Chapter 4 · Part 2 of 2Loops: Doing Things Again and Again

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Repeat Until: while Loops

Name: Repeat Until: while Loops
Author: StemPy Lab

A for loop is for a number of rounds that you know. A while loop is for the cases where you do not know: keep saving until there is enough, keep asking until the answer is right, keep going until something happens. In this course you write while loops, learn what makes them stop, leave a loop early with break, skip a round with continue, and ask the user again and again. A lesson on the loops that go wrong shows how to choose between for and while, and how to find an endless loop and a loop that is off by one. A guessing game with hints and a limit of tries is the project. The practice runs your loops and tells you which results are wrong.

5 lessons, about 2 h 30 min5 simulations14 problems10 practice questionsPython in every lesson

Created by StemPy Lab

Course outline (5 lessons)

Lesson 1about 35 min
The while loop
A while loop keeps repeating its indented lines as long as its condition is True. Python checks the condition before each round, and the loop ends when the condition is False. Something inside the loop must change, so that the condition can become False, or the loop never stops. Use while when you do not know the number of rounds: saving until there is enough, doubling until a size is passed, or removing the digits of a number one at a time.
1 simulation2 practice questions4 Python programs8 problems4 Python labs1 question set1 note
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Lesson 2about 30 min
Stopping early: break and continue
break leaves a loop at once, and the program continues with the line after the loop. continue skips the rest of the current round and starts the next one. Both are used with an if inside the loop. while True with a break inside is a loop that stops from the middle, and is useful when the stopping test comes in the middle of the body. A sentinel is a special value, such as 0, that means stop.
1 simulation2 practice questions3 Python programs3 problems1 Python lab
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Lesson 3about 30 min
Loops that wait for the user
A loop can ask the user again and again until the answer is right. A guessing game starts with a value that is not the secret, so that the loop begins, asks inside the loop, counts the guesses, and gives a hint. To check an answer, use while True with a break when the answer is good. A sentinel such as 0 ends a list of numbers. On this site, the answers go into the Input box before Run, one on each line, and a loop that asks many times needs as many lines as it asks, or the program stops with an EOFError.
1 simulation2 practice questions3 Python programs2 problems2 Python labs
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Lesson 4about 30 min
Choosing the loop, and loops that go wrong
Use a for loop when the number of rounds is known or when you go through the items of something, and use a while loop when the rounds depend on a condition. Many jobs can be done with either. The two usual faults of loops are the endless loop, where nothing in the body moves the condition towards False, and the off-by-one fault, where a loop makes one round too many or too few because of < and <=. To find them, follow the loop with a trace table, print the variable inside the loop, and test with a small number of rounds first.
1 simulation2 practice questions2 Python programs1 problem1 Python lab
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Lesson 5about 25 min
Project: the guessing game
In this project we write a number guessing game with hints and a limit of tries. We write the rules first, plan the loop with the conditions that end it (the player wins, or the tries are used up), build the game from the simple version of the last lesson, test the cases that end the game in different ways, and extend it with a random secret, a best score and a play-again question. The project uses a while loop with two reasons to stop, input(), if, elif and else, and a flag variable.
1 simulation2 practice questions1 Python program1 Python lab1 note
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while Loops: Predict the Output

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Repeat Until: while Loops

Course outline (5 lessons)

The while loop

Stopping early: break and continue

Loops that wait for the user

Choosing the loop, and loops that go wrong

Project: the guessing game

Test Your Learning

Your teacher

Discussion (0)