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The if-else statement

The if-else statement

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The if-else statement

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  1. The if-else statement

  2. 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

  3. 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"); }

  4. 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

  5. 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" ); } } }

  6. 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

  7. Introducing the if-else statement (cont.) • Example: Enter a:1 Enter b:2 Enter c:5 -1.0+2.0i -1.0-2.0i

  8. Introducing the if-else statement (cont.) • Shortcoming: • We have to negate the if-condition ourselves... • This can introduce unnecessary errors

  9. 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

  10. The if-else statement in Java • The if-else statement: • The if-else statement is the second conditional statement in Java • The if-else statementselectsone of two statements to be executed based on a given condition

  11. Syntax and meaning of the if-else-statement • Syntax of the if-else-statement: if ( CONDITION ) ONE- statement else ONE-statement

  12. 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

  13. Syntax and meaning of the if-else-statement (cont.) • Following the condition clause, you can write (only) one statement • Following the then-part, you must specify the keywordelse followed by (only) one statement • This statement will only be executed if the condition is true • This is the condition of the if-else-statement • This statement will only be executed if the condition is false

  14. 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

  15. 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)

  16. Computer Jargon: else-part (cont.) • Schematically:

  17. 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

  18. 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;

  19. 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" ); } } }

  20. 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

  21. 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

  22. Programming example: find maximum of 2 numbers (cont.) • Algorithm:

  23. Programming example: find maximum of 2 numbers (cont.) 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 ); } } • Java program:

  24. 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

  25. 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

  26. Program example: find maximum of 3 numbers (cont.) • Algorithm:

  27. 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

  28. 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 ); } }

  29. 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

  30. 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

  31. Programming example: leap year (cont.) • Algorithm:

  32. 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;

  33. 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); } }

  34. 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

  35. 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;

  36. Common errors in if-else-statements (cont.) • Compiler message: Error01.java:18: 'else' without 'if' else ^

  37. 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;

  38. 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" );

  39. 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" );

  40. 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;

  41. Common errors in if-else-statements (cont.) • Compiler message: Error03.java:17: ';' expected max = a ^

  42. Common errors in if-else-statements (cont.) • Reason: • The then-part is ONE statement • A statement must be ended with a semicolon

  43. 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; }

  44. Common errors in if-else-statements (cont.) • Compiler message: Error04.java:20: 'else' without 'if' else ^

  45. Common errors in if-else-statements (cont.) • Reason: • The then-part is ONE statement • A statement must be ended with a semicolon

  46. 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; }

  47. Common errors in if-else-statements (cont.) • Compiler message: Error04.java:20: 'else' without 'if' else ^

  48. 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; }

  49. 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 !!!

  50. 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) }