counting Vowel Letters • Task: • Complete the count_vowels() function which counts the number of vowels in a string • Arguments: a string • Returns: the number of vowel letters • Cases: A, E, I, O, U, a, e, i, o, u 0 count_vowels('123') 3 count_vowels('Hello world') 11 len('Hello world') 10 count_alphas('Hello world') COMPSCI101

  17. Case Study 2: Counting Vowel Letters • Algorithm: COMPSCI101

  18. Case Study 2: Membership operators • The In Operator • Evaluates to true if it finds a variable in the specified sequence and false otherwise. a = 10 b = 2 list = [1, 2, 3, 4, 5 ] if ( a in list ): print ('a is a member of list') else: print ('a is not a member of list') if ( b in list ): print ('b is a member of list') else: print ('b is not a member of list') a is not a member of list b is a member of list COMPSCI101

  19. Try Demo 6 Case Study 2:Counting Vowel Letters • Algorithm: for letter in word: if letter in vowels: COMPSCI101

  20. Exercise 2Counting 3-letter words • Task: • Complete the count_3letters() function which counts the number of three-letter words in a list • Information Required: a list of strings • Information produced: the number of 3-letter words in a list • Cases: 2 ['dog','letter', 'stop' ,'see'] 0 ['stop', 'input'] 0 ['happy'] COMPSCI101

  21. Exercise 2Counting 3-letter words • Algorithm: for word in words if len(word) == 3: count += 1 COMPSCI101

  22. Case Study 3Comparing Strings • Task: • Complete the words_after()function which finds all the words in a list which are alphabeticallylater than the given parameter • Arguments: a string and a list of strings • Returns: a sub list • Cases: words_after('b', ['a', 'alien', 'b', 'banter', 'class', 'classic', 'zebra']) ['banter', 'class', 'classic', 'zebra'] words_after('z', ['zebra', 'a', 'alien', 'b', 'banter', 'class', 'classic'] ['zebra'] COMPSCI101

  23. Try Demo 7 Case Study 3Comparing Strings • Algorithm: new_words = [] if word > another_string: new_words.append(word) COMPSCI101

  24. Summary • A Boolean type variable can store a True or False value. • The relational operators (<, <=, ==, !=, >, >=) which work with numbers and strings, yield a Boolean value. • The part of the loop that contains the statements to be repeated is called the loop body • In designing loops, you need to consider both the loop-control structure and the loop body. • Examples: for word in words: if word > another_string: new_words.append(word) for letter in word: if letter.isalpha(): count += 1 COMPSCI101

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