CSC 142/Chapter 3

Chapter 3: Parameters and Objects

Sections:

3.1 Parameters

3.2 Methods returning values

3.3 Using objects

3.4 Projectile trajectory

Section 3.1: Parameters

Definitions

Definitions:

• Parameter/parametric/parameterize
• Formal parameter
• Actual parameter
• Method signature
• method overloading
• Parameters = arguments (language)

Material

Parameterization:

• Form of abstraction
• What we are doing is generalizing a task
• Not just solving x2 + 4x + 2 = 0, solving ax2 + bx + c = 0
• Example code for parameterizing print statements
• How do you use/declare parameters in a class method?
• How not to declare parameters - not writeSpaces(int numLines)
• Differentiating between TYPE DECLARATION/DEFINITION, and the ACTUAL METHOD CALL
• Parameters: data --> method
• Each method is defined to have its own scope, as denoted by {}. So how to pass data through the scope?

Scope:

• Scope means, all variables from outside the function are unknown
• Data is passed in as fresh
• Parameters are variables that are predefined in that fresh variable space
• Scope ties in with return values - once you do a calculation, how do you get the result out?
• Return values/return variables.
• Overloading methods: so that we can handle variations in the input data.

Section 3.2: Methods that Return Values

Definitions

Definitions:

• Return
• Object
• class

Material

Example of when you need to return a value: square root function

• Method syntax: how to declare a function's return type (void --> int/double/etc)

Real life example: Java Math (static class, static methods)

• Math constants E and PI
• Casting and math functions
• How to explore the Java API
• Just focus on Math, don't get overwhelmed

Math example:

• Carl Gauss, sum of first 100 integers
  • Excellent example of how different ways of tackling problems (different algorithms) can lead to vast speedups
  • Formula:

$ \sum_{i=1}^{n} i = \dfrac{ n(n+1) }{2} $

To program this formula:

public static int sum(int n) {
    return (n * (n+1)) / 2;
}

How return statements work: value is returned where function call happens, so we need to assign the result of the function call to a variable.

Math example: Pythagorean theorem

• find the hypotenuse, given sides a and b

public static double hypotenuse( double a, double b ) {
    double c = Math.sqrt( Math.pow(a, 2) + Math.pow(b, 2) );
    return c;
}

How to debug, if this isn't working? Split calculation into more parts: csquared, c, etc.

Section 3.3: Using Objects

Definitions

Definitions:

• Object
• Class
• Index
• Immutable object
• Exception
• Console input
• Constructor
• Token
• Whitespace
• Package
• Declaration

Material

Primitive types and objects:

Outline:

• String objects (and return values and mutability)
• Interactive programs and user input (scanner0
• Sample interactive program

This is where we focus on Strings

• Tokenization, strings, etc - that gets at arrays, which is Chapter 7.

String tokenization section for ciphers.

Metaphor: Why experimentation is necessary in programming.

• We talked about James Gosling.
• Does your brain work like his?
• Does your brain work like Java compiler?
• Nope! So we can't just think our way through problems
• Need to be improving our mental compiler, our mental model of what the code will do
• Iterative refinement

In-Class Worksheet

In-class worksheet: Caesar Cipher

• How Caesar cipher works
• Specifications: give them a main method, and call the cipher. They define the cipher method.

Handout for Caesar Cipher:

• Give them a hint on how to do the char shift
• Encourage playing around/experimentation

Section 3.4: Projectile Trajectory

Material

Case study with complex example, covering:

• Input parameters
• methods that return values
• mathematical calculations
• scanner for user input

Basic projectile problem: given an initial velocity and angle, answer questions;

• Highest point, and the time for that point
• Time to ground
• Distance (horizontal distance) traveled
• Basically: x/y path lengths, and times
• Print usage/intro
• Print pretty table

Program:

• User input
  • Get vel at t=0
  • Get angle at t=0
  • Get steps to display
• Prep calc
• For t in Nsteps
  • Increment t
  • Increment x
  • Increment y

Chapter 3 Summary

Assessment Material

Parameters:

• Notation
• Purpose
• How they work

Return values:

• Notation
• Purpose
• How they work

Objects

• LOTS of definitions nad new concepts
• Use of String objects to understand these concepts

Projectile trajectory:

• Focusing on a complex application/program

Chapter 3 Code

Lecture Code

Quadratic equation solver

• Takes arguments a, b, c
• No return (two primitive values requires object)
• Will cover later

Sum of integers

• Sum of first n integers, ties in with Carl Freidrich Gauss, algorithmic thinking, pattern-finding

Pythagorean theorem

• hypotenuse
• let's recap what we have so far: we've solved a projectile gravity equation, solved a quadratic equation, summed up the first N integers, solved the Pythagorean theorem
• no limit to the kinds of mathematics we can implement
• very deep and profound connection between programming and mathematics

Scope

• Illustrate a few simple scope examples

Worksheet Code

Caesar cipher

• Char shift is the key here, getting them to think about how to implement modular arithmetic AND how to use that integer offset to shift chars
• Get them to link chars with integers
• Hint: try executing this Java code: "'a' + 5"

Chapter 3 Homework

HW Summary

(Recommended) Self-check problems: #3, #7, #9, #13, #14, #20, #26

(Required) Exercises: #1, #6, #16, #19

(Required) Projects: (none)

HW Details

Self-check:

• 3 - spotting mistakes
• 7, 9 - mystery program
• 13 - math operations
• 14 - mystery return
• 20 - string method calls
• 26 - get a phrase from user and repeat phrase

Exercises:

• 1 - print formatting digits
• 6 - function find larger of abs of 2 numbers
• 16 - triangle area using Heron's formula
• 19 - reverse string

Chapter 3 Goodies

Puzzle 3

Puzzles/Fall 2016/Puzzle 3: Et three, brute? (Caesar cipher cryptograms with no spaces (frequency or brute force)

Profiles

Carl Freidrich Gauss

• Adding up the sum of all integers from 1-100
• Add up 100 numbers: pair off, and get 101 each time. 100 and 1, 99 and 2, etc. Then 100*101/2 = 10100/2 = 5050
• First approach: don't manage the complexity, just dive in and have at it
• Gauss: found an alternative pattern
• By adding last and first, then second last and second first, and so on, got same number for each
• Utilizing this turned a sum problem into a multiplication problem
• Lesson: by changing your pattern of thinking/problem solving, can open up new, better, faster approaches

Julius Caesar

• History lesson: why important
• Sometimes, connecting seemingly unconnected topics can lead to sudden insights
• Need for security, military applications of cryptography
• Back to the definition of programming: "turning a task that requires repeated computation into something that a machine can execute"
• Computation is as old as civilization itself