L.1: WHILE LOOPS

OBJECTIVES

I can:

• represent iterative processes using a while loop

• execute a return or break statement inside an iteration statement to halt the loop and exit the method or constructor

• develop an algorithm

• identify if an integer is or is not evenly divisible by another integer

• determine a minimum or maximum value

• compute a sum, average, or mode

When you play a song, you can set it to loop, which means that when it reaches the end it starts over at the beginning.

A loop in programming, also called iteration or repetition, is a way to repeat one or more statements.

If you didn't have loops to allow you to repeat code, your programs would get very long very quickly (lots of copying and pasting)!

Using a sequence of code, selection (ifs), and repetition (loops), the control structures in programming, you can construct an algorithm to solve almost any programming problem!

WHILE LOOPS

A while loop executes the body of the loop as long as (or while) a Boolean condition is true.

When the condition is false, we exit the loop and continue with statements after the body of the wile loop.

If the condition is false the first time you check it, the body of the loop will not execute.

Notice that the while statement loops a lot like an if statement, but it runs more than once.

The curly braces { } are optional when there is just 1 statement following the condition, but required if there is more than 1 statement in the loop.

On the exam, they will always use curly braces, which is a good practice to follow.

Here's what the flow of control looks like in a Java while loop.

Notice that while the condition is true, the loop body is repeated:

THREE STEPS TO WRITING A WHILE LOOP

The simplest loops are counter-controlled loops as shown below, where the loop control variable is a counter that controls how many times to repeat the loop.

There are 3 steps to writing a loop using this loop control variable as seen below in a loop that counts from 1 to 10:

Remember these three steps to writing a while loop:

1. Initialize the loop variable (before the while loop)

2. Test the loop variable (in the loop header)

3. Change the loop variable (in the while loop body)

The following while loop counts from 1 to 5.

Question: How could we modify it to count from 2 to 10?

Answer:

Initialize the variable count with 2 instead of 1.

Change the condition tested from count <= 5 to count <= 10.

Note: Java doesn't require your code to be indented to make it clear which statements are in the body of the while loop, if statement, etc., but it is standard practice to use indentation to make your code easier to read.

On the exam, the grader will use indentation to determine the meaning of your code, even if you forget the open or close curly brace.

TRACING LOOPS

A really important skill to develop is the ability to trace the values of variables to keep track of how they change during each iteration (cycle) of a loop.

You can create a tracing table that keeps track of the variable values each time through the loop as shown below.

This is very helpful on the exam.

Studies have shown that students who create tables like this do much better on code tracing problems on multiple choice exams.

For the trace table above, iteration 0 would be used to track the values stored in the variables before the loop.

Watch the following video for a tracing demo.

When you are tracing through code, pretend to be the computer running the code line by line, repeating the code in the loop, and keeping track of the variable values and output.

COMMON ERRORS WITH LOOPS

One common error with loops is infinite loops.

An infinite loop is one that never stops (the condition is always true).

The infinite loop above is pretty obvious, but most infinite loops are accidental.

The usually occur when you forget to change the loop variable.

Another common error with loops is a off-by-one error where the loop runs one too many or one too few times.

This is usually an issue with the test condition and using the incorrect relational operator (for example, using > instead of >=).

INPUT-CONTROLLED LOOPS

You can used a while loop to repeat the body of the loop a certain number of times, as shown above.

However, a while loop is typically used when you don't know how many times the loop will execute.

It is often used for a input-controlled loop where the user's input indicates when to stop.

For example, in the Magpie chatbot lab, the while loop stops when you type in "Bye".

The value entered by the user to stop the loop is often called the sentinel value for the loop.

If you were to type in "Bye" right away, the loop will never run.

Here is another example which calculates the average of positive numbers.

Notice that -1 is used as the sentinel value.

The while loop will continue to run while the user does not input -1.

SUMMARY

• Iteration statements (loops) change the flow of control by repeating a set of statements zero or more times until a condition is met.

• Loops often have a loop control variable that is used in the boolean condition of the loop. Remember the 3 steps of writing a loop:

  • Initialize the loop variable

  • Test the loop variable

  • Change the loop variable

• In loops, the Boolean expression is evaluated before each iteration of the loop body, including he first. When the expression evaluates to true, the loop body is executed. This continues until the expression evaluates to false which signals to exit the loop. If the Boolean expression evaluates to false initially, the loop body is not executed at all.

• A loop is an infinite loop when the Boolean expression always evaluates to true so that the loop never ends.

• Off by one errors occur when the iteration statement loops one time too many or one time too few.

• If the Boolean expression evaluates to false initially, the loop body is not executed at all.

• Input-controlled loops often use a sentinel value that is input by the user like "bye", "q", or -1 as the condition for the loop to stop. Input-controlled loops are not on the exam, but are very useful to accept data from the user.

• There are standare algorithms to compute a sum or average.

EVIDENCE

1) Complete the following Google Form. This form must be 100% correct and includes released AP practice questions. To stop working and return later, hit submit! You can "edit your response" and continue where you left off.

L.1 Evidence 

2) You are encouraged to work in pairs for this programming challenge!

Create a "number guessing game" in which the computer picks a random number from 0 - 100 and the player has to guess it.

After each guess, the computer should give you clues like "Too high" or Too low".

Pseudocode is an English description or plan of what your code will do step by step. 

Here is the pseudocode for the guessing game:

> Choose a random number from 0-100

> Get the first guess from the user using the Scanner

> Loop while the guess is not equal to the random number

> If the guess is less than or greater than the random number, print out "Too high!" or "Too low!".

> Get a new guess from the user and store it in the same "guess" variable.

> If the guess is equal to the random number, print out "You got it!"

Make sure to add a counter variable which keeps track of the number of guesses that the user took and print out something like "You guessed the number in 7 tries!".

If the user guesses in less than 7 tries, tell them "You did great!". If the user guesses in more than 7 tries, politely ask them something like, "what happened?".

When you finish and run your program, what is a good guessing strategy for guessing a number between 0 and 100? One great strategy is to always split the guessing space into two and eliminating half, so guessing 50 for the first guess. This is called a binary search algorithm. If your guess is between 0-100, you should be able to guess the number within 7 guesses.