Introduction to Computing Using Java

1
Introduction to Computing Using Java

• Branching if and if/else

2
Branching/Selection

• Apart from sequential execution, branching is
  another important programming concept.
• It makes our programs go through different
  statements and produce different results.
• Two categories in Java
• if-statement
• switch-statement (not discussed here)

3
How Does it Work?

• For example
• ??????,????? ??????
• Java equivalent
• boolean raining cloudThickness gt 0.8
• if (raining)
• System.out.println(Bring umbrella)
• else
• System.out.println(Hands free)

4
if-(then)-else

• Dont invent the keyword then!
• if (condition)
• true_statement
• else
• false_statement
• next_statement
• condition is a boolean expression.
• true_statement and false_statement may be
  compound statements.

5
if-(then)-else

• Dont invent the keyword then!
• if (condition)
• true_statement
• else
• false_statement
• next_statement
• If the condition is true, true_statement part is
  executed, false_statement otherwise.
• In both cases, the program continues (joins) at
  the next_statement.

6
Optional else-part

• The following is also valid
• if (condition)
• true_statement
• next_statement
• In this case, if the condition is true, the
  true_statement part will be executed.Otherwise,
  nothing happens.
• The program always continues at the
  next_statement.

7
Dice Example Again

• int diceOne 5, diceTwo 6, diceThree 6
• if ((diceOne diceTwo) (diceTwo
  diceThree))
• System.out.println(Eat all!)
• else
• if (diceOne diceTwo diceThree lt 10)
• System.out.println(Small!)
• else
• System.out.println(Big!)
• Nested if-statements.

One statement
8
Successive else-ifs

• int marks
• char grade
• if (marks gt 80)
• grade A
• else
• if (marks gt 70)
• grade B
• else
• if (marks gt 60)
• grade C
• else
• if (marks gt 50)
• grade D
• else
• grade E

• int marks
• char grade
• if (marks gt 80) grade A
• else if (marks gt 70) grade B
• else if (marks gt 60) grade C
• else if (marks gt 50) grade D
• else grade E
• Just a matter of style.

9
Whose else?

• double GPA
• if (GPA gt 3.5)
• if (GPA gt 4.0)
• System.out.println(Distinction)
• else
• System.out.println(Credit)
• Which if does this else belong to?

10
Whose else?

• double GPA
• if (GPA gt 3.5)
• if (GPA gt 4.0)
• System.out.println(Distinction)
• else
• System.out.println(Credit)
• Which if does this else belong to?

11
Whose else?

• double GPA
• if (GPA gt 3.5)
• if (GPA gt 4.0)
• System.out.println(Distinction)
• else
• System.out.println(Credit)
• Which if does this else belong to?
• Correct!

12
Solution to Dangling else Issue

• Rule of thumb
• An else-part attaches to the nearest available
  if, which has not already been matched.
• double GPA
• if (GPA gt 3.5)
• if (GPA gt 4.0)
• System.out.println(First Hon)
• else
• if (GPA gt 3.7)
• System.out.println(Second Upper)
• else
• System.out.println(Second Lower)

This pair of braces is a must!
13
Omitting then-statement

• How can we omit the then-part and keep the
  else-part?
• Empty then-statement
• if (condition)
• else
• System.out.println(Is it OK?)
• Use of boolean negation (better approach).
• Left to you as an exercise.

14
End Note

• Readings and References
• Section 2.6 The Scanner class from Java
• Section 3.6 Formatting output
• Sections 5.1, 5.2 Conditionals (if/ else)
• Exercise
• 5.1, 5.2, 5.3, 5.4, 5.5, 5.10
• Programming Projects
• 5.10