The if-else statement
Introducing the if-else statement
• Programming problem:
• Re-write the a,b,c-formula program to solve for complex number solutions when b2 - 4ac < 0
Introducing the if-else statement (cont.)
• Algorithm:
input a, b, c;
Det = b*b - 4*a*c; // Compute the determinant
if ( Det >= 0 ) { print -b/(2a) + sqrt(Det)/(2a); // Real number solutions print -b/(2a) - sqrt(Det)/(2a); }
if ( Det < 0 ) { print -b/(2a) "+" (sqrt(-Det)/(2a) + "i"); // Complex number solutions print -b/(2a) "-" (sqrt(-Det)/(2a) + "i"); }
Introducing the if-else statement (cont.)
• Java program:
import java.util.Scanner;
public class Abc3 { public static void main(String[] args) { double a, b, c, Det, re, im; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c
Introducing the if-else statement (cont.)
Det = b*b - 4*a*c;
if ( Det >= 0 ) { System.out.println( (-b + Math.sqrt( Det ) ) / (2*a) ); System.out.println( (-b - Math.sqrt( Det ) ) / (2*a) ); }
if ( Det < 0 ) { re = -b/(2*a); // Compute real part im = Math.sqrt( -Det )/(2*a); // Compute imaginary part
System.out.println( re + "+" + im + "i" ); System.out.println( re + "-" + im + "i" ); } } }
Introducing the if-else statement (cont.)
• Example Program: (Demo above code) – Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/Abc3.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Abc3.java
• To run: java Abc3
Introducing the if-else statement (cont.)
• Example:
Enter a:1
Enter b:2
Enter c:5
-1.0+2.0i
-1.0-2.0i
Introducing the if-else statement (cont.)
• Shortcoming:
• We have to negate the if-condition ourselves...
• This can introduce unnecessary errors
Introducing the if-else statement (cont.)
• Solution:
• Extend the if-statement with an alternative statement
• The alternative statement is only executed when the if-condition is false
The if-else statement in Java
• The if-else statement:
• The if-else statement is the second conditional statement in Java
• The if-else statement selects one of two statements to be executed based on a given condition
Syntax and meaning of the if-else-statement
• Syntax of the if-else-statement:
if ( CONDITION ) ONE-statement
else
ONE-statement
Syntax and meaning of the if-else-statement (cont.)
• Explanation:
• The keyword if announces (to the Java compiler) that we started an if-else-statement
• A conditional clause ( CONDITION ) follows the keyword if
• This is the condition of the if-else-statement
Syntax and meaning of the if-else-statement (cont.)
• This is the condition of the if-else-statement
• Following the condition clause, you can write (only) one statement
• Following the then-part, you must specify the keyword else followed by (only) one statement
• This statement will only be executed if the condition is true
• This statement will only be executed if the condition is false
Syntax and meaning of the if-else-statement (cont.)
• Note:
The way that the Java compiler decide whether a conditional statement is:
is by the presence/absence of the keyword else.
•An if-statement
•An if-else-statement
Computer Jargon: else-part
• The statement following the keyword else in an if-else-statement is called
• The else-part of the if-else-statement (Or else-part for short)
Computer Jargon: else-part (cont.)
• Schematically:
The a,b,c-formula using an if-else-statement
• Programming problem:
• Algorithm:
• Re-write the a,b,c-formula program to solve for complex number solutions when b2 - 4ac < 0
The a,b,c-formula using an if-else-statement (cont.)
• Java program:
import java.util.Scanner;
public class Abc4 { public static void main(String[] args) { double a, b, c, Det, re, im; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c
Det = b*b - 4*a*c;
The a,b,c-formula using an if-else-statement (cont.)
if ( Det >= 0 ) { System.out.println( (-b + Math.sqrt( Det ) ) / (2*a) ); System.out.println( (-b - Math.sqrt( Det ) ) / (2*a) ); } else { re = -b/(2*a); // Compute real part im = Math.sqrt( -Det )/(2*a); // Compute imaginary part
System.out.println( re + "+" + im + "i" ); System.out.println( re + "-" + im + "i" ); } } }
The a,b,c-formula using an if-else-statement (cont.)
• Example Program: (Demo above code) – Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/Abc4.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Abc4.java
• To run: java Abc4
Programming example: find maximum of 2 numbers
• Programming problem:
• Read in 2 number a and b
• Assign to the variable max the largest value of a and b
Programming example: find maximum of 2 numbers (cont.)
• Algorithm:
Programming example: find maximum of 2 numbers (cont.)
• Java program: import java.util.Scanner;
public class Max01 { public static void main(String[] args) { double a, b, max; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b
if ( a >= b ) max = a; else max = b;
System.out.println( "max value = " + max ); } }
Programming example: find maximum of 2 numbers (cont.)
• Example Program: (Demo above code) – Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/Max01.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Max01.java
• To run: java Max01
Program example: find maximum of 3 numbers
• Programming problem:
• Read in 3 number a, b and c
• Assign to the variable max the largest value of a, b and c
Program example: find maximum of 3 numbers (cont.)
• Algorithm:
Program example: find maximum of 3 numbers (cont.)
• Java program:
import java.util.Scanner;
public class Max01 { public static void main(String[] args) { double a, b, max; Scanner in = new Scanner(System.in); // Construct Scanner object a = in.nextDouble(); // Read in next number into a b = in.nextDouble(); // Read in next number into b c = in.nextDouble(); // Read in next number into c
Program example: find maximum of 3 numbers (cont.)
if ( a >= b ) // Find max(a,b) max = a; else max = b;
if ( c > max ) // Check c > max ? max = c;
System.out.println( "max value = " + max ); } }
Program example: find maximum of 3 numbers (cont.)
• Example Program: (Demo above code) – Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/Max02.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Max02.java
• To run: java Max02
Programming example: leap year
• Leap year description (Wikipedia):
• In the Gregorian calendar, the current standard calendar in most of the world, most years that are evenly divisible by 4 are leap years.
• Years that are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years
Programming example: leap year (cont.)
• Algorithm:
Programming example: leap year (cont.)
• Program in Java:
import java.util.Scanner; public class LeapYear01 { public static void main(String[] args) { int year; boolean leap;
Programming example: leap year (cont.)
Scanner in = new Scanner(System.in); // Construct Scanner object year = in.nextInt(); // Read in year if ( year % 4 == 0 ) leap = true; else leap = false; if ( year % 100 == 0 ) leap = false; if ( year % 400 == 0 ) leap = true; System.out.println("Year is leap year ? " + leap); } }
Programming example: leap year (cont.)
• Example Program: (Demo above code) – Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/LeapYear01.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac LeapYear01.java
• To run: java LeapYear01
Common errors in if-else-statements
• Common error 1: bogus semicolon after the if-condition
Example:
if ( a >= b ) ; // Bogus ; max = a; else max = b;
Common errors in if-else-statements (cont.)
• Compiler message:
Error01.java:18: 'else' without 'if' else ^
Common errors in if-else-statements (cont.)
• because the compiler "reads" the program as follows:
if ( a >= b ) // A correct if-statement ; // Note: the if-statement ended here ! max = a; // A correct assignment statement
else // else ? Where is the if ??? max = b;
Common errors in if-else-statements (cont.)
• Common error 2: forgetting to use statement block in the then-part
Example:
if ( Det >= 0 ) System.out.println( (-b + Math.sqrt( Det ) ) / (2*a) ); System.out.println( (-b - Math.sqrt( Det ) ) / (2*a) ); else re = -b/(2*a); // Compute real part im = Math.sqrt( -Det )/(2*a); // Compute imaginary part
System.out.println( re + "+" + im + "i" ); System.out.println( re + "-" + im + "i" );
Common errors in if-else-statements (cont.)
• The Java compiler will report the error 'else' without 'if' because syntactically, the program is read as follows:
if ( Det >= 0 ) System.out.println( (-b + Math.sqrt( Det ) ) / (2*a) ); // If-statement
System.out.println( (-b - Math.sqrt( Det ) ) / (2*a) ); // Print
else // Else without if re = -b/(2*a); // Compute real part im = Math.sqrt( -Det )/(2*a); // Compute imaginary part
System.out.println( re + "+" + im + "i" ); System.out.println( re + "-" + im + "i" );
Common errors in if-else-statements (cont.)
• Common error 3: missing semicolon after the then-part
Example:
if ( a >= b ) max = a // Missing semicolon !!! else max = b;
Common errors in if-else-statements (cont.)
• Compiler message:
Error03.java:17: ';' expected max = a ^
Common errors in if-else-statements (cont.)
• Reason:
• The then-part is ONE statement
• A statement must be ended with a semicolon
Common errors in if-else-statements (cont.)
• Common error 4: bogus semicolon after the block in the then-part
• Example:
if ( a >= b ) { max = a; }; // bogus semicolon !!! else { max = b; }
Common errors in if-else-statements (cont.)
• Compiler message:
Error04.java:20: 'else' without 'if' else ^
Common errors in if-else-statements (cont.)
• Reason:
• The then-part is ONE statement
• A statement must be ended with a semicolon
Common errors in if-else-statements (cont.)
• Common error 4: bogus semicolon after the block in the then-part
Example: if ( a >= b ) { max = a; }; // bogus semicolon !!! else { max = b; }
Common errors in if-else-statements (cont.)
• Compiler message:
Error04.java:20: 'else' without 'if' else ^
Common errors in if-else-statements (cont.)
• Because syntactically, the program is read as follows:
if ( a >= b ) // An if-statement { max = a; }
; // An empty statement !!!
else // Else without if... { max = b; }
Programming advice
• As you can see, forgetting a ";" and adding an extra ";" can cause serious syntax errors
The best thing is to stick to one useful form: use block !!!
Programming advice (cont.)
• I always write if-statements and if-else-statements using blocks first:
if ( ..... ) { (leave empty first) }
----------------------------------------------
if ( ..... ) { (leave empty first) } else { (leave empty first) }
Programming advice (cont.)
I fill in the statements in the then-part and else-part later.
So even when the if-part and/or else-part consist of 1 statement, I write them as blocks.