CT_Week3_Conditionals

CT Week 3: Conditionals (Decision Making)

0. Prerequisites

NOTE

What you need to know:

• Relational Operators: $>,<,==$.
• Logical Operators: AND, OR, NOT.

Quick Refresher

• If Statement: Executes code only if condition is True.
• Else: Executes code if the condition is False.
• Nested If: An if inside another if.

---

1. Core Concepts

1.1 The Structure

1. Simple If:

   If (Condition) {
       Do Something
   }
   

2. If-Else:

   If (Condition) {
       Do A (True Path)
   } Else {
       Do B (False Path)
   }
   

3. If-Else If-Else (Ladder):
   • Checks conditions one by one.
   • Crucial: As soon as one matches, it executes that block and skips the rest.

1.2 Independent vs Mutually Exclusive

• Series of Ifs (Independent):
  • If A > 10 ...
  • If A > 20 ...
  • Result: Both can run.
• If-Else If (Mutually Exclusive):
  • If A > 10 ...
  • Else If A > 20 ...
  • Result: Only the first one runs.

---

2. Pattern Analysis & Goated Solutions

Pattern 1: The “Ladder” Trap (First Match Wins)

Context: “What is printed?” Code:

Marks = 95
If Marks > 35:
    Print "Pass"
Else If Marks > 90:
    Print "Grade A"

TIP

Mental Algorithm:

1. Check 1st Condition: 95 > 35? Yes.
2. Execute: Print “Pass”.
3. Skip Rest: The Else If is IGNORED because the first If was taken.
   • Fix: Always order from Specific (High) to General (Low). Check >90 first.

Pattern 2: Nested Logic (The “And” Equivalent)

Context: “When does X become 1?” Code:

If City == "Chennai":
    If Gender == "F":
        X = 1

TIP

Mental Algorithm:

1. Translate to Logic: Nested Ifs are equivalent to AND.
2. Condition: City == "Chennai" AND Gender == "F".

Pattern 3: Finding the “Else” Owner

Context: “Which ‘If’ does this ‘Else’ belong to?” Code:

If A:
    If B:
        Print "X"
Else:
    Print "Y"

TIP

Mental Algorithm:

1. Indentation: Look at the alignment.
2. Matching: The Else belongs to the If at the same indentation level.
   • Here, Else matches If A.
   • So “Y” prints only if A is False.

---

3. Practice Exercises

1. Logic: If True then If False then Print A else Print B. Output?
   • Hint: Outer True enters. Inner False goes to Else. Prints B.
2. Ladder: X=10. If X>5: A=1. If X>8: A=2. Final A?
   • Hint: These are separate Ifs (not Else If). Both run. A becomes 1, then A becomes 2. Final 2.
3. Trace: X=5. If X>10: Y=1 Else: Y=2. Final Y?
   • Hint: $5>10$ is False. Goes to Else. Y=2.

---

🧠 Level Up: Advanced Practice

Question 1: Modulo & Logic

Problem:

If (LetterCount % 2 == 0) { P = P + 1 }
Else {
    If (PartOfSpeech == "Adverb") { Q = Q + 1 }
}

Interpretation:

• P counts words with Even letter count.
• Q counts Adverbs that have Odd letter count (since they fell into the Else block).

Question 2: The “At Least One” Condition

Problem: Count students who scored < 80 in at least one subject. Logic:

• If (Math < 80) OR (Phy < 80) OR (Chem < 80) then Count = Count + 1. Contrast: “In exactly two subjects” would require:
• ((M<80) + (P<80) + (C<80)) == 2. (Treating True as 1).

Question 3: Nested vs Chained

Scenario:

• Nested: If A { If B { ... } } → Requires A AND B.
• Chained: If A { ... } Else If B { ... } → Checks B only if A is False. Trap: Using Chained when you want to check both independently.