CS-I MidTerm Review

Hao Jiang

Computer Science Department

Boston College

About Programs Data and method (procedures, routines, functions) are two

fundamental elements of programming

Two basic tasks in constructing a program Define data types and data elements Define procedures that process data

Two schemes of building a program Procedure oriented (what we have been learning) Object oriented (to be covered soon)

Data Types Java is a strong typed language: every data element (literal or

variable) has a “fixed” type

We have learned built-in types in Java, e.g., int, double, String (We will learn how to define our data types later)

Before using a variable, you must make sure that its type has been clearly “declared”

It is important to choose suitable data types in information processing.

Built in Data Types Numerical types are used for math computing

Numeric Types: byte, short, int, long, float, double “int” literals: 10, -5 “long” literals: 1034678L “double” literals: 1.234, -2.5, 1.0e10 “float” literals: 1.234f

Numerical Operators Operators:

Binary operators: +, - , * , / , % Unary operators: ++, -- (only for variables), +, -

Built in Data Types Boolean types are for logic computing

Boolean Types: boolean liberals: true, false

Operators: &&(and), ||(or), ! (not)

Comparison Boolean type can be generated by comparison operations:

>, <, >=, <=, ==, !=

For example, expression (a +c >= b) has a boolean type

Built in Data Types Character and string types are for text processing

Character Type: char literals: ‘a’, 3’, ‘+’, ‘/n’, ‘/t’, ‘#’

String Type: String literal: “Hello World”, “1234”, “1+2” Operator: +

“123” + ”abc”=“123abc” “123” + 4 = “1234” “123” + ‘4’ = “1234” Pay attention to the difference of

1 + 2 + “123” “123” + 1 + 2

The Precedence of Operators

( )!, unary -, unary +, ++(post), --(post)++(pre), --(pre)*, /, %+, -<, <=, >, >===, !=&&||=

High

low

Example: a++ > 5 || b < 6((a++) > 5) || (b < 6)

a && b || c && d <=> (a &&b) || (c && d)

Type in Java Every entity in Java that has a “value” has association with

specific data types.

Literals: 1.5, 2000, 1.0e20 Variables: n, m, counter, isPostive Expressions: counter++, n+m Functions: input and output must have clearly declared

variables with certain types.

Variables Variables are data elements whose values can “change” Variables are referenced by names Each variable must be declared to be of some “type” Assignment operation “=“

int n; double x, y;n = 10 + 5 - 4;int m = 20;n = m + n;x = 100 / n;n = x; illegal operation

n = (int) x;

promotion

variable = expression;

+=-=*=/=

Special Assignments

How can you changethe value of a variable?

The Scope of Variables Variable Scope: the part of code where you can reference a

variable.

{ int y; int x; { int z; }

}

Scopeof yScope

of x

{ int x; int y;

}

Differentx, y fromthe previous block

Scopeof z

A block in Java isusually enclosed by { }for instance: A function body,or blocks in if, for or while statements.

Procedures Java defines different language constructs for writing

procedures – an action or sequential of actions on data elements. The flow control statements

Straight-line code “If statement” for conditional branches “Loops” such as “while” loops or “for” loops for iterative

procedures.

Function (routine) Function is a basic tool in Java for “information hiding” The key of a function is the “interface”:

the input and output of the function.

The Flow of a Program

Straight line code

If conditionf()Elseg()

loops

Main() f()

g()

Straight Line Code

Logic group1

Logic group 2

Logic group 3

Good program structure

Logic group 3

Logic group 2

Logic group1

Bad program structure

If Statements

Nested if statement:

if (isA) doA;if (isB) doB;if (isC) doC;

if (isA) doA; else if (isB) doB; else if (isC) doC;

If (condition) { statements}else { statements}

or

If (condition) { statements}

Loops

while statement:while (condition) { statements}

for statement:for(initialization; condition; update) { statements}

• while loop is a general one. If you do not know the number ofIterations, you should use a while loop.

• For loop is more compact when dealing with counting up ordown a control variable.

• Prefer “for” loop when possible.

• Do not use “for” loop is “while” loop is more appropriate.

While Loops

double s = 0;int n = 0;while ( !StdIn.isEmpty() ) { double x = StdIn.readDoulbe(); s += x; n ++;}s = s / n;

while(s.charAt(0) ==‘ ‘) s = s.substring(1);

While (n%2 == 0) { n = n/2;}

Average:

Removewhite space:

Removefactor 2:

For Loops int N = 100; int s = 0; for (int i = 0; i < N; i++) { s += i; } double p = 0; for (int k = N ; k >= 1; k --) { p = p + (1.0/K) ; }

for (int i = 0; i < N; i++) { StdOut.println(); for (int j = 0; j < N; j ++) { if ( i % 2 == 0 && j % 2 == 0) StdOut.print(“*”); else if (I % 2 == 1 && j % 2 ==1) StdOut.print(“*”); else StdOut.print(“ “); }}

Sum:

Harmonic number:

Checkerboard:

More about Loops We can solve many problems using iterative methods.

But, how do I know a loop is correct?

There are quite a lot of things going on in a loop. Interestingly, the most important feature you should pay attention to is not something that changes but something that does not change.

Loop Invariant

int N = 100; int s = 0; for (int i = 0; i < N; i++) { s += i; }

loop invariant: After each iteration, s equals the summation from 0 to i.

int v = 1; while (v < N/2) v = 2*v;

loop invariants: At the beginning of each loop, v < N/2 After each loop v < N v is a power of 2 (true for both the beginning and the end of a loop)

1. The loop will terminate somehow2. Some loop invariant holds in each each loop.3. When the loop terminates, we obtain the desired result .

Checking a Loop

public static boolean isPowerOf2(int n) { while (n % 2 == 0) { n = n / 2; } if (n == 1) { return true; } else { return false; }}

Input and Output Standard Input and Standard output

Read from standard input, e.g., StdIn.readInt() Write to standard output, e.g., StdOut.print(x) Redirection: we can read from a file by using standard input and output.

Data Processing “Boston” 2 “San Diego” 40

… --------------------------------------------------------- int lowestTemperature = 200; String coldestCity = “”; while (!isEmpty()) { String city = StdIn.getString(); int temperature = StdIn.getInt(); if (temperature < lowestTemperature) { lowestTemperature = temperature; coldestCity = city; } }

Function Information Hiding

Using functions, we can hide the details of a procedure. This is the basic way of constructing large systems.

The most important feature of a function is its “interface” public static TYPE functionName(TYPE a1, TYPE a2, …) output input

Design by contract Each function is a “contract”.

From the client side (the caller): the function requires that the client must fulfill some condition before calling some procedure. For instance, function Math.sqrt(x) requires that x >= 0.

The function is a service provider: it must complete some specified task. For instance, Math.sqrt(x) will compute the square root of x when the function returns.

The client condition is termed as “precondition” and the service condition is called “postcondition”.

Math Functions Math functions

double x = -1000.0;double y = Math.abs(x);

Others: Math.sin() Math.cos() Math.random() Math.round() Math.max()…

String Functions The following functions can be used to process strings

String s = “abcd”;Char c = s.charAt(2);Int l = s.length();String t = s.substring(0,2);t = s.substring(2);

c = ‘c’l = 4

t = “ab”t = “cd”

abcdefg1234567

position0

Position13

Recursion Recursion is an important method to construct functions to

solve problems.

To construct a recursive function: You should have a method to “reduce” your problem into smaller

same problems that can be combined to solve the current problem.

You know how to solve the trivial base problem. You make sure that the reeducation plan will NOT go infinitely.

Recursion Example:

Example: A function that returns a string that reverse the original one, e.g. if the input is “abc” the output is “cba”.

Base case: If the input is has a single character, return itself.Reduction plan: Reverse the substring starting from the second character and then connected with the first character.

String reverseString(String str){ if (str.length() == 1) return str; string s = reverseString(str.substring(1)) + str.charAt(0); return s;}

Recursion Example: Example: Test whether a number is a power of 2.

boolean isPowerOfTwo(int n){ if (n == 1) return true; if (n % 2 == 1) return false; return isPowerOfTwo(n/2); }

Iteration or Recursion? Even though iteration is more appealing at first thought,

recursion solution is usually easier to construct (bug free).

Some times recursion is not efficient (to compute a Fibonacci sequence).

The Procedure based Programming

Get initial time

Draw the Panel

Start the Clock

int hour = Integer.parseInt(args[0]);int minute = Integer.parseInt(args[1]);int second = Integer.parseInt(args[2]);

drawFace()

while (true) { clearHandsDrawingRegion(); drawHands(hour, minute, second); hour = updateHour(hour, minute, second); minute = updateMinute(hour, minute, second); second = updateSecond(hour, minute, second); wait(1);}

Function blocks Procedures in Java

The clock:

public static void main(String[] args) { int hour = Integer.parseInt(args[0]); Int minute = Integer.parseInt(args[1]); int second = Integer.parseInt(args[2]); drawFace(); while (true) { clearHandsDrawingRegion(); drawHands(hour, minute, second); hour = updateHour(hour, minute, second); minute = updateMinute(hour, minute, second); second = updateSecond(hour, minute, second); wait(1); }

public class Clock { public static void drawMarkers() { … } public static void drawLetters() { … } public static void drawFace() { drawMarkers(); drawLetters(); } public static void clearHandsDrawingRegion() { … } public static void drawHands(int h, int m, int s) { … } public static int updateHour(int h, int m, int s) { … } public static int updateMinute(int h, int m, int s) { … } public static int updateSecond(int h, int m, int s) { … }

The structureof the wholeprogram