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WIT COMP1000 while Loops Wentworth Institute of Technology - - PowerPoint PPT Presentation

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Wentworth Institute of Technology Engineering & Technology WIT COMP1000 while Loops Wentworth Institute of Technology Engineering & Technology Loops Often, you need to repeat the same computation, action, or sequence of steps many

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Wentworth Institute of Technology

Engineering & Technology

WIT COMP1000

while Loops

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Loops

§ Often, you need to repeat the same computation, action, or sequence of steps many times § Example: Writing “I will not expose the ignorance of the faculty.” 100 times

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Loops

§ Of course, you could use 100 println() statements to accomplish this, but that's a lot

  • f copy and pasting work

§ Instead, programming languages have control flow mechanisms called loops that allow you to loop over (repeat) the same section of code as many times as you need § Two of the most common types of loops are while loops and for loops

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

while Loops

§ while loops are used to repeat a set of Java statements while some condition is true § Example:

int iteration = 1; while (iteration <= 100) { System.out.println("I will not expose the ignorance of the faculty."); iteration = iteration + 1; }

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Generic Form of the while Loop

while (BOOLEAN EXPRESSION) { STATEMENT1; STATEMENT2; … } § The loop body executes over and over as long as the expression is true

» Expressions are the same as for if/else if statements

§ Each repetition is called an iteration of the loop

Loop body

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Example Behavior of a while Loop

Current value of input_value: ? 7 3 1 0

int input_value; System.out.print("Enter an integer: "); input_value = input.nextInt(); while (input_value > 0) { System.out.println(input_value); input_value = input_value / 2; } System.out.println("Done."); Console Output

Enter an integer: 7 3 7 1 Done.

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Another Example

int i = 1; int sum = 0; while (i <= 4) { sum = sum + i; i = i + 1; } System.out.println(sum);

Current value of i: 1 2 3 4 5

Console Output

10

Current value of sum: 0 1 3 6 10

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Wait, i = i + 1??

§ Yes, that is valid Java! § Always remember that “=“ is NOT a statement

  • f fact, it is a one time assignment of a value to

a variable § For example, if i currently has a value of 3, then it will plug that in to the right hand side of the equal sign, add one to get 4, then assign 4 back to the variable i § The same is true for sum = sum + i

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

§ When the program reaches a while loop for the first time, it checks the condition

» If it is true it begins running the statements inside the loop (in the loop body, between the curly braces) » If the condition is false, it skips past the while loop entirely

§ When it executes the last statement inside a loop and reaches the }, it goes back to the original while line and checks the condition again

» The condition is only checked when the while line itself is executing, not after each statement inside the loop body

Notes

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Exercise

§ Write a program that prints out all the numbers from 0 to N, where N is provided by the user. That is, ask the user for a number then print

  • ut all the numbers from 0 to that number,

each on their own line.

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Answer

Scanner input = new Scanner(System.in); System.out.print("Enter N: "); int n = input.nextInt(); int i = 0; while (i <= n) { System.out.println(i); i = i + 1; }

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Infinite Loops

§ Always be careful to ensure that your loop conditions will be false eventually § Loops that have conditions that are always true are infinite loops, and are usually a mistake § You can halt a program stuck in an infinite loop by pressing the terminate button (red square) in the console window

int iteration = 1; while (iteration < 100) { System.out.println("This will repeat forever..."); }

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Increment/Decrement Operators

§ Java includes shorthand increment and decrement

  • perators that are often useful with loops (and

plenty of other times) § ++ is the increment operator, used to increase a variable's value by one

»Example: count++; // same as count = count + 1;

§ -- is the decrement operator, used to decrease a variable's value by one

»Example: i--; // same as i = i - 1;

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

do-while Loops

§ A while loop body might be executed zero times if the condition is never true § If you need to always execute the body at least

  • nce, use a do-while loop

§ Example:

int input_value; do { System.out.print("Enter 1 to print this message again: "); input_value = input.nextInt(); } while (input_value == 1);

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Generic Form of the do-while loop

§ Note that you need a semicolon after the while (EXPRESSION) in do-while loops, but NOT in while loops

do { STATEMENT1; STATEMENT2; … } while (BOOLEAN EXPRESSION);

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

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Example: Sanitizing Inputs

double input_value; do { System.out.print("Enter a positive number: "); input_value = input.nextDouble(); } while (input_value <= 0); System.out.printf("The square root is %.3f%n", Math.sqrt(input_value));

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WIT COMP1000

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

String Example

String input_value; do { System.out.print("Enter y to print this message again: "); input_value = input.next(); } while (input_value.equals("y"));

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WIT COMP1000

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Wentworth Institute of Technology

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  • Do. Learn. Succeed.

Exercise

§ Write a program that uses a do-while loop to read integer values from the user until a value between 1 and 100 (inclusive) is entered

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WIT COMP1000

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Wentworth Institute of Technology

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  • Do. Learn. Succeed.

Answer

int input_value; do { System.out.print("Enter a number between 1 and 100 (inclusive): "); input_value = input.nextInt(); } while (input_value < 1 || input_value > 100);

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Wentworth Institute of Technology

Engineering & Technology

  • Do. Learn. Succeed.

Take Home Points

§ Use while loops to repeat a series of statements so long as some condition is true § Use do-while loops if you need to guarantee that the loop body executes at least once § Be wary of infinite loops § Use increment/decrement operators as shortcuts to add or subtract one from a variable