Worked Example: Lists - Reverse Traverse

Section 11.6 Worked Example: Lists - Reverse Traverse

Subgoals for Evaluating Lists.

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.
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.
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.
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
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.
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.
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.
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.

Subsection 11.6.1

Problem: Evaluate these statements and determine their output

alpha = [15, 24, 7, 6, -4, 0, 13]
print(len(alpha))
for i in range(len(alpha)-1, -1, -1):
    alpha[i] = alpha[i]+1
for i in range(len(alpha)):
    print(alpha[len(alpha)-1-i])

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Subsection 11.6.2 SG1: Declaring and initialization a List

alpha = [15, 24, 7, 6, -4, 0, 13]

alpha is declared as a list of integers.
This statement allocates 7 slots for integers because there are 7 literal values given in the initialization list.

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The figure shows a table with 8 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 6. The second row contains the values 15, 24, 7, 6, -4, 0, and 13. — Figure 11.6.1.
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Subsection 11.6.3 SG2: Determine access or action

Within the loop (SG7), there is both access (SG3) and changing of elements (SG5). We are also printing out the entire list by passing it as an argument to print (SG8).

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Subsection 11.6.4 SG8: Passing a list as an argument

print(len(alpha))

The statement print(len(alpha)) passes the entire list to len, which returns the length of the list (in this case 7). Then, that value is passed to print, will print the value 7 and then a newline return, so that the next output will begin on a new line.

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Subsection 11.6.5 SG7: Traversing the list

The first for loop iterates through the list by using the range function with the following parameters:

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The first parameter is the starting index, which is the last index of the list: len(alpha)-1.
The second parameter is the ending index, which is -1 (the loop will stop before this value).
The third parameter is the step size, which is -1 (the loop will decrement by 1 each time).

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for i in range(len(alpha)-1, -1, -1):

The loop control variable i will take on the values 6, 5, 4, 3, 2, 1, and 0. The loop will iterate 7 times, once for each index in the list. The first iteration will access the last element of the list (index 6), the second iteration will access the second to last element (index 5), and so on. The last iteration will access the first element of the list (index 0).

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Subsection 11.6.6 SG5 and SG3: Changing and accessing elements

The statement >alpha[i] = alpha[i]+1 accesses the element at index i in the list alpha, adds 1 to it, and assigns the result back to the same index. This means that the value at index i is changed to its original value plus 1.

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The resulting array is:

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This is the same table structure as before, but the values in the second row are now 16, 25, 8, 7, -3, 1, and 14. — Figure 11.6.2.
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Subsection 11.6.7 SG7: Traversing the List

The second for loop iterates through the list in forward order by using the single parameter len(alpha) in the range function. This means that the loop will iterate from 0 to 6 (the length of the list minus 1).

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for i in range(len(alpha)):

However, the statement inside the loop is print(alpha[len(alpha)-1-i]). This means that the loop will print the elements of the list in reverse order. The first iteration will print the last element of the list (index 6), the second iteration will print the second to last element (index 5), and so on. The last iteration will print the first element of the list (index 0).

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14 1 -3 7 8 25 16

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Answer.

The complete output of the program is:

7 14 1 -3 7 8 25 16

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Subsection 11.6.8 Practice Pages

Lists-WE3-P1

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You have attempted 1 of 2 activities on this page.

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