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CS1 Python Subgoals

Section 4.11 Nested If Statements

Subgoals for Evaluating Selection Statements.

  1. Diagram which statements go together by indentation
  2. For conditional, determine whether expression is true
    1. If true, follow true branch
    1. If false, follow next elif/else branch or exit conditional if no else branch
  3. Repeat step 2 as necessary
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Subsection 4.11.1

Given the following declarations:
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alpha = 2
delta = 3
eta = 0
gamma = 0
omega = 2.5
theta = -1.3
kappa = 3.0
rho = 0.0
Evaluate these statements and determine the value of all variables used.
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if omega < kappa:
    if alpha < delta:
        eta = 5
    if alpha < eta:
        gamma = 4
rho = -1.0
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Subsection 4.11.2 SG1: Diagram which statements go together.

In this diagram, the first thing to note is the parent/outer if-statement highlighted in blue.
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Inside the true branch of the parent/outer if-statement, there are two sequential if-statements.
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The final single statement is highlighted in green, and it is not part of the nested if-statements, so it will always be executed.
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Figure 4.11.1.
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Subsection 4.11.3 SG2: For if statement, determine whether true or false

Because there are 2 sequential if-statements, we start with the first one, and then repeat SG2 and SG3 for the other.
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First we evaluate (omega < kappa):
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(2.5 < 3.0) is True
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Subsection 4.11.4 SG3: If true, follow true branch. If false, follow else branch (OR do nothing if there is no else branch).

The true branch contains two sequential if-statements, so we must repeat SG2 and SG3 for each of them.
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Figure 4.11.2.
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Subsection 4.11.5 SG2: For if statement, determine whether true or false

Start with the first if-statement in the inner sequence.
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First we evaluate (alpha < delta):
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(2 < 3) is True
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Subsection 4.11.6 SG3: If true, follow true branch. If false, follow else branch (OR do nothing if there is no else branch).

The condition is TRUE so we follow the true branch.
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eta = 5
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Subsection 4.11.7 SG2: For if statement, determine whether true or false

Now we do the second if-statement in the inner sequence.
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First we evaluate (alpha < eta):
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(2 < 5) is True
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Subsection 4.11.8 SG3: If true, follow true branch. If false, follow else branch (OR do nothing if there is no else branch).

The condition is TRUE so we follow the true branch.
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gamma = 4
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Next, the final sequential statement after the entire selection structure is always executed:
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rho = -1.0
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Answer.
omega = 2.5, kappa = 3.0, alpha = 2, delta = 3, eta = 5, gamma = 4, rho = -1.0

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