Chapter 4Statement Forms

Statement types

1. Simple statementsexpression;

println(“The total is “ + total + “.”);(method call)

2. Compound statements{

statement1;...

statementn;}

3. Control statements

Conditional statements: if, switchIterative statements: for, whileExample: Add more than 2 integersThree startegies

Duplicating the code in Add2Integers;Repeating the input cycle a predetermined number of

times;Repeating the input cycle until the user enters a special

sentinel value;

public class Add4Integers extends ConsoleProgram { public void run() { println(“This program adds four integers.”); int n1 = readInt(“ ? “); int n2 = readInt(“ ? “); int n3 = readInt(“ ? “); int n4 = readInt(“ ? “); int total = n1 + n2 + n3 + n4; println(“The total is “ + total + “.”); }}

• Difficult to generalize• Cumbersome to add 100 values

The repeat-N-times idiom

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

statements to be repeated

}• header line: for (int i = 0; i < N; i++)• body: statements to be repeated

/* * file: AddNIntegers.java * ------------------------------ * This program adds a predefined number of integers * and them prints the total. To change the number of * integers, change the definition of N_VALUES */

Import acm.program.*;

public class AddNIntegers extends ConsoleProgram {

public void run() { println(“This program adds “ + N_VALUES + “integers.”); int total = 0; for (i = 0; i < N_VALUES; i++) {

int value = readInt(“ ? “); total += value;

} println(“The total is “ + total + “.”); }

/* specifies the number of values */ private static final int N_VALUES = 4;}

Tracing the program

public void run() { println(“This program adds “ + N_VALUES + “integers.”); int total = 0; for (i = 0; i < N_VALUES; i++) {

int value = readInt(“ ? “); total += value;

} println(“The total is “ + total + “.”); }

N_VALUES (4)total (0)i (0)value ( )

The repeat-until-sentinel

Read numbers until the user signals the end of input data.

Sentinel: A special value, not a legitimate data value, signals the end of input data.

Examples• Add a list of integers

sentinel: 0• Average grades

Sentinel: -1

Repeat-until sentinel idiom

while (true) { prompt user and read in a value if (value == sentinel) break; rest of the loop body}

A sample output

This program adds a list of integers.Enter values, one per line, using 0to signal the end of the list. ? 1 ? 2 ? 3 ? 0This total is 6.

/* * File: AddIntegerList.java * -------------------------------- * This program reads integers one per line until the * user enters a special sentinel value to signal the * end of the input. At that point, the program prints * the sum of the numbers entered so far. */

import acm.program.*;

public class AddIntegerList extends ConsoleProgram {

public void run() { println(“This program adds a list of integers.”); println(“Enter values, one per line,”); println(“using “ + SENTINEL + “ to signal the end of the list.”);

int total = 0; while (true) { int value = readInt(“ ? “); if (value == SENTINEL) break; total += value; } println(“The total is “ + total + “.”); }

/* Specifies the value of the sentinel */ private static final int SENTINEL = 0;}

Tracing the program

int total = 0;

while (true) { int value = readInt(“ ? “); if (value == SENTINEL) break; total += value; } println(“The total is “ + total + “.”); }

SENTINEL (0)total (0)value ( )

Average grade

Changes• new variable: int count• new variable: double average• SETINEL = -1• comments

public class AverageGrade extends ConsoleProgram {

public void run() { println(“This program calculates average grades.”); println(“Enter grades, one per line,”); println(“using “ + SENTINEL + “ to signal the end of the list.”);

int total = 0; int count = 0; while (true) { int value = readInt(“ ? “); if (value == SENTINEL) break; total += value; count++; } double average = (double) total / count; println(“The average is “ + average + “.”); }

/* Specifies the value of the sentinel */ private static final int SENTINEL = -1;}

Zeros are not counted

int total = 0;

int count = 0; while (true) { int value = readInt(“ ? “); if (value == SENTINEL) break; if (value > 0) { total += value; count++; } } double average = (double) total / count; println(“The average is “ + average + “.”);

What if count = 0? int total = 0; int count = 0; while (true) { int value = readInt(“ ? “); if (value == SENTINEL) break; if (value > 0) { total += value; count++; } } if (count == 0) { println(“No grades entered.”); } else { /* count > 0 */ double average = (double) total / count; println(“The average is “ + average + “.”); }

Example: IsLeapYear.java /* * File: IsLeapYear.java * -------------------------- * * This program reads in a year and determines whether it is a * leap year. A year is a leap year if it is divisible by four, unless * it is divisible by 100. Years divisible by 100 are leap years only * if divisible by 400. */

Import acm.program.*;

public class IsLeapYear extends ConsoleProgram { public void run() { println(“This program checks for leap years.”); int year = readInt(“Enter year: “); boolean isLeapYear = (((year % 4) == 0) && ((year % 100) != 0)) || ((year % 400) == 0); if (isLeapYear) { println(year + “ is a leap year.”); } else {

println(year + “ is not a leap year.”); } }}

Four forms of if statementsSingle-line

if (condition) statementMultiline

if (condition) { statements;}

if-elseif (condition) { statement1} else { statement2

}

Cascadingif (condition1) {

statements1} else if (condition2) { statements2 ...} else {

statements}

Example: Sign of x

If x > 0, sign(x) = 1; if x < 0, sign(x) = -1; if x == 0, sign(x) = 0.

Using a table

x > 0 < = 0

< 0 = 0

sign(x) 1 -1 0

The ? : operator

condition ? expressionT : expressionFequivalent to

if (condition) { expressionT } else {

expressionF}

Examples

• Max = (x >y)? x : y;

• println(nItems + “item” + ((nItems == 1) ? “” : “s”) + “ found.”);

The switch statementswitch (e) { case c1: statements1 break; case c2: statements2 break; ... more case clauses ... default: statements break;}Useful when program must choose among several cases.

Good programming practice

• Although a break is not necessary for each case, it is advised to include a break at the end of each case.

• Although a default clause is optional, it is advised to include a default clause unless you are absolutely sure that you have covered all the cases (using a table).

Examplepublic void run() { println(“This program shows the number of days in a month.”); int month = readInt(“Enter numeric month (Jan = 1): “); switch (month) { case 2: println(“28 days (29 in leap years)”); break; case 4: case 6: case: 9 case 11: println(“30 days”); break; case 1: case 3: case 5: case 7: case 8: case 10: case 12: println(“31 days”); break; default: println(“Illegal month number”); break; }}

Tracing the program.

The while statement

while (condition) { statements}• The conditional test is performed only at the beginning of a

loop cycle. If the condition happens to become false at some point in the middle of the loop, the program doesn’t notice the fact until it has executed a complete cycle. At that point, the program evaluates the test condition again. If it is still false, the loop terminates.

• If you must quit loop inside a cycle, use break. See later.

Example: DigitSum

Pseudo code:

public void run() { println(“This program sums the digits in an integer.”); int n = readInt(“Enter a positive integer: “); int dsum = 0; for each digit in the number, add that digit to dsum; println(“The sum of the digits is “ + dsum + “.”);}

Find digits in an integer1729 % 10 = 91729 / 10 = 172

...1 % 10 = 11 / 10 = 0

/* for each digit in the number, add that digit to dsum */while (n > 0) { dsum += n % 10; n /= 10;}

public void run() { println(“This program sums the digits in an integer.”); int n = readInt(“Enter a positive integer: “); int dsum = 0; while (n > 0) { dsum += (n % 10); n = n /10; } println(“The sum of the digits is “ + dsum + “.”);}

Trace the program.

Loop terminationg

• When you use a while loop in a program, it is important to make sure the loop eventually terminates.

while (n >= 0) { dsum += n % 10; n /= 10;}Infinite loop!• Find out the command sequence on your computer

which stops an infinite loop.

The loop-and-half patternswhile (true) { prompt user and read in a value if (value == sentinel) break; process the data value}or the sentinel-based loop

Prompt user and read in a valuewhile (value != sentinel) { process the data value prompt user and read in a value}

Two drawbacks of the sentinel-based loop:

1.The order of the operations in the loop in unnatural, hard to follow. (Process the data value, then read in a new value.)

2.Duplication of code presents a serious maintenance problem. Subsequent edits to one copy might not be made in the other.

The for statement

for (init; test; step) { statements}

Example: countdownInt START = 10;for (int t = START; t >= 0; t--) { println(t);}println(“Liftoff!”);

The expressions init, test, and step are each optional,

but semicolons must appear.

• If init is missing, no initialization is performed;• If test is missing, it is assumed to be true;• If step is missing, no action occurs between loop

cycles.

for ( ; ;) is equivalent to while (true)

Effect of each of the following for statements

for (int i = 0; i < N; i++)

for (int i = 1; i <= 10; i++)

for (int n = 99; i >= 1; n -= 2)

for (int x = 1; x <= 1024; x *= 2)

Simple graphic animation

Moving a square diagonally.

/* File: AnimatedSquare.java * ------------------------------------ * This program animates a square so that it moves from the * upper left corner of the window to the lower right corner. */

import acm.graphics.*;import acm.program.*;

public class AnimatedSquare extends GraphicsProgram {

public void run() { GRect square = new Grect(0,0,SQUARE_SIZE, SQUARE_SIZE); square.setFilled(true); add(square); double dx = (double) (getWidth() – SQUARE_SIZE) / N_STEPS; double dy = (double) (getHeight() – SQUARE_SIZE) / N_STEPS; for (int i = 0; i < N_STEPS; i++) { square.move(dx, dy); pause(PAUSE_TIME); }

/* private constants */ private static final int N_STEPS = 1000; private static final int PAUSE_TIME = 20; private static final int SQUARE_SIZE = 50;}

for vs whilefor (init; test; step) { statements}is equivalent toInit;while (test) { statements step;}

Prefer for over while, when you have choice.

Nested for statements2D applications.Example: Checkerboard

/* * File: Checkerboard.java * -------------------------------- * This program draws a checkerboard. The dimensions of * checkerboard are specified by the constants N_ROWS and * N_COLIMNS, and the size of the square is chosen so that the * checkerboard fills the available vertical space. */

import acm.graphics.*;import acm.program.*;

public class Checkerboard extends GraphicsProgram {

public void run() { double sqSize = (double) getHeight() / N_ROWS; for (int i = 0; i < N_ROWS; i++) { for (int j = 0; j < N_COLUMNS; j++) { double x = j * sqSize; double y = i * sqSize; GRect sq = new GRect(x, y, sqSize, sqSize); sq.setFilled(((i + j) % 2) != 0); add(sq); } /* for j */ } /* for i */

/* private constants */ private static final int N_ROWS = 8; private static final int N_COLUMNS = 8;}

Using for with floating-point data

double h = 0.1;for (double x = 1.0; x <= 2.0; x += h) { println(x);}

Output:1.01.11.2000000000000002...1.9000000000000008

Why?Decimal 0.1 cannot be exactly represented in binary.

During the assignment h = 0.1, decimal 0.1 is rounded to it closest double floating-point, which is slightly larger than 0.1.

Caution: more rounding errors occur during addition.

for (double x = 0.0; x <= 0.5; x += h) { println(x);}The final value of x is 0.5 exact.