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CS1 Python Subgoals

Section 11.3 Worked Example: Lists - Traverse

Subgoals for Evaluating Lists.

  1. Declaring and initializing a list
    1. Set up a one dimensional table (i.e., one row) with 0 to size - 1 elements.
    2. Upon instantiation of a list, all elements contain values from the initializer list (i.e., the list of values inside the square brackets). If no initializer list is provided, the list is empty.
  2. Determining access, slicing, changing, adding, or whole list actions.
    1. Determine the list name and the action to be performed.
    2. If the list is on the left hand side of an assignment statement WITHOUT square brackets, it is a Whole List Action for List Assignment.
    3. If the list is on the left hand side of an assignment statement WITH square brackets, it is a Changing value of a List Element.
    4. If the append or insert method is being used on an instance of a list, it is an Adding a Value to a List Element.
    5. If the list is an expression WITHOUT square brackets, it is a Whole List Action for Passing a List as an Argument.
    6. If the list is an expression WITH square brackets, then check if there is a colon in the square brackets. If there is a colon inside of the square brackets, it is a Slicing Multiple Values from a List. Otherwise, it is an Accessing List Element.
  3. Accessing list element
    1. Determine value of index for element to be accessed; a positive value if counting from the beginning, or a negative value if counting from the end.
    2. listName[index] returns value stored at that index.
    3. Index must be between 0 and len(listName)-1, inclusive, or a negative value; otherwise an IndexError exception occurs at runtime.
  4. Slicing multiple values from a list
    1. Determine the range of indexes for the elements to be sliced
    2. listName[startIndex:endIndex] returns a new list containing the elements from startIndex to endIndex-1 (inclusive)
    3. Negative numbers can be used for startIndex and endIndex to count from the end of the list
    4. Omitting startIndex starts from the beginning of the list, and omitting endIndex goes to the end of the list
  5. Changing value of a list element
    1. Evaluate expression within [] brackets to determine the index of the element to be changed, and the list to change.
      Determine value of index for element to be changed; a positive value if counting from the beginning, or a negative value if counting from the end.
    2. Determine the expression of RHS (right-hand side) of the assignment statement.
    3. The lists’ value is now changed to match the value calculated from the RHS of the assignment statement.
  6. Adding a value to a list element
    1. Check whether the append or insert method is being used. Note that either way you do not use an assignment statement, it is just a method call.
    2. If append is used, the new value is added to the end of the list.
    3. If adding a value elsewhere in the list, use the insert method to add the new value at the specified index. Existing values starting from that index are shifted to the right.
  7. Traversing a List
    1. Determine the list that is being iterated over. If the expression also involves a range(len(list_name)), then the list is being traversed by index. The range function can either take one argument (the length of the list) or 3 arguments (the starting index, ending index, and step size). If the range function takes one argument, it will start at 0 and go to the length of the list - 1. If the range function takes 3 arguments, it will start at the first argument and go to the second argument - 1, incrementing by the third argument. Otherwise, if range is not used, then the list is being traversed by value.
    2. Determine the loop control variable that is being used to iterate over the list. The loop control variable will take on each value or index in the list, one at a time, depending on whether we are iterating by value or by index.
    3. The loop control variable is used to access the list element in the body of the loop. If iterating by index, the loop control variable is used as an index to access or update the list element. If iterating by value, the loop control variable is used directly to access the list element (no updates are possible).
    4. The list can also be added to with the append or insert methods. The append method adds a new value to the end of the list, while the insert method adds a new value at the specified index. Existing values starting from that index are shifted to the right.
  8. Whole list actions
    1. Passing a list as an argument
      1. Determine that the entire list must be passed as an argument to a method by consulting documentation.
      2. When calling a function, put variable name that represents the list as an argument in the method call. Remember that when passing a list as an argument that changes made by the function to the list are persistent. The list itself is not copied, so the function does not have its own copy of the list. However, the one exception to this is if you assign the argument to reference a different list in memory; then you will no longer be modifying the original list.
    2. List Assignment
      1. Determine that the reference to the list needs to be changed, not just its contents.
      2. The LHS of the assignment is the list reference needing to be changed.
      3. The RHS of the assignment is the new list reference.
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Subsection 11.3.1

Problem:
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Given the initialized list:
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alpha = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
Evaluate these statements and determine the value of all variables. If any error occurs, give the reason.
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for i in range(len(alpha)):
    alpha[i] = i * 10
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Subsection 11.3.2 SG1: Declaring and initialization of list 

alpha = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
The figure shows a table with 11 columns and 2 rows. The top row is labeled with the indexes of the list, and the bottom row is labeled with the values of the list. The first column is labeled 0, and the last column is labeled 9. The second row contains all zeros.
Figure 11.3.1.
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  • alpha is declared as a list of integers.
  • This statement allocates 10 slots for integers (first line is indexes, second line is values).
  • Declared size of list is 10, so last valid index value is 9.
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Subsection 11.3.3 SG2: Determine access or action

In this example, a list is declared and instantiated to hold 10 new integers. We are traversing the list, so we will need SG7 Within the loop. Then, since we will be changing values within the list, we also look to SG5.
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Subsection 11.3.4 SG7: Traversing the list

The loop header is:
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for i in range(len(alpha)):
The list expression is range(len(alpha)). The len function returns the number of elements in the list, which is 10. The range function returns a list of integers from 0 to 9. So the loop control variable (i) will take on the values 0, 1, 2, ..., 9. The loop will iterate 10 times, once for each index in the list.
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Subsection 11.3.5 SG5: Change array element

The body of the loop is:
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alpha[i] = i * 10
The statement alpha[i] = i * 10 assigns the value of i * 10 to the element at index i in the list alpha. Notice that the loop control variable (i) is used as both the index for the element within the array to be changed and for its value on the RHS of the assignment statement.
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  • When i is 0 (the first iteration of the loop), 0 is copied into alpha[0].
  • When i is 1 (the second iteration of the loop), 10 (1 * 10) is copied into alpha[1].
  • This pattern continues until i is 9, where 90 (9 * 10) is copied into alpha[9].
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Note that we have to verify that the index values are all within the list bounds. They are fine in this case.
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The resulting list is:
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The table from before, but now the second row contains the values 0, 10, 20, 30, 40, 50, 60, 70, 80, and 90.
Figure 11.3.2.
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Subsection 11.3.6 Practice Pages

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You have attempted 1 of 2 activities on this page.
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