Computer Science: Programming Basics

Computer Science: Programming Basics

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Programming basics is the discipline of converting a problem into an ordered set of machine-executable instructions. The pipeline moves from problem analysis → algorithm → flowchart/pseudocode → source code → translation → testing. Three control structures govern every program: sequence, selection (if/else, switch), and iteration (while, do-while, for). A compiler translates the whole source file into object code; an interpreter executes it line by line. Variables store typed data (primitive types: int, float, char, boolean; non-primitive: string, array), and operators — arithmetic, relational, logical, assignment — combine them into expressions. Watch the = vs == trap and integer-division truncation (e.g., 7/2 = 3). Errors split into syntax, runtime, and logical — syntax is caught at compile time, runtime crashes mid-execution, logical produces wrong answers silently.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Algorithm Representation

An algorithm is a finite, ordered, unambiguous set of steps that solves a problem. Before writing code, NAT-I expects you to read two representations:

• Pseudocode — language-agnostic text using keywords like INPUT, OUTPUT, IF … THEN … ELSE, WHILE, ENDWHILE.
• Flowchart — geometric symbols governed by ANSI/ISO standard shapes:

Symbol	Shape	Meaning
Oval / pill (terminator)	⬭	Start / Stop
Parallelogram	▱	Input / Output
Rectangle	▭	Process / Assignment
Diamond	◇	Decision (Yes/No branch)
Arrow (flowline)	→	Sequence of execution

Control Structures

Every flowcharts reduces to three patterns:

1. Sequence — statements execute top-to-bottom.
2. Selection — if (condition) … else … picks a branch; switch dispatches on discrete values.
3. Iteration — for (counted), while (pre-test), do-while (post-test; runs at least once).

Data, Variables, Operators

A variable is a named memory cell bound to a data type. Naming rules: must begin with a letter or _, no spaces, no reserved keyword. Operators have fixed precedence — arithmetic > relational > logical > assignment; parentheses override. && (AND), || (OR), ! (NOT) return booleans.

Translation Phases

Source code → Lexical analysis → Syntax analysis → Semantic analysis → Code generation → Object code → Linking → Executable. An assembler translates assembly; an interpreter skips object-code production and runs statements directly (slower start, easier debugging).

Errors and the SDLC

Syntax errors are flagged at compile time. Runtime errors (divide-by-zero, index out of bounds) crash mid-execution. Logical errors compile and run cleanly but yield wrong output — the hardest to catch. The Software Development Life Cycle (SDLC) spans planning, design, coding, testing, deployment, and maintenance; debugging sits inside the testing phase.

NAT-I MCQs typically ask you to identify the flowchart symbol for I/O, predict loop output, or spot the operator-precedence mistake.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Pre/Post-Increment Edge Case

++i increments then returns the new value; i++ returns the old value then increments. In int x = 5; printf("%d", x++); the output is 5, but x afterwards is 6. Examiners test this inside expressions like a = ++i + i++;, where order matters.

Off-by-One and Loop Boundary

A loop for (i = 0; i < n; i++) runs exactly n times with indices 0…n−1. Writing i <= n adds one extra iteration — the classic off-by-one bug. Reverse iteration for (i = n-1; i >= 0; i--) requires careful unsigned-vs-signed handling in C/C++.

Arrays and Strings

An array stores homogeneous elements at contiguous indices 0 to length−1. Accessing arr[n] is undefined behavior — caught as ArrayIndexOutOfBoundsException in Java or a segfault in C. String concatenation (s1 + s2) allocates new memory; indexing (s.charAt(i)) is O(1).

Modular Programming

Functions isolate reusable logic, accept parameters, and return values. Pass-by-value copies the argument; pass-by-reference hands the original variable. A void function returns nothing. Recursion replaces iteration when a problem has self-similar substructure.

Worked Micro-Example

Trace int a = 7, b = 2; float c = a / b;

• Integer division: a / b = 3 (truncated).
• Implicit cast to float stores 3.0, not 3.5.
• Correct form: float c = (float)a / b; → 3.5.

Common Traps in NAT-I

• Confusing = (assignment, returns assigned value) with == (equality check).
• Skipping parentheses around mixed && / ||, so a > b || a < c && d parses as a > b || (a < c && d).
• Forgetting to update the loop variable, producing an infinite loop.
• Dividing by zero at runtime — not a syntax error.

Practice Prompts

1. Given the pseudocode i = 1; WHILE i < 10: i = i*2; PRINT i, predict the output.
2. Identify which flowchart symbol denotes a decision and what the two outgoing arrows represent.

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