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CS1 Python Subgoals

Section 6.1 Write Loops

Subgoals for Writing a Loop.

  1. Determine purpose of loop
    1. Pick a loop structure (while, for, do_while)
  2. Define and initialize variables
  3. Determine termination condition
    1. Invert termination condition to continuation condition
  4. Write the loop body
    1. Update Loop Control Variable to reach termination

Subsection 6.1.1

Problem: Write a loop that will prompt the user to continue to enter numbers until a sentinel value (-1) is entered, and then find the maximum value entered.

Subsection 6.1.2 1. Determine purpose of loop to pick loop structure (for, while)

Generally, while loops are a good choice for sentinel loops, since you cannot know up front how many times the code will need to iterate.

Subsection 6.1.3 2. Define variables before loop that have dependencies within the loop

The user must input at least one number in order for us to find a maximum. The variable user_input will also be used as the loop control variable, so we must define it before the loop begins.
user_input = int(input("Enter a value, enter -1 to end."))
The maximum value must be updated during the loop based on its previous value, and will be printed after the loop, so that variable will need to be initialized prior to the loop.
maximum = user_input

Subsection 6.1.4 3. Based on loop structure

The terminiation condition for this loop is when the user’s inputted value is equal to the sentinel. We need to invert that condition to identify the continuation condition instead. That continuation condition must involve the loop control variable, which in this case will be user_input.
while user_input != -1:

Subsection 6.1.5 4. Write loop body

First, we must check whether the latest maximum is greater than the user_input; if so, we must update the maximum. This check is unnecessary during the first iteration of the loop, but is critical on every subsequent iteration.
while user_input != -1:
    if user_input > maximum:
        maximum = user_input
We then need to update the user_input loop control variable so that the loop has a chance to end at some point based on the user’s value.
while user_input != -1:
    if user_input > maximum:
        maximum = user_input
    user_input = int(input("Enter a value, enter -1 to end."))
Answer.
user_input = int(input("Enter a value, enter -1 to end."))
maximum = user_input

while user_input != -1:
    if user_input > maximum:
        maximum = user_input
    user_input = int(input("Enter a value, enter -1 to end."))

print(maximum)
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