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COMPSCI 101 S1 2014 Principles of Programming

COMPSCI 101 S1 2014 Principles of Programming. 18 Iterating through sequences using index values. Recap. A list stores data as a sequence len () returns the number of elements in a list Each element of the list can be accessed We use a for loop to iterate through the contents of a list

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COMPSCI 101 S1 2014 Principles of Programming

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  1. COMPSCI 101 S1 2014Principles of Programming 18 Iterating through sequences using index values

  2. Recap • A list stores data as a sequence • len() returns the number of elements in a list • Each element of the list can be accessed • We use a for loop to iterate through the contents of a list • Index • Elements are the individual items in each sequence • Lists can be indexed to get individual entries. Value of an index can go from 0 to (length of list - 1). • Negative indices can be used to index the list from the right. COMPSCI101

  3. Learning outcomes • At the end of this lecture, students should be able to: • Iterate through sequences using index values • Examples • Case Study 1: Printing in reverse order • Case Study 2: Calculating difference between adjacent elements • Case Study 3: Is the list sorted? • Exercise • Printing elements of a list using negative indices COMPSCI101

  4. 18.1 Iterating sequences Using an Element Variable • Many times you need to do something to every element in a sequence. • You can do a traversal using a for loop as follows: for ELEMENT_VAR in SEQUENCE: STATEMENT my_list = [10, 20, 30, 40, 50] If no more items in sequence Element variable 10 20 30 40 50 for element in my_list: print(element)

  5. 18.1 Iterating sequencesUsing Index Values • Iterate through the elements of a list using index values • Example: • index: 0, element: my_list[0], (10) • index: 1, element: my_list[1], (20) • … • Index: index, element: my_list[index] • Therefore, available index values: 0, 1, 2, 3, 4 my_list = [10, 20, 30, 40, 50] Index position 10 20 30 40 50 for index in ????: print(my_list[index]) COMPSCI101

  6. 18.1 Iterating sequencesUsing Index Values • Using index values: • It is not convenient to use a list of numbers • A better approach: 10 20 30 40 50 Available index values for index in [0,1,2,3,4]: print (my_list[index]) elements i.e. range(0,5) Range object: [0, 1, 2, 3, 4] for index in range(len(my_list)): print (my_list[index]) COMPSCI101

  7. Case Study 1Printing in reverse order Demo 1 • Using the range function() • Example: • Positive indices: 4, 3, 2, 1, 0 • Negative indices: -1, -2. -3, -4, -5 50 40 30 20 10 my_list = [10, 20, 30, 40, 50] for index in range(4, -1, -1): print (my_list[index]) for index in range(-1, -6, -1): print (my_list[index]) COMPSCI101

  8. Case Study 1Printing in reverse order Demo 2 • Task: • Complete the print_reverse_list()function which prints a list in reverse order using positive indices • Arguments: an integer list my_list = [10, 20, 30, 40, 50] my_list = [10, 20, ...] for index in range(4, -1, -1): print (my_list[index]) for index in range(start, -1, -1): print (my_list[index]) COMPSCI101

  9. Case Study 1Printing in reverse order Demo 2 • Task: • Complete the print_reverse_list_neg()function which prints a list in reverse order using negative indices • Arguments: an integer list my_list = [10, 20, 30, 40, 50] my_list = [10, 20, ...] for index in range(-1, -6, -1): print (my_list[index]) end = -len(my_list) -1 for index in range(-1, end, -1): print (my_list[index]) COMPSCI101

  10. Exercise 1Printing elements of a list using… • Odd indices • Task: • Complete the print_odd_list()function which prints elements of a list using odd indices • Arguments: an integer list 10 30 50 print_odd_list([10, 20, 30, 40, 50]) 10 30 print_odd_list([10, 20, 30, 40]) for index in range(0, len(numbers), 2): print (numbers[index]) COMPSCI101

  11. Case Study 2Calculating the difference between adjacent items • Task: • Complete the calc_difference_list()function which calculates the difference between adjacent items in a list • Arguments: an integer list • Case: 14 13 5 14 2 13 calc_difference_list([ 0, 5, 19, 21, 34]) 5 2 COMPSCI101

  12. Case Study 2Calculating the difference between adjacent items Demo 3 • Algorithm: • Iterate through the elements of a list using index values • A for loop + range • Calculate the difference: calc_difference_list([ 0, 5, 19, 21, 34]) for index in range(0, len(numbers)-1): numbers[index+1]-numbers[index] COMPSCI101

  13. Case Study 3Is the list sorted? • Task: • Complete the is_sorted()function which returns true if the list is increasing, false otherwise. • Arguments: an integer list • Returns: true if it is in increasing order, false otherwise • Cases: True is_sorted([3, 5, 9, 11]) False is_sorted([3, 9, 5, 11]) COMPSCI101

  14. Case Study 3Is the list sorted? • Algorithm: • for each iteration • Compare adjacent values, if they are not in order, stop and return False • return True 3 > 5? 5 > 9? 9 > 11? COMPSCI101

  15. Case Study 3Is the list sorted? • Algorithm: • for each iteration • Compare adjacent values, if they are not in order, stop and return False • return True 3 > 9? 9 > 5? STOP COMPSCI101

  16. Case Study 3Is the list sorted? Demo 4 • Algorithm: • Iterate through the elements of a list using index values • A for loop + range • Sorted? is_sorted([3, 5, 9, 11]) for index in range(len(numbers)-1): numbers[index] numbers[index+1] numbers[index] > numbers[index+1] COMPSCI101

  17. Exercise 2 • Task: • Complete the mult3()function which takes a list of integers as a parameter and prints only the multiples of 3, one per line • Use the range function to access all elements in a list. • Arguments: an integer list • Returns: a sub-list of number Cases: mult3([3, 1, 6, 2, 3, 9, 4, 9]) 3 6 3 9 9 COMPSCI101

  18. Summary • A list stores data as a sequence • len() returns the number of elements in a list • Each element of the list can be accessed using the index operator • Positive index: count from the left, starting with 0 • Negative index: count from right, starting with –1 • We use a for loop to iterate through the contents of a list COMPSCI101

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