Variables, Expressions, and Data Types – Comprehensive Study Notes chapter 2

Variables and Assignments

• A variable = named storage location that holds a value until reassigned.
  • Notation: $x$, $numPeople$, $userAgeYears$, etc.
  • Box analogy: think of a labelled box whose content can be replaced.
• Assignment statement: $\text{variable} = \text{expression}$
  • Left side must be a variable; right side can be literal, variable, or any valid expression.
  • Read as “is assigned with”, NOT “equals”.
  • Examples
  • $x = 5$
  • $y = a$ (copies $a$’s current value)
  • $z = w + 2$
• Overwriting: new assignment replaces old value until another assignment occurs.
  • E.g. $x = 6$ then $x = x + 1$ → $x = 7$, old 6 lost.
• Variable can appear on both sides: $count = count - 1$, $balance = balance * 1.05$.
• Important invalid forms
  • $5 = x$ ❌ (left side not variable)
  • $x + y = 5$ ❌ (left side expression, not variable)

Identifier Rules & Naming Conventions

• Valid characters: letters, digits, underscore; must start with a letter.
• Case-sensitive: $numCats$ ≠ $NumCats$.
• Reserved words (integer, float, Get, Put, if, while, …) cannot be used.
• Style conventions
  • lowerCamelCase: $numApples$, $peopleOnBus$.
  • underscoreseparated: $num</em>apples$.
  • Be meaningful & concise; avoid cryptic abbreviations and excessive length.
  • Choose best identifier per purpose (e.g., $numberOfJellyBeansInJar$ better than $nmJlyBnsInJr$).

Integer Variables & Declarations

• Declaration syntax (Coral): integer userAgeYears.
• Default initial value in Coral = 0.
• Range ≈ $-2\text{B}$ to $+2\text{B}$ (32-bit).
• Typical use: countable quantities (cars, pizzas, days).

Assignment Semantics & Tracing

• Example trace:
  1. x = 3, y = 8, z = 8
  2. x = 2, y = 6, z = 0
  3. x = 9
     Final: $x = 9,\; y = 6,\; z = 0$.
• Participation activities reinforce manual tracking.

Expressions & Operators

• Expression = combination of literals, variables, operators, parentheses; evaluates to a value.
• Literals: integers (5), floats (3.14), strings ("Hi").
• Arithmetic operators
  • Addition $+$
  • Subtraction / Unary minus $-$
  • Multiplication $*$
  • Division $/$
  • Modulo $$ (integer remainder)
• Validity examples
  • $x + 1$ ✔️
  • $2 * (x - y)$ ✔️
  • $3x$ ❌ (implicit multiplication not allowed; must write $3 * x$).

Precedence Rules & Parentheses

1. Parentheses $(\;)$
2. Unary minus (negation)
3. $*$ and $/$ (left→right)
4. $+$ and $-$ (left→right)

• Good practice: add parentheses to clarify intent, e.g. y = (m * x) + b.
• Calories formulas (with explicit parentheses):
  • Men: $\text{calories} = \big((\text{Age} \times 0.2017) - (\text{Weight} \times 0.09036) + (\text{HR} \times 0.6309) - 55.0969\big) \times \dfrac{\text{Time}}{4.184}$.
  • Women analogous with different constants.

Floating-Point (float) Numbers

• float = real number; decimal can “float”. Examples: 98.6, 0.0001, -666.667.
• Declaration: float milesTraveled (default 0.0).
• Typical use: measured values or fractions of counts.
• Literal style: always include leading digit (0.5 not .5).
• Output formatting: Put var to output with N decimal places → rounds display only.
  • Example: 0.125 with 1 decimal → 0.1 (rounded).
• Scientific notation output possible: 2.99792e+08.
• Division by zero (float)
  • Non-zero ÷ 0 → $\infty$ or $-\infty$.
  • 0 ÷ 0 → NotANumber.

Math Functions

• Built-in Coral functions:
  • $\text{SquareRoot}(x)$ → $\sqrt{x}$.
  • $\text{RaiseToPower}(x, y)$ → $x^y$.
  • $\text{AbsoluteValue}(x)$ → $|x|$.
• Functions can be nested: RaiseToPower(2.0, RaiseToPower(x, y) + 1).
• Mass-growth example: $\text{finalMass} = m_0 \times (1 + r)^{t}$ using RaiseToPower.
• Pythagorean calculation uses chained calls.

Random Numbers

• RandomNumber(low, high) → uniform integer in [low, high].
  • #values = $\text{high} - \text{low} + 1$.
  • Dice: RandomNumber(1, 6).
  • Coin: RandomNumber(0, 1).
• Pseudo-random: deterministic sequence seeded by SeedRandomNumbers(seed).
  • Omit seeding → auto-seed via current time (different each run).
  • Fixed seed useful for reproducible tests.
• Seat-moving example: pick row 1-20 and column sets 1-15 vs 16-30.

Integer Division & Modulo (%)

• Integer division $/$ truncates toward 0.
  • $10 / 4 = 2$, $5 / 9 = 0$.
• Float division occurs if either operand is float.
• Divide by zero (integer) → runtime error, program terminates.
• Modulo $a \% b$ returns remainder.
  • $23 \% 10 = 3$, $70 \% 7 = 0$.
• Phone-number breakdown example uses / and % to split digits.

Type Conversion & Casting

• Implicit conversion (automatic)
  • In mixed expression, int → float so operation is float.
  • Assigning float to int → fraction truncated.
• Explicit cast: multiply int by 1.0 to force float before division.
  • Correct: (1.0 * a) / b yields float.
  • Common error: 1.0 * (a / b) still performs int division first.
• Resulting type examples:
  • 3 / 2 → int 1.
  • 3.0 / 2 → float 1.5.

Constants

• Constant = named immutable value.
  • Syntax (conceptual): const float SOUND_SPEED = 761.207.
• Convention: ALL_CAPS with underscores.
• Benefit: readability & single-point change (e.g., PRICE_DISCOUNT).

Data Types Overview

• Integer — whole numbers, counts.
• Float — real numbers, measurements.
• Boolean — true / false states.
• Character — single symbol.
• String — sequence of characters.
• Array — ordered list of same-typed items.
• Coral fully supports integer, float, array; Booleans arise from comparisons; strings as literals for output.

Example Programs & Incremental Development

• Health-data app developed in stages:
  1. Age in days: $\text{ageDays} = \text{ageYears} \times 365$.
  2. Add leap-year days: $+ \lfloor\text{ageYears}/4\rfloor$.
  3. Convert to minutes: * 24 * 60.
  4. Estimate heartbeats: multiply by avg 72 bpm.
• Travel-time calculator: flying vs driving for input miles.
• Lightning-distance estimator using constants SEC_PER_HOUR and SOUND_SPEED.
• Numerous zyLabs: house pricing, caffeine decay (half-life), divide-by-x, driving costs, simple statistics, musical note frequencies, phone-number parsing.

Style Guidelines & Readability

• One statement per line; blank line separates declarations from executable code.
• Single space around operators (a + b, not a+b).
• Unary minus exemption: x = -y.
• Avoid “magic numbers”; use constants.

Common Errors & Pitfalls

• Confusing assignment with equality.
• Invalid left-hand side of =.
• Forgetting that / between ints truncates.
• Casting result instead of operands.
• Omitting parentheses, relying on ambiguous precedence.
• Using % or / with zero divisor.
• Writing 3x instead of 3 * x.

Key Takeaways

• Think of variables as labelled containers; assignment updates contents.
• Master precedence rules but prefer explicit parentheses.
• Choose integer vs float based on count vs measure.
• Use built-in math & random utilities; seed for reproducibility.
• Employ constants and naming conventions for maintainable code.
• Understand type conversions to avoid subtle integer-division bugs.
• Practice incremental development and thorough testing.