2.6.1 Group PA Part 1: Design the Chart

Chapter 2 Group PA — Part 1 of 3: Design

This is the assessment for the whole chapter, and you are doing it in pairs. Three
thirds: design, build, demo. This is the design third, it is worth 25%, and it comes
first. Neither of you opens an editor until this chart is green.

What you'll be assessed on here:

• Agreeing on what the program does before either of you types
• Turning a word problem into pseudocode, then into a chart
• Using the five shapes released through §2.2 — oval, rectangle, diamond, parallelogram,
  and the loop-setup hexagon (§2.2)

Step 1: pick one problem

Four problems. Pick one. They are the same difficulty and the same shape: take one or
two inputs, walk something with a loop, decide inside the loop, classify one result with
a switch, guard the input that can be bad, report a running total and a verdict. Check
with the pair next to you first — two adjacent tables should not build the same one.

1. Locker Sweep

You are checking a row of lockers numbered 1..n. Each locker starts closed. You walk
down the row once: if a locker number is a multiple of 3, you continue (skip it);
otherwise you flip its state (closed → open, open → closed). After the walk, classify
locker number 7 — open, closed, or never touched — using a switch. Guard against
a bad n (throw if n < 1).

2. Word Audit

You are analyzing a word. Walk it character by character: continue past any space
character; for every other character, classify it as vowel / digit / other using a
switch and keep three running counts. After the loop, report all three counts and
the word itself. throw on an empty string.

3. Savings Run

You are saving for a target. Each week you deposit a fixed amount. continue past the
weeks where the week number is a multiple of 4 (those are "no-deposit" weeks). break
the moment your running total reaches or exceeds the target. After the loop, use a
switch to classify the outcome — hit early, hit on the last week, or missed — and
report the final balance. throw on a negative target.

4. Coin Drawer

You are making change from an amount. The drawer has a limited supply of quarters,
dimes, nickels, and pennies. A while loop dispenses the largest denomination that
fits until the amount reaches zero or the drawer runs dry (break when the drawer is
empty). The switch names each denomination as it is dispensed. Report the count of
each coin. throw on a negative amount.

Whichever you pick, your program also needs one piece of text — the name of the
locker bank, the word being audited, the savings goal, or the coin system. Pick one
and write it down now; Part 2 asks for it.

Step 2: pseudocode it, on paper, together

Before anyone touches the canvas. Both of you, one sheet, out loud. Use the keywords
from 2.2 and the flowchart conventions from 2.2.4 and 2.2.7.

START
INPUT n
INPUT name
SET count TO 0
FOR i = 1 TO n
    IF rule(i) THEN
        CONTINUE
    END IF
    FLIP state of i
    IF i == 7 THEN
        record state
    END IF
END FOR
CLASSIFY state OF 7 WITH switch
REPORT total
END

Step 3: chart it

One screen, one keyboard, both of you looking at it. Five shapes (Appendix D §D.2):

The toolbar also offers a parallelogram for input and output. The checker counts
rectangles — a parallelogram or hexagon is not one. Use them for what they mean,
never in place of a rectangle, or "Uses at least two task rectangles" goes red on a
chart that looks finished.

What "green in four minutes" means

It means you drew a straight line and called it a design. Eleven checks run, and two of
them exist specifically to stop that: your chart needs at least two task rectangles
and at least one decision diamond. A chart that passes those and still took four
minutes is missing the loop setup or the reporting step. The checker also demands eight shapes in total -- a straight line with one diamond does not clear it.

The part people get wrong

Both exits of the diamond have to arrive somewhere. A locker that is not a multiple
of 3 is still an answer, and the program has to say so. An arrow that leads nowhere is
your chart claiming the program hangs.

The loop return arrow goes to the hexagon, not to the diamond. The hexagon is the
loop header; the return arrow closes the loop by pointing back to the hexagon. Drawing
it back to the diamond is the most common mistake and makes the chart illegal.

The hexagon counts as one shape for min-nodes, not two. Do not draw the three
parts of the for loop (init, condition, increment) as separate shapes — the hexagon
is one shape that means "this is a loop header with its setup inside it."

Before you submit

Press Check my diagram. Everything runs in your browser and tells you which shape is
wrong. Fix anything red and press it again — there is no penalty for redrawing, exactly
as in 2.2.7.

Green means your drawing is a legal flowchart. Whether it solves your problem is for
the two of you to read. Before you call it done, walk one set of real numbers through it
out loud, and then a second set that comes out on the other side of the diamond.