CT Week 1: Flowcharts & Pseudocode

0. Prerequisites

NOTE

What you need to know:

• Logic: Basic “If this, then that” reasoning.
• Arithmetic: Basic addition, subtraction, comparison ($>,<,=$).

Quick Refresher

• Algorithm: A step-by-step recipe to solve a problem.
• Pseudocode: Code written in plain English (not a specific language like Python).
• Flowchart: Visual diagram of an algorithm.

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1. Core Concepts

1.1 Flowchart Symbols

1. Oval (Start/Stop): The beginning and end.
2. Parallelogram (Input/Output): Reading a card, Printing a value.
3. Rectangle (Process): Calculations ($A=A+1$).
4. Diamond (Decision): Questions with Yes/No answers ($A>B?$).
   • Crucial: Always has 2 arrows coming out (True/False).

1.2 The “Pile” Metaphor (Dataset)

• Pile 1: The input stack of cards (Data).
• Pile 2: The processed stack (Done).
• Iteration: “Pick card from Pile 1 $\to$ Process $\to$ Move to Pile 2 $\to$ Repeat”.
• Stop Condition: “If Pile 1 is empty, stop.”

1.3 Variables

• Containers: Boxes that hold one value at a time.
• Initialization: Setting a starting value ($A=0$).
• Assignment: Updating the value ($A=A+1$).
  • Note: The old value is erased!

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2. Pattern Analysis & Goated Solutions

Pattern 1: The “Dry Run” Table (Master Skill)

Context: “What is the value of A at the end?” Code:

Step 1: A = 0, B = 0
Step 2: Read Card X
Step 3: If X > 10, A = A + 1
Step 4: Else, B = B + 1

Data: 5, 12, 8.

TIP

Mental Algorithm (The Trace Table):

1. Draw Columns: One for every variable ($A,B$) and the Input ($X$).
2. Row by Row: Go through each data item.
3. Update: Cross out old values, write new ones.

Example (Detailed Solution)

Data: 5, 12, 8. Trace:

Step	Input (X)	Condition (X > 10?)	A	B
Init	-	-	0	0
Card 1	5	No	0	1
Card 2	12	Yes	1	1
Card 3	8	No	1	2

Answer: A = 1, B = 2.

Pattern 2: Counting vs Summing

Context: “Does variable A count the cards or sum the values?”

TIP

Mental Algorithm:

• Counting: Look for A = A + 1. (Adds 1 for every match).
• Summing: Look for A = A + X.Value. (Adds the content of the card).

Example (Detailed Solution)

Code: If X.City == "Chennai" then A = A + X.Marks Analysis:

1. It checks for “Chennai”.
2. It adds X.Marks (a value), not 1. Answer: Sum of marks for students from Chennai.

Pattern 3: The “Min/Max” Logic

Context: “Find the highest mark.”

TIP

Mental Algorithm:

1. Init: Set Max = 0 (or a very small number).
2. Loop: Compare every card X with Max.
3. Update: If X > Max then Max = X.
   • Logic: If I see something bigger than my current champion, the new guy becomes the champion.

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3. Practice Exercises

1. Trace: $A=0$. Data: 2, 4, 6. Loop: $A=A+X$. Final A?
   • Hint: $0+2=2\to 2+4=6\to 6+6=12$.
2. Logic: If A > B then C = 1 else C = 0. What does C=1 mean?
   • Hint: A is strictly greater than B.
3. Flowchart: What shape is “Is A > 10?”
   • Hint: Diamond (Decision).

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🧠 Level Up: Advanced Practice

Question 1: The Initialization Trap

Problem: A procedure counts bills > 500.

• Step 2: Initialize count = 0.
• Step 5: If Bill > 500 increment count.
• Step 6: Move to Pile 2 and repeat from Step 2. Bug: Repeating from Step 2 resets count to 0 for every card. The final result will only be 0 or 1 (based on the last card). Fix: Repeat from Step 3 (Check if Pile 1 empty).

Question 2: Nested Logic Flow

Problem:

• Step 5: If Date in Jan-Jun, increment A.
• Step 6: If Date in Jul-Dec, increment B.
• Step 8: If A < B then set C = 1. Meaning: C becomes 1 only if there are more students born in the second half (Jul-Dec) than the first half.

Question 3: Data Sanity

Problem: Identifying “Bad Data” in a dataset.

• Row 1: Card Number is “Twelve” (String instead of Int). → Incorrect Data Type.
• Row 4: Letter Count is -5. → Invalid Value (Count cannot be negative). Takeaway: Always check if data matches the expected type and logical constraints.