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Unformatted text preview: WEEK 2 Repetition Statements & Arrays CS 180 Prof. Sunil Prabhakar Department of Computer Science Purdue University Review Looping constructs: while, dowhile, for nested looping statements Arrays Declare and use arrays Arrays as arguments and return values Multidimensional arrays. 3 while ( number <= 100 ) { sum = sum + number; number = number + 1; } The while Statement while ( <boolean expression> ) <statement> 4 Control Flow of while next statement; true false sum = sum + number; number = number + 1; int sum = 0, number = 1 previous statement; number <= 100 ? while ( number <= 100 ) { sum = sum + number; number = number + 1; } 5 More Examples Keeps adding the numbers 1, 2, 3, … until the sum becomes larger than 1,000,000. Computes the product of the first 20 odd integers. int sum = 0, number = 1; while ( sum <= 1000000 ) { sum = sum + number; number = number + 1; } 1 int product = 1, number = 1, count = 20, lastNumber; lastNumber = 2 * count  1; while ( number <= lastNumber ) { product = product * number; number = number + 2; } 2 6 String inputStr; int age; inputStr = JOptionPane.showInputDialog ( null , "Your Age (between 0 and 130):" ) ; age = Integer.parseInt ( inputStr ) ; while ( age < 0  age > 130 ) { JOptionPane.showMessageDialog ( null , "An invalid age was entered. Please try again." ) ; inputStr = JOptionPane.showInputDialog ( null , "Your Age (between 0 and 130):" ) ; age = Integer.parseInt ( inputStr ) ; } Example: Testing Input Data Priming Read 7 Watch Out for Pitfalls 1. Watch out for the offbyone error (OBOE). 2. Make sure the loop body contains a statement that will eventually cause the loop to terminate. 3. Make sure the loop repeats exactly the correct number of times. 4. If you want to execute the loop body N times, then initialize the counter to and use the test condition counter < N or initialize the counter to 1 and use the test condition counter <= N . Loop Pitfall  1 Infinite Loops Both loops will not terminate because the boolean expressions will never become false. int count = 1; while ( count != 10 ) { count = count + 2; } 2 int product = 0; while ( product < 500000 ) { product = product * 5; } 1 9 Overflow An infinite loop often results in an overflow error. An overflow error occurs when you attempt to assign a value larger than the maximum value the variable can hold. In Java, an overflow does not cause program termination. With types float and double , a value that represents infinity is assigned to the variable. With type int , the value “wraps around” and becomes a negative value. 10 Loop Pitfall  2 Using Real Numbers Loop 2 terminates, but Loop 1 does not because only an approximation of a real number can be stored in a computer’s memory....
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 Spring '08
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 Statistics, Array, person

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