3.9.1 Group PA Part 1: Design the Chart

Chapter 3 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 the function list before either of you types — what goes in, what comes
  back (3.2.2)
• Turning a word problem into a set of function contracts, then into a chart
• Using the six shapes released through §3.1 — oval, rectangle, diamond, parallelogram,
  the loop-setup hexagon (§2.2), and the double-rail subroutine (3.1.2)

Step 1: pick one problem

Four problems. Pick one. They are the same difficulty and the same shape: an array
of record objects, and five functions over it — build one record, total the list, select
a subset, transform it to one line per record, and change one record in place — then
save and load the list. Check with the pair next to you first — two adjacent tables
should not build the same one.

1. Snack Shack

Your stock is an array of items, each { name, price, qty }. Write and use:

• makeItem(name, price, qty) — builds one item object and returns it
• totalValue(list) — walks the list and returns the total dollar value
• lowStock(list, level) — builds and returns a NEW array of items whose qty is
  below level
• receiptLines(list) — returns a NEW array with one line per item, name xqty
• sellItem(list, name) — finds the item and lowers its qty by one, never below zero.
  This one changes the list it was handed.
• saveStock(list) and loadStock() — the whole list goes to localStorage as JSON
  under a key, and comes back.

2. Lap Times

You are logging a meet. Each runner is { name, laps, bestLap }. Same five jobs: total
every lap logged across the whole team; list the runners above a target time; one result
line per runner (name: 3 laps, best 42.1); log one more lap for a named runner
(mutates); save and reload.

3. Seed Tray

You are tracking a tray of varieties, each { variety, planted, sprouted }. Total the
seeds planted; list the varieties whose sprout rate is under a target percent; one label
line per variety; record one more sprout for a named variety (mutates); save and reload.

4. Library Cart

A cart of titles, each { title, copies, dueInDays }. Total the copies on the cart;
list what is due back within a limit; one spine line per title; check out one copy of a
named title (mutates — the copy count goes down, never below zero); save and reload.

Whichever you pick, your program also needs one string of text — the shack's name,
the meet name, the tray label, or the library branch. Pick one and write it down now;
Part 2 asks for it.

Step 2: write the contracts, on paper, together

Before any shape is drawn. One row per function, both of you, out loud. This is 3.2.2
made to do work — a function's signature is its contract.

Then pseudocode, one function per block. The keywords from 2.2 and the conventions from
3.1 still apply.

START
LOAD stock FROM storage          -- or use the seed list
REPORT totalValue(stock)
PUT receiptLines(stock) ON SCREEN
PUT lowStock(stock, LEVEL) ON SCREEN
SELL one of NAME
SAVE stock
END

Step 3: chart it

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

The checker counts rectangles. A parallelogram, a hexagon and a double-rail are all
not rectangles — that is why "Uses at least two task rectangles" exists, and why the
floor is low. Use each shape for what it means, never in place of a rectangle.

The double-rail is one shape, not three. Do not draw the inside of totalValue on
this chart — the double-rail means "a function defined somewhere else does this, then we
come back." Drawing the body inline is the most common way to end up with a chart that
has no functions in it.

What "green" means

Eleven checks run. The two that stop a four-minute straight line: at least two
task rectangles and at least one decision diamond. The checker also demands ten
shapes in total — a straight line with one diamond does not clear it, and neither does
a chart whose only shapes are the calls.

The part people get wrong

The loop return arrow goes to the hexagon, not to the diamond. Same as 2.6.1. The
hexagon is the loop header; the return arrow closes the loop by pointing back at it.

A double-rail has one arrow in and one arrow out. The function ran and the flow
came back. If your chart shows the double-rail branching, the branch belongs in the
caller's logic, before or after the call.

The hexagon counts as one shape for min-nodes, and so does the double-rail. One
shape, one meaning, one count.

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 3.1.12.

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 record through it out loud —
and then walk the list, because the loop is where this chapter's programs stop being
straight lines.