Transcript 2013 - Zoo - Yale University

CS 112 Introduction to
Programming
Pre-Midterm Exam Review Session
Computer Science Department
Yale University
March 27, 2013
Admin
 Assignments
Assignments 1-5: Returned (via Assignments tab)
 Assignment 6: Due Mar. 26 (yesterday), 11:55 PM
 Assignment 7: Will Be Posted Shortly

 Midterm – Friday, Mar. 29 during class
 Office Hours – Every Day this Week
 Check the CS112 website for the schedule
 Send us feedback!
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Midterm Structure
 Content

List of topics covered available on the class website
 Question Types
 Writing Methods or Code Snippets
 Making Corrections to Code
 Analyzing “Black Box” Algorithms
 Discussing Code Implications, Trade-offs, etc.
 Study
 Lectures thru Mar. 25
 Homework thru Assignment 6
 Readings Listed in the Class Schedule (thru Mar. 25)
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Review Session Overview
 Core Content Areas
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Basic Java Architecture
Primitive Data Types
Variables
Flow Control
Program Analysis Using Assertions
Methods
Standard Java Classes
 Sample Questions
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Basic Java Architecture
 Java is architecture-neutral – compile once,
and your code works anywhere via the “Java
Runtime Environment”
 Using Java:
 runs a Java .class file via the Java runtime
 javac  compiles Java source code (.java files) into
Java “bytecode” (.class files)
 jar  enables compression of multiple files (e.g.,
.java, .class, etc) into a single file
 java
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Primitive Data Types
Data Type
Description
Examples
int
Numeric: Whole numbers
int x = 123;
double, float Numeric: Includes Decimal
double x = 123.001;
boolean
True/False (yes/no, on/off)
boolean x = true;
char
Single letter/number character
char x = ‘a’;
String
Multiple letter/number characters
String x = “Hello
world”;
 Traditional order of operations applies
 Remember:
Numeric operations require numeric data types
 Typecasting numeric values is a “safe” approach
 Integer math is non-conventional! (e.g., rounding)
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Primitive Data Types
Operator
Description
Examples
=
Assignment operator
int x = 123;
==, !=
Tests for equality/inequality
if (x == y)
<=, >=
Less than/greater than
comparators
if (a >= b)
+, -, *, /, %
Addition, subtraction, division,
multiplication, and modulus
int x = 1701 – 30;
+=, -=, *=,
/=, %=
In-line arithmetic/assignment
operators
x += 2;
!
The “not” operator – reverses the
true/false result of an expression
If (!(a == b))
||
Boolean “or” operator
if (a == b || b == c)
&&
Boolean “and” operator
if (a == b && b == c)
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Variables
 Distinguish local from global variables
public class Hello{
public static String name = “World”; // Global Variable
public static void main(String args[]){
int numTimes = 5; // Local Variable
for (int i = 0; i < numTimes; i++){
System.out.println(“Hello “ + name);
}
}
}
 Use the final keyword for constants!
public static final double PI = 3.14159;
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Flow Control
Conditional Statements
Loops
if/else if/else
for
switch
while
 All control statements rely on evaluation of
boolean expressions
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Determines whether to enter/continue or skipover/exit
 Interchangeability of if/else if/else with
switch, and for with while
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Recognize appropriateness of each
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Flow Control
for (initialization; test; update) {
header
statement;
statement;
...
body
statement;
}
======================================
initialization;
while (test) {
header
for is more appropriate
statement;
when number of
statement;
iterations is known in
...
advance! Otherwise, use
body
statement;
while.
update;
}
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Flow Control
 Several different loop designs
 Continuous – iterates ad infinitum
 Early Exit – use of the break keyword causes the
loop to “stop early”
 Sentinels – a certain condition is tested on each
iteration and when it is no longer true, the loop exits
(most common)
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Flow Control
Optional
Optional
if (test) {
statement;
}
else if (test) {
statement;
}
else {
statement;
}
switch (variable) {
case value:
statement;
break;
case value:
Optional
statement;
break;
default:
Optional
statement;
break;
}
 A switch statement is only capable of testing
for equality! (i.e., cannot use other comparison
operations)
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Program Analysis Using Assertions
 Recognize that testing for a particular
condition in one place ensures that a condition
is true elsewhere
int studentGradeLevel = 9;
if (studentGradeLevel >= 9) {
// Student is grade 9 or higher
...
statement;
// Student is still grade 9 or higher
}
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Methods
 Definition Format
public static <return-type> <name> (<type> <name>, …) {
<statement>(s);
}
 Calling and Passing Parameters
 rand.nextInt();  no parameters
 rand.nextInt(50);  50 is the parameter
 Because nextInt() is overloaded (i.e., has more than one
definition), both calls work!
 Overloading methods
 Allows multiple methods to share the same name
 Overloaded methods must differ in terms of parameter data
types and/or order
 Variables declared inside methods are local
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Standard Java Classes
Data
Type
Relevant Methods
Examples
String
length()
Returns length (number of characters) in the String
charAt(int x)
Returns the character in the String at position x
substring(int
startPos, int endPos)
Returns a String containing the characters in the
string from startPos to endPos, inclusive
round(double x)
Returns the integer-rounded value of x
max(int a, int b) /
min (int a, int b)
Returns the larger/smaller of a or b
sqrt(double x)
Returns the square root value of x
Math
Scanner next(), nextInt(),
nextDouble()
Random
Reads next String/int/double from Scanner
hasNext(),
hasNextInt(),
hasNextDouble()
Determines whether there is another
String/int/double to be read from the Scanner
nextInt(int x)
Returns a randomly-generated integer between 0
(inclusive) and x (exclusive)
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Standard Java Classes
 Other common methods:
System.out.print();
 System.out.println();
 System.out.printf();
 Review printf placeholders
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 DrawingPanel and Graphics
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Standard Java Classes
Method name
Description
g.setColor(Color);
set Graphics to paint any following
shapes in the given color
g.drawLine(x1, y1, x2, y2);
line between points (x1, y1), (x2, y2)
g.drawOval(x, y, width, height);
outline largest oval that fits in a box of
size width * height with top-left at (x, y)
g.drawRect(x, y, width, height);
outline of rectangle of size
width * height with top-left at (x, y)
g.drawString(text, x, y);
text with bottom-left at (x, y)
g.fillOval(x, y, width, height);
fill largest oval that fits in a box of size
width * height with top-left at (x, y)
g.fillRect(x, y, width, height);
fill rectangle of size width * height
with top-left at (x, y)
http://download.oracle.com/javase/6/docs/api/java/awt/Graphics.html
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Sample Questions
 Identify the syntax and logic problems in the
below code, as well as stylistic issues:
class Hello{ static
public void main(String args[]){
int x = 0;
Add quotes
while (x < 5){
System.out.println(Hello world!);
Use a
x += 1;
x++ is shorter
for
loop
}
instead
public
return true;
}
Method is void, thus
should not have a
return value
}
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Sample Questions
 What does the below method do?
public static void doSomething(){
for (int i = 0; i < 8; i++){
for (int j = 0; j < i; j++){
System.out.print(“+”);
}
for (int j = 0; j < 8 – i; j++){
System.out.print(“*”);
}
System.out.println(“”);
}
}
Prints:
********
+*******
++******
+++*****
++++****
+++++***
++++++**
+++++++*
Note that this line is not completely filled by plus
signs – be cognizant of off-by-one issues with loops!
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Questions?
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Thank You!
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