CS 110 Notes Repetition Repetition Iteration Looping o Repeat End o Ability to repeat a set of statements Called the loop body Stop repeating Set number of times o Counted loop o Iterates through a series of values Condition is reached o Input matches a sentinel o Counter limit is reached o JAVA has three types of loops For iterates thru a series of values Used for counted loop While based on condition Executes the statements in the loop body while the condition is true Executes the loop body 0 or more times Do based on a condition Executes the loop body while condition is true Executes the loop body 1 or more times o Infinite Loops Occur when the stopping condition is never reached Can occur on any loop type Counted loop o Repeats the loop body a set of times o Examples Determine grades for an entire class of students in the class Calculate baseball statistics of players on the team Spin roulette wheel 5 times Find the largest of 10 values Average length of 5 words o Require a counter variable Initialize the counter variable Test have we reached the set number of times Execute the loop body Increment the count variable for counter initalValue Condition Increment Statements in the loop body Copyright C Tanner 2010 1 CS 110 Notes Repetition for int i 0 i 5 i System out println i for int i 5 i 0 i System out println i Sum of 5 values 2 int nextNum int sum 0 for int i 0 i 5 i nextNum sc nextInt sum nextNum System out println the sum of the values is sum Write a loop that displays each even integer between 4 and 6 with commas between them for int i 4 i 6 i 2 System out println i When we run this we see there is a trailing comma rewrite this so as to remove the trailing comma Infinite Counted loops for int i 1 i 0 i condition is always true and will never become false for int i 0 i 10 i simple typo should have been i Using the counter variable outside the loop body o When declared within the for loop statement for int i 0 It can not be used outside the loop body because its scope exists only within the loop body Copyright C Tanner 2010 CS 110 Notes Repetition 3 Declare outside of for loop statement int i for i 0 i 10 i if i 11 i exists since it was declared outside the loop body and can be referenced as such Sample Programs Words java Roulette java Largest java Event Controlled loops o Counted loop executes a set number of times and uses a counter variable to keep track of the number of times Can also use the counter variable within the loop to iterate through a set of values o Event controlled loop is based on a condition and looping continues while the condition evaluates to true once the condition evaluates to false we exit the loop o Event controlled loop can be used to create counted loops as well o While statement while condition Statements loop body Test the condition If true execute the body Since we test the condition first the variable on which the condition is based must have a value prior to the original test Since we retest the condition after each iteration of the loop body the condition variable must be reassigned a value prior to retesting or we will have an infinite loop While loop is sometimes called a zero loop because if the condition tests false the first time the body of the loop is never executed or executed zero times Initialize condition variable Copyright C Tanner 2010 CS 110 Notes Repetition 4 while condition Loop body Update condition variable Counted loop example count 0 while count 5 System out println count count System out print enter the number of employees numEmployees sc nextInt countEmp 0 while countEmp numEmployees get gross pay calculate net pay display gross and net pay rates countEmp Events o State occurs boolean variable while state has not been reached o Sentinel Data value used in input stream to indicate end of data Value used as the sentinel can not be a legitimate value in the input while inputValue sentinel value not all applications can use a sentinel entering temperatures o what value is not possible When no sentinel can be used we base the loop on user input to do more o Example go on number of employees but we don t know the number System out print Enter an Employee Y N Copyright C Tanner 2010 CS 110 Notes Repetition 5 more sc next charAt 0 while more y more Y get gross pay calculate net pay display gross and net pay System out print Enter an Employee Y N more sc next charAt 0 Example of sentinel loop value sc nextInt while value sentinel Process data value sc nextInt Sample programs Circle2a java Minmax2 java Stats java Do loops o Another loop which is based on a condition o Form do stmts while condition Execute the body Test the condition If true execute the body If false exit the loop Called a one loop because the statement body is always execute at least one time Example display the digits of an integer and sum the digits sum 0 Copyright C Tanner 2010 CS 110 Notes Repetition 6 do digit num 10 sum digit num 10 while num 0 Why use the do instead of while in this instance There is at least one digit in the number When not to use a do Page 308 25 Add the input values up to but not including the sentinel of 1 total 0 do number sc nextInt total number while number 1 System out println the sum of the numbers enters is total Try with 12 5 30 48 1 Non Collected Homework page 307 24 b c e f How many times will the loop execute and what is its output a x 5 y 50 do x 10 while x y System out println x y d x 5 y 35 while x y x 10 System out println x y Sample Program Ckbook java Copyright C Tanner 2010 CS 110 Notes Repetition 7 End of File o Sometimes there is no possible sentinel value and don t want to prompt with do you want another o Can signal end of file EOF CTRL z windows CTRL d UNIX o Scanner class has a method call hasNext which returns true if there is data in the buffer and false if the buffer contains and EOF signal System out print enter a grade or CTRL D to end while sc hasNext grade sc nextInt process the grade System out print enter a grade or CTRL D to end This is the only time you input at the start of a while loop body Nested controls o Loops can be nested for i 0 i 5 i for j 0 j 5 j System out println i j Sample Programs …