CS 211 Control Flow Arrays

1. CS 211Control FlowArrays

2. Today’s Lecture • Review Chapter 2 • Go over exercises

3. Control Flow • Boolean expressions • if / if-else • switch • while, do-while • for (original, Iterator versions) • (break, continue)

4. Boolean Expressions Control flow uses boolean expressions to navigate blocks of code. How do we get booleans? • directly, with true and false. • using relational operators: <, <=, >, >=, ==, != • using boolean operators: &&, ||, ! • calling a method that returns a boolean e.g. myScanner.hasNext() • any expression, as long as it results in true or false.

5. Block Statement • As we introduce blocks of code for the branches of if-else's and switch statements, and for the bodies of loops, we want to group many statements together. • In Java, we place curly braces { } around multiple statements to group them. • It is so common to use them with control structures that it seems like {}'s are part of their syntax, but it is a separate statement structure all on its own.Example: { stmt1; stmt2;}

6. 'Elif' in Java? There is no 'elif' in Java: just chain "if else" statements together: if (be1) s1 else if (be2) s2else if (be3) s3 else s4

7. Switch Statement Syntax:switch (expr) { case val1: stmt1 case val2: stmt2 ... default: stmtD // this 'default' case is optional }Semantics: • expr must be integral (whole number) or char (no Strings or booleans or floats or objects!). All case values must be constants. • evaluate expr, and compare against each case value in orderuntil exact match is found. • execute allstmt's after matching case! (thus break is common at the end of each case) • default: no value; stmtD always runs if no other case values equaled the switch expression.

8. Do-While Loop Syntax: do stmt while (boolexpr); Semantics: • evaluate stmt (no matter what). • evaluate boolexpr; if true, repeat (evaluate stmt again). If false, do-while is done. • Note: semicolon after (boolexpr) is required! ; • Note: stmt runs at least once (unlike while loop, whose stmt might not run at all). Example: int x = 0; //consider also x = 500; do System.out.println(x++); while (x<100);

9. For Loop Syntax:for (initializer; guard; incrementer)stmtSemantics: • initializer is a statement. Runs exactly once, before everything else. (If a variable is declared, its scope is only within loop. Variable doesn't have to be declared, it can already exist). • guard is a boolean expression. Each iteration (including first), this is checked: true => run stmt; false => exit loop. • incrementer is a statement. Runs after stmt, each time that stmt runs. • Note: initializer, guard, and incrementer could each be omitted!E.g., for (;;) stmtExample: for (inti = 0; i<10; i=i+1) System.out.println(i);

10. Let’s Go Over the Exercises

11. Arrays

12. Today’s Lecture • Review Chapter 3 • Go over exercises

13. Array Types • The array type is indicated with [ ]'s. • Monomorphism: Just as variables can only hold one type of value, Java arrays can only hold one specified type of value, in every slot. • Example array types: int[] double[] boolean[][] Person[] • The type doesn't record the dimension lengths, but an array value will specify the (unchanging) lengths.int[][][] xs = new int[3][4][5]; //a 3x4x5 structure of ints.

14. Accessing/Modifying Arrays Brackets [ ] are used to access and update values in arrays.int a = xs[4]; //accesses 5thelt. of xs. xs[0] = 7; //replaces 1stxselt. with a 7. Any expression of type int may be used as an index, regardless of the type in the array:xs[ a+4 ] xs [ sc.nextInt() ]xs[ i ] ys[ i ][ j ]The length of an array is available as an attribute: xs.lengthys[i].length

15. Let’s go over the exercises

16. Questions?