The for Loop, Accumulator Variables, Seninel Values, and The Random Class

1. The for Loop, Accumulator Variables, Seninel Values, and The Random Class CS0007: Introduction to Computer Programming

2. Review • General Form of a switch statement: switch(SwitchExpression){ caseCaseExpression1: //One or more statements break; caseCaseExpression2: //One or more statements break; default: //One or more statements } • CaseExpressions must be of type… • char, byte, short, or int.

3. Review • If you want to display a floating-point number in a particular format use • The DecimalFormatClass • printf • A loopis… • a control structure that causes a statement or group of statements to repeat. • Two looping structures talked about so far… • whileLoop • do-while Loop • The difference between the two… • whileLoop is pretest • do-whileLoop posttest

4. The for Loop • You can do any kind of looping with what we learned up to this point. • For instance, how can we make a do-while loop without the do-while looping structure? • while and do-while are conditionally-controlled loops. • A Conditionally-Controlled Loop executes as long as a particular condition exists. • However, sometimes you know exactly how many iterations a loop must perform. • A loop that repeats a specific number of times is called a count-controlled loop. • For example, you may ask for information about the 12 months about a year. • You can turn conditionally controlled loops into count-controlled loops, but Java provides a structure specifically for this called the for loop.

5. The for Loop • The for loop has three elements: • It must initialize a control variable to a starting value. • It must test the control variable to see when the loop terminates. • It must update the control variable during each iteration. • General Form of a for loop: for(Initialization; Test; Update) Statement or Block • Initialization – an initialization expression that happens once when the loop is first reached. • Normally used to initialize the control variable • Test – boolean expression known as the test expression that controls the execution of the loop. • As long as this is true, the loop with iterate again • Note: the for loop is a pretest loop • Update – expression known as the update expression that executes at the end of every iteration • Usually used to change the control variable.

6. for Loop Flowchart Initialization Expression Update Expression Test Expression True Statement or Block False

7. The for Loop for(int count = 0; count < 5; count++) System.out.println("Hello!"); • This will print “Hello!” 5 times. • First, count is initialized to 0. • count is often called a counter variable because it keeps count of the number of iterations. • Then, count < 5 is tested. • It is true so the body is executed. • Then, count is incremented. • This happens 5 times until count = 5 which makes count < 5false. • Note that count is declared inside of the loop header, this makes it have block-level scope in the loop. • This implies that it can be used in the body of the loop. • The counter variable can be declared outside of the header.

8. for Loop Example • New Topics: • for Loop

9. The forLoop Notes • Remember: the for loop is a pretest loop. • Use the update expression to modify the control variable, not a statement in the body of the loop (unless there is no way to avoid it) • You can use any statement as the update expression: • count-- • count += 2 • You can declare the loop control variable outside of the loop header, and it’s scope will not be limited to the loop. intcount; for(count= 0; count < 5; count++) System.out.println("Hello!"); count = 99;

10. Prefix and Postfix Increment and Decrement Operators • We talked about the ++ and -- operators before • x++ • x-- • These are known as postfix increment and decrement operators, because they are placed after the variable. • There is also prefix increment and decrement operators: • ++x • --x • What’s the difference? • When the increment or decrement takes place. intx = 1, y; y = x++; • y is 1 x is 2. • The increment operator happened after the assignment operator. intx = 1, y; y = ++x; • yis 2 xis 2. • The increment operator happened beforethe assignment operator.

11. User-Controlled for Loop • Sometimes, you may want the user to determine how many times the loop should iterate. • Example: UserControlledForLoop.java

12. Multiple Statements in the Initialization and Update Expressions • Java allows multiple statements to be executed in the initialization and/or update expression portion of the for loop. • Example: MultipleForLoop.java

13. Running Totals and Accumulator Variables • Programming tasks often require you to keep a running total of some data. • This can often be done by looping and keeping track of the running total in a single variable. • A variable that keeps track of a running total is called an accumulator variable. • Example: AccumulatorVariable.java

14. Sentinel Value • The previous example required the user to enter in beforehand how many days they sold. • We can allow the user to keep entering until they decide to quit by looping until they enter a sentinel value. • A Sentinel Value is a special value that cannot be mistaken for normal input that signals that a loop should terminate. • We’ve done this before… • SoccerLeague.java • Example: SentinelValue.java

15. Nested Loops • Just like in if statements, loops can be nested. • This is required when a repetition of statements itself must be repeated a number of times. • Example: NestedLoop.java • sleep()

16. break and continue • Java provides two keywords that can be used to modify the normal iteration of a loop: • break – when encountered in a loop, the loop stops and the program execution jumps to the statement immediately following the loop. • continue – when encountered in a loop, the current iteration of the loop stops immediately. • Example: BreakAndContinue.java

17. The Random Class • Some application require randomly generated numbers • The Java API provides a class called Random that does exactly that. • Need to import it: importjava.util.Random; • To create an object: Random identifier = new Random(); • The random class provides many methods for generating random numbers, namely: • nextDouble() – Returns the next random number as a double between 0.0 and 1.0. • nextInt() – Returns the next random number as an int within in the range of int (-2,147,483,648 to 2,147,483,648) • nextInt(int n) - Returns the next random number as an intwithin in the range of 0 and n.

18. Random Class Example • New Topics: • Random Class