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COMP 110-001 Flow of Control: Branching 1

Learn about the if-else statement, boolean expressions, and the order of actions in a program. Explore branching and simple order concepts using real-life examples.

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COMP 110-001 Flow of Control: Branching 1

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  1. COMP 110-001Flow of Control: Branching 1 Yi Hong May 19, 2015

  2. Today • The if-else statement • Boolean expressions

  3. Flow of Control • The order in which a program performs actions • Continuation (unconditional) • Until now, actions are taken sequentially • More complicated flow of control: • A branching statement chooses an action from a list of two or more possible actions • A loop statement repeats an action again and again until some stopping condition is met

  4. Simple order Leave Home • Continuation (unconditional) • Perform actions sequentially • A single path in the flow chart Check Time Take Bus Reach School

  5. Branching Leave Home • Choose a path in the flow chart by checking conditions Check Time Before 7am? Yes No Take Bus Take Subway Reach School

  6. Branching Leave Home • Choose a path in the flow chart by checking conditions Check Time Before 7am? Yes No Take Bus Take Subway Reach School

  7. Branching Leave Home • Choose a path in the flow chart by checking conditions Check Time Before 7am? Yes No Take Bus Take Subway Reach School

  8. Another Example Compute #days • In homework 1 Print #days #days==1 Yes No Print “day” Print “days” Compute #hours

  9. The if-else Statement • A branching statement that chooses between two possible actions • Syntax if (Boolean_Expression) Statement_1 else statement_2 Example: if (days == 1) system.out.print(“ day”); else System.out.print(“ days”);

  10. The if-else Statement • What if you have multiple statements in each branch? if (Boolean_Expression){ Statement_1.1 Statement_1.2... } else { Statement_2.1 Statement_2.2 ... }

  11. Java Example importjava.util.*;publicclassFlowChart{publicstaticvoid main(String[] args){System.out.println("Give me an integer:");Scanner keyboard = new Scanner(System.in);intinputInt = keyboard.nextInt();if( inputInt > 5){System.out.println("Big number");}else{System.out.println("Small number");}}} Start Prompt User for int Is user input greater than 5? NO YES Print: “small number” Print: “big number”

  12. Java Comparison Operators

  13. Boolean Expressions • if (boolean_expression) { statements } • True or false • “atomic” expressions 5 == 3 // always false myInt <= 6 // depends on the value of myInt // depends on the value of myInt, anotherInt myInt!= anotherInt

  14. Review of Boolean Operators • The effect of the boolean operators && (and), || (or), and ! (not) on boolean values

  15. Use && for and • Form a larger boolean expression out of many smaller boolean expressions • Syntax • (Sub_Expression_1) && (Sub_Expression_2) && … • Will be true if and only if ALL statements are true

  16. Use || for or • Also form a larger boolean expression out of many boolean expressions • Syntax • (Sub_Expression_1) || (Sub_Expression_2) || … • Will be true if ONE expression is true

  17. Example in Homework 1 • Check if #days is not 1 • days ! = 1 • !(days == 1) • days < 1 || days > 1 • !(days >= 1 && days <= 1)

  18. Comparison for Objects • Call object’s method equals() method • E.g.: Compare two strings • str1 = str2; ( ✗ assignment statement) • str1 == str2; (Do NOT, == tests whether they are stores in the same memory location) • str1.equals(str2); (GOOD)

  19. Compare Strings • Syntax • String.equals(Other_String) • String.equalsIgnoreCase(Other_String) • Example String name1 = “COMP110”; String name 2 = new String(“COMP110”); if (name1.equals(name2)){ System.out.println(“The same”); } else { System.out.println(“Different”); }

  20. If Without Else Wake up • You can use just an if statement if (Boolean_Expression) { Statement_1.1 Statement_1.2... } the rest of your code Check Time Before 7am? Yes No Have Breakfast Leave Home

  21. A Different Implement for Homework 1 Print #days dayText = “ days” Print #days #days==1 #days==1 Yes No Yes No Print “day” Print “days” dayText=“ day” Print dayText

  22. A Different Implement for Homework 1 Print #days System.out.print( days ); String dayText = “ days”; if (days==1){ dayText= “ day”; } System.out.print( dayText ); dayText = “ days” #days==1 Yes No dayText=“ day” Print dayText

  23. Nested if-else Statements • An if-else statement can contain any sort of statements within it • You can nest an if-else statement within another if-else statement if (boolean expression) { … } else if (boolean expression){ stuff goes here } else { more stuff } if (boolean expression) { if (booleanexpression){ stuff goes here } else { more stuff } } else { … }

  24. Pseudocode in Flowchart Format Start Prompt User for int What is the integer? inputInt == 0 inputInt > 1 inputInt == 1 Print: “how may I help you” Print: “hello” Print: “how are you”

  25. importjava.util.*;publicclassFlowChart {publicstaticvoid main(String[] args) {System.out.println("Give me an integer:"); Scanner keyboard = new Scanner(System.in);intinputInt = keyboard.nextInt();if ( inputInt == 0)System.out.println(”hello"); else if ( inputInt == 1) System.out.println(”how are you"); else if (inputInt > 1) System.out.println(”how may I help you"); else System.out.println(“Negative”);} }

  26. Next Class • More if / else statements • The switch statements

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