Session 6 - while Loops

Discussion prompt

Before we open Session 6 - while Loops: without looking back, what was the main idea of Session 5 - Comparisons & Logic (and / or / not), and what could you do by the end of it that you could not do before?

Hint: One sentence for the idea, one for the skill. If the second one is blank, that is the part to revisit.

Discussion prompt

A while loop runs its indented block again and again, as long as its condition is true.

That is stated as though it always holds. Do one of two things: produce a case where it fails, or say precisely what rules such a case out. "It just does" is not on the menu.

Hint: Hunt at the extremes first — zero, one, negative, empty, equal. If every extreme survives, the reason they survive is the proof.

Discussion prompt

Explain Repeat-until, out loud by analogy to something with no Python Fundamentals in it at all — a queue, a recipe, a map, a bank balance, whatever fits. Then say where your analogy breaks.

Hint: An analogy that never breaks is not an analogy, it is the same idea wearing a hat. Find the seam — that is the part that is actually new.

Discussion prompt

Explain The three moving parts to a student a year behind you. No notation, no jargon they have not met — and it still has to be true.

Hint: If your explanation needs a symbol they have never seen, you are describing the notation rather than the idea.

Put in order

Put the moves of Count 1 to 5 into the order they have to happen.

1. Before the loop: i starts at 1
2. Each pass: print i, then add 1
3. Trace every pass

Why: These are the moves of the worked example in the order it makes them, and each one is set up by the one before it. The initial value. The condition will be checked against this first.

Comparison matrix

From Count 1 to 5: refill the i column from what you know. The rest of the table is as it appeared.

Predict first

The table runs: True | 3 | T-minus 3 · True | 2 | T-minus 2 · True | 1 | T-minus 1

In A countdown, given the rows so far: what is the next one — the row where check c > 0 is False?

Correct: False | 0 | Liftoff!

Why: The relationship between the columns, not the individual numbers, is what generates the next row. c starts at 3 and c -= 1 lowers it each pass, heading toward 0.

Comparison matrix

From A countdown: every row here is a choice with a cost. Fill the c column, then say which row you would actually pick and what you give up for it.

Step through it

Step through A multiplication table one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: i is 1
2. Step 2: i is 2
3. Step 3: i is 3
4. Step 4: i is 4
5. Step 5: i is 5

Predict first

A student writes this, and it looks reasonable:

i stays 1 forever

It is wrong. Say what breaks — and say it before you turn the page.

Correct: There is no i += 1, so i <= 5 is always true.

Update the variable the condition depends on.

Why: There is no i += 1, so i <= 5 is always true. It prints 1 endlessly and never stops on its own.

Annotate

Walk the callouts on Trap: forgetting to update. Each one is a place this is easy to get subtly wrong.

• There is no i += 1, so i <= 5 is always true. It prints 1 endlessly and never stops on its own.
• Now i climbs 1..5 and the loop ends when i is 6. Real output: 1 through 5, then it stops.

Draw it

Draw what <= versus < in the condition just did — the shape of it, not the line-by-line working. One picture, labels only where you need them. Then check it against the steps: anything you could not draw is a step you followed rather than understood.

Put in order

Put the moves of Sum 1 through 5 into the order they have to happen.

1. total starts at 0, before the loop
2. Each pass adds the current n, then advances n
3. Trace the running total

Why: These are the moves of the worked example in the order it makes them, and each one is set up by the one before it. total grows 0 -> 1 -> 3 -> 6 -> 10 -> 15.

Step through it

Step through Sum 1 through 5 and watch the numbers move. Now imagine the input ten times bigger: which column is the one that stops this being practical?

1. Step 1: n is 1
2. Step 2: n is 2
3. Step 3: n is 3
4. Step 4: n is 4
5. Step 5: n is 5

Sort into buckets

These are the pieces of Session 6 - while Loops, out of order. Put each one back under the part of the lesson it belongs to.

Repeating with while

A loop repeats a block; Repeat-until, out loud; The three moving parts

The Infinite Loop

If nothing changes, it never stops; The exit must get closer; Watch the comparison at the edge

Building Up a Result

An accumulator variable; Sum 1 through 5; The total is a bucket you fill

s1

Repeating with while is where Session 6 - while Loops puts A loop repeats a block, Repeat-until, out loud, The three moving parts. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s2

The Infinite Loop is where Session 6 - while Loops puts If nothing changes, it never stops, The exit must get closer, Watch the comparison at the edge. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

s3

Building Up a Result is where Session 6 - while Loops puts An accumulator variable, Sum 1 through 5, The total is a bucket you fill. Knowing which part of the lesson a problem belongs to is most of knowing which method to reach for.

Sort into buckets

Every line below is part of the definition of iteration or of sentinel — one or the other, never both. Put each where it belongs.

iteration

One pass through the loop body.; A loop that runs its block five times has five iterations.

sentinel

A chosen value that signals 'stop'.; The loop condition compares the input against it

b1

One pass through the loop body. A loop that runs its block five times has five iterations.

b2

A chosen value that signals 'stop'. The loop condition compares the input against it: while answer != "quit".

Fill in the blanks

From Repeat until they quit — one line has had its right-hand side removed. Put it back.

message = ""
while message != "quit":
message = input("You: ")
print("Bot heard:", message)

Why: message is what everything below it consumes, so the wrong expression here fails later and somewhere else. message = "" makes the first check true so the loop runs at least once.

Step through it

Step through Sum numbers until 0 one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: x is 5
2. Step 2: x is 8
3. Step 3: x is 2
4. Step 4: x is 0

Match the pairs

Match each term to the definition this lesson gave it — not the one you would guess from the word.

• t1. iteration
• t2. accumulator
• t3. sentinel
• t4. flag

• d1. One pass through the loop body. A loop that runs its block five times has five iterations.
• d2. A variable that builds up a result across iterations - a running total, a count, or a joined string.
• d3. A chosen value that signals 'stop'. The loop condition compares the input against it: while answer != "quit".
• d4. A bool variable that records a yes/no state, like running = True. Flip it to change what the loop does next.

Why: These are the working definitions of iteration, accumulator, sentinel, flag as Session 6 - while Loops uses them. Pairing them correctly is the test of whether you could state each one with the slide switched off.

Fill in the blanks

From A menu loop with a flag — one line has had its right-hand side removed. Put it back.

running = True
while running:
c = input("1 = play, 3 = quit: ")
if c == "3":
running = False
print("quitting")
else:
print("you chose", c)

Why: running is what everything below it consumes, so the wrong expression here fails later and somewhere else. While running is True the menu repeats; choosing 3 sets running = False.

Step through it

Step through while True with break and watch the numbers move. Now imagine the input ten times bigger: which column is the one that stops this being practical?

1. Step 1: k is 0
2. Step 2: k is 1
3. Step 3: k is 2
4. Step 4: k is 3

Comparison matrix

From Print only the odds: refill the even? column from what you know. The rest of the table is as it appeared.

Predict first

A student writes this, and it looks reasonable:

Looping forever with no way out.

It is wrong. Say what breaks — and say it before you turn the page.

Correct: The condition is always True and there is no break, so it prints hi forever until you force-quit.

Give it an exit: a break on some condition.

Why: The condition is always True and there is no break, so it prints hi forever until you force-quit.

Discussion prompt

The rule this trap just fixed:

After 3 prints, the break ends the loop. Real output: hi three times, then it stops.

Now break it on purpose. Build a case that violates it and follow the consequences until something visibly fails. Where does the failure first show up — and would you have noticed it if you had not been looking?

Hint: The dangerous rules are the ones whose violation still produces an answer. If yours fails loudly, try to find one that fails quietly.

Predict first

The secret is 7; the user types 3, 9, then 7:

Commit before you compute: what does Guess the number come out to? A rough magnitude and the right form is enough — the point is to have something concrete to be wrong about.

Correct: Each wrong guess gives a hint; the right one ends the loop

Why: A prediction you can defend turns the computation into a check rather than a leap of faith — and an answer that contradicts it is caught on the spot. Verified by execution: higher, lower, then You got it!

Step through it

Step through Guess the number one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: guess is 3
2. Step 2: guess is 9
3. Step 3: guess is 7

Predict first

Same game, now counting attempts. The user types 3, 9, 7:

Commit before you compute: what does Count the guesses too come out to? A rough magnitude and the right form is enough — the point is to have something concrete to be wrong about.

Correct: Read the output

Why: A prediction you can defend turns the computation into a check rather than a leap of faith — and an answer that contradicts it is caught on the spot. Verified by execution: three guesses, prints Got it in 3 tries.

Discussion prompt

Work backwards. The example finished here:

Read the output

What was it asked to do, and what must it have been given? Reconstruct the problem from its answer.

Hint: Every quantity in the result had to enter somewhere. Account for each one.

Discussion prompt

Explain Building a string in a loop to a student a year behind you. No notation, no jargon they have not met — and it still has to be true.

Hint: If your explanation needs a symbol they have never seen, you are describing the notation rather than the idea.

Predict first

The table runs: 1 | 1- · 2 | 1-2- · 3 | 1-2-3- · 4 | 1-2-3-4-

In Build 1-2-3-4-5, given the rows so far: what is the next one — the row where i is 5?

Correct: 5 | 1-2-3-4-5

Why: The relationship between the columns, not the individual numbers, is what generates the next row. result grows one piece per pass; the if avoids a trailing dash.

Step through it

Step through Build 1-2-3-4-5 one row at a time. What is driving the change, and what would the row after the last one be?

1. Step 1: i is 1
2. Step 2: i is 2
3. Step 3: i is 3
4. Step 4: i is 4
5. Step 5: i is 5

Discussion prompt

Explain A loop condition can use and / or by analogy to something with no Python Fundamentals in it at all — a queue, a recipe, a map, a bank balance, whatever fits. Then say where your analogy breaks.

Hint: An analogy that never breaks is not an analogy, it is the same idea wearing a hat. Find the seam — that is the part that is actually new.

Predict first

The secret is 1234; the user types 0000, then 1234:

Commit before you compute: what does Limited password attempts come out to? A rough magnitude and the right form is enough — the point is to have something concrete to be wrong about.

Correct: Trace the attempts

Why: A prediction you can defend turns the computation into a check rather than a leap of faith — and an answer that contradicts it is caught on the spot. Verified by execution: 0000 fails, 1234 sets unlocked = True, loop ends after 2 attempts.

Draw it

Draw what Limited password attempts just did — the shape of it, not the line-by-line working. One picture, labels only where you need them. Then check it against the steps: anything you could not draw is a step you followed rather than understood.

Discussion prompt

You can stop from the top - by making the while condition false - or from the middle - with a break.

That is stated as though it always holds. Do one of two things: produce a case where it fails, or say precisely what rules such a case out. "It just does" is not on the menu.

Hint: Hunt at the extremes first — zero, one, negative, empty, equal. If every extreme survives, the reason they survive is the proof.

Fill in the blanks

From The same limit with break — one line has had its right-hand side removed. Put it back.

secret = "1234"
attempts = 0
while attempts < 3:
pw = input("PIN: ")
attempts += 1
if pw == secret:
print("Unlocked")
break

Why: pw is what everything below it consumes, so the wrong expression here fails later and somewhere else. The while limits attempts to 3; the break leaves the instant the PIN matches.

Annotate

Walk the callouts on The same limit with break. Each one is a place this is easy to get subtly wrong.

• The while limits attempts to 3; the break leaves the instant the PIN matches.
• Verified by execution: after the correct 1234, prints Unlocked and breaks. Both styles solve the same problem.

Discussion prompt

Outside this lesson: where does Session 6 - while Loops actually turn up? Name one concrete situation — a job, a piece of software someone ships, a decision somebody has to make — and say which part of Avoiding infinite loops is doing the work in it.

Hint: Vague is the failure mode here. "Engineering" is not a situation; "deciding whether this build is fast enough to ship" is.

Check your understanding

How many numbers does this print?

• A. Three (1, 2, 3) (correct)
• B. Four (1, 2, 3, 4)
• C. Two (1, 2)
• D. It never stops

Answer: A

Why: i runs 1, 2, 3. When i becomes 4, 4 <= 3 is false and the loop ends before printing 4. Verified by execution.

Why B tempts people

4 is never printed - the condition i <= 3 is false at i = 4, so the loop stops first.

Why C tempts people

i = 3 still satisfies 3 <= 3, so 3 does print. That is three numbers, not two.

Why D tempts people

i += 1 moves i toward 4, so the condition eventually fails - it does stop.

Check your understanding

What does this do?

• A. Prints go forever (infinite loop) (correct)
• B. Prints go three times
• C. Prints go once
• D. Prints nothing

Answer: A

Why: i never changes, so i < 3 stays true forever and go prints without end. Adding i += 1 in the body would make it stop. Verified by reasoning (an infinite loop).

Why B tempts people

There is no i += 1, so i stays 0 and the loop never advances toward 3.

Why C tempts people

The condition is true every pass, so it prints repeatedly, not once.

Why D tempts people

0 < 3 is true, so the body runs - and keeps running.

Check your understanding

What does this print?

• A. 10 (correct)
• B. 6
• C. 4
• D. 15

Answer: A

Why: It adds 1 + 2 + 3 + 4 = 10. total climbs 1, 3, 6, 10. Verified by execution.

Why B tempts people

6 is the total after n = 3 (1 + 2 + 3). The loop also runs for n = 4, adding 4.

Why C tempts people

4 is only the last number added, not the sum of all of them.

Why D tempts people

15 would be 1 + 2 + 3 + 4 + 5, but the loop stops after 4 (n <= 4).

Check your understanding

What does this print?

• A. 0 then 1 (correct)
• B. 0, 1, 2
• C. 0 through 9
• D. nothing

Answer: A

Why: It prints 0, then 1. When n reaches 2, the break fires before the print, ending the loop. Verified by execution.

Why B tempts people

2 is never printed - the break runs first, before the print on that pass.

Why C tempts people

The break exits early at n = 2, so it never reaches 9.

Why D tempts people

n = 0 and n = 1 both print before the break, so it is not empty.

Comparison matrix

From Check: break: every row here is a choice with a cost. Fill the action column, then say which row you would actually pick and what you give up for it.

Check your understanding

What does this print?

• A. 1, 3, 4 (correct)
• B. 1, 2, 3, 4
• C. 2
• D. 1, 3

Answer: A

Why: continue skips the print only when n == 2, so 2 is omitted and 1, 3, 4 print. n is advanced before the check, so the loop still ends. Verified by execution.

Why B tempts people

2 is skipped by the continue, so it does not print.

Why C tempts people

continue skips 2; it does not print only 2. The others print normally.

Why D tempts people

4 also prints - the loop runs while n < 4, and n reaches 4 on the last pass.

Check your understanding

What prints in total?

• A. got hi, then got quit (correct)
• B. only got hi
• C. got hi, got quit, then loops again
• D. nothing

Answer: A

Why: The sentinel is only checked at the top of the loop. On the pass where quit is typed, the body still runs and prints got quit; the next check then ends the loop. Verified by execution.

Why B tempts people

quit is read and printed within the same pass before the condition is re-checked.

Why C tempts people

After printing got quit, the top-of-loop check msg != "quit" is false, so it does not loop again.

Why D tempts people

The loop runs at least once (msg starts as ""), so it prints.

Comparison matrix

From Check: the sentinel: refill the prints column from what you know. The rest of the table is as it appeared.

Draw it

One page, no notation unless you need it: draw how these connect — Repeating with while · The Infinite Loop · Building Up a Result · Loops that Listen · break and continue · Build a Game. Put an arrow wherever one of them is what makes another possible, and label the arrow with why.