Using Shortcut Arithmetic Operators Accumulator A variable that

Using Shortcut Arithmetic Operators • Accumulator: • A variable that is used to total. Its value is repeatedly increased by some amount. • Java provides shortcuts for incrementing and accumulating: += -= *= /= %= 1 add and assign subtract and assign multiply and assign divide and assign remainder and assign Java Programming, Sixth Edition y += 2 y -=2 y*=2 y/=2 y%=2 y = y+ 2; y = y-2; y=y*2 y=y/2 y=y%2

Increasing and Decreasing a Variable Different ways to increase by 1 int count = 0; count = count + 1; used as an accumulator count +=1; used as an accumulator count++ ; increment operator ++ Different ways to subtract 1 count = count -1 count -= 1 count-decrement operator --

When would you use an accumulator? • When you want to average a set of grades. total. Grades += grade; total. Deposit += deposit; sum+= num You would use a while statement that will let the user continually enter numbers until you type a certain value.

Categories of loops • definite loop: Executes a known number of times. – The for loops are definite loops. – Examples: • Print "hello" 10 times. • Find all the prime numbers up to an integer n. • Print each odd number between 5 and 127. • indefinite loop: One where the number of times its body repeats is not known in advance. – Examples: • Prompt the user until they type a certain value. • Print random numbers until a prime number is printed. • Repeat until the user has types "q" to quit.

Repetition Statements • Repetition statements or loops allow us to execute a statement multiple times • Like conditional statements, they are controlled by boolean expressions • Java has three kinds of repetition statements: – the while loop – the do loop // not tested on AP exam – the for loop 5 -5

"while" loops • A while statement has the following syntax: condition true statement while (condition) statement; • If the condition is true, the statement is executed; then the condition is evaluated again • The statement is executed over and over until the condition becomes false

The while Statement • An example of a while statement: int count = 7; while (count < 5){ System. out. println (count); count++; } • If the condition of a while loop is false initially, the statement is never executed • Therefore, the body of a while loop will execute zero or more times © 2004 Pearson Addison. Wesley. All rights reserved 5 -7

Infinite Loops • The body of a while loop eventually must make the condition false • If not, it is called an infinite loop, which will execute until the user interrupts the program • An example of an infinite loop: © 2004 Pearson Addison. Wesley. All rights reserved int count = 1; while (count <= 25){ System. out. println (count); count = count - 1; } 5 -8

Two types of loops 1. Count Controlled Loop • There are three parts to a count-controlled loop: • 1) initialize the variable • 2) test the variable • 3) increment the variable

While loop int x = 1; // initialize the variable while(x < 5) // test the variable condition { // begin loop x++; // increment variable System. out. print(x); // print statement is inside loop } // loop ends

Trace loop int x = 1; while(x < 5) { x++; // increment x System. out. print(x ); } Starting x = 1 (1 < 5) X Is true? x++ print 1 Yes 2 2 2 Yes 3 3 3 Yes 4 4 4 Yes 5 5 5 no stop Print 2345

accumulator int m = 2; total = 0; // initialized outside the loop while(m < 6) { total += m; // accumulator total = total + m; m++; // increment control variable } System. out. println(total); // print statement is outside the loop

scope int m = 2, total = 0; while(m < 6) { // body of loop total = total+m; m++; } // loop ends System. out. println(total); total needs to be accessed outside the loop Variables need to be declared outside the loop that need to be seen or used outside the loop Any variable created inside { } of method, if statement, for statement, while statement is not visible outside the curly braces. { }

Trace int m = 2; total = 0; while(m < 6) { total = total+m; m++; } System. out. println(total); Check condition 2 < 6 m is true? total 2 yes 2 3 yes 5 4 yes 9 5 yes 14 6 no get out of loop total = 14 print 14

Decrement int num = 10; while ( num > 0 ) // decrease until no longer > 0 { System. out. println( num); num--; //decrement } // end while loop System. out. println("Loop ended");

accumulator variable An accumulator is a variable that keeps a running total. It will add something to itself. m b+2 int b = 5 , ans = 0; 5 5+2 = 7 7 7+2=9 while(b<11) 9 9 + 2 = 11 11 stop { b=b+2; ans=ans+b; } System. out. println(ans); 27 ans=ans+b 7+0 = 7 7 + 9=16 16 + 11 = 27

accumulator variable An accumulator is a variable that keeps a running total. It will add something to itself. k int k=3 , tot = 0; while(k<11) { tot = tot+k; k++; } System. out. println(tot); 3 4 5 6 7 8 9 10 11 tot+k 3+0=3 4 4+3=7 5 5 + 7 = 12 6 6 + 12 = 18 7 7 + 18 = 25 8 8 + 25 = 33 9 9 + 33 = 42 10 10 + 42 = 52 11 stop 52 k++

accumulator variable An accumulator is a variable that keeps a running total. It will add something to itself. z int z=2 , sum = 0; 2 3 while(z<9) 4 5 { 6 7 z++ 8 9 sum = sum+z } System. out. println(sum); zz++ 3 4 5 6 7 8 9 stop 42 sum=sum+z 3+0 4+3=7 5 + 7 = 12 6 + 12 = 18 7 + 18 = 25 8 + 25 = 33 9 + 33 = 42

Decrement int num = 5; while ( num > 0 ) { System. out. print( num); num--; } // end System. out. println("Loop ended"); Check condition(5>=0) Start loop num = 5; num is true? print 5 yes 5 4 yes 4 3 yes 3 2 yes 2 1 Yes 1 0 no get out of loop Print 5 4 3 2 1 Loop ended

Writing while conditions • If what you want is to execute the loop 10 times, write the condition number < 10 and not as number <= 9 In Java generally you would more likely want to loop not from 1 to 10, but from 0 to 9. All counting in Java tends to start at zero rather than one. This is a convention that most Java programmers adopt.

Trace int b=2, sum=0; while(b<9) { b++; sum=sum+b; } System. out. print(sum); b= sum=

Nested Loop If you have a nested loop, it passes through the first loop and will continue to execute the second loop until finished, then goes back to the first loop int x = 6; int y, q; while (x < 10) { y = 1; while (y <= 10) { y++; q = x + y; } x += y; } System. out. println (q); x= Outer loop condition y= Inner loop condition y<=10 y++ q=x+y x+=y;