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A14.4 System design

Software development, law and ethics · A level · OCR H446 1.2.3, AQA 7517 4.13.1.2, Eduqas A500QS 1.6 · about 40 min

BugBotLab

What this lesson is about

Structure charts, data flow diagrams, designing data, algorithms and the user interface, and printing a structure chart with recursion.

Questions 6 marks in all

  1. [1 mark]In a structure chart, what does a small arrow with an open circle show?

    1. AA data couple: data passed between two modules
    2. BA loop
    3. CA decision
    4. DAn external entity
    Answer: A. Data couples show parameters passed down and return values passed up; a filled circle is a control flag.

  2. [1 mark]In a data flow diagram, which of these flows breaks the rules?

    1. AData flowing directly from one data store to another
    2. BData flowing from an external entity to a process
    3. CData flowing from a process to a data store
    4. DData flowing from a process to an external entity
    Answer: A. Every data flow must start or end at a process, since only a process can move or change data.

  3. [1 mark]Which of these should a design plan, according to AQA?

    Tick every answer that is true.

    1. AThe data structures for the data model
    2. BThe algorithms
    3. CThe modular structure
    4. DThe human user interface
    5. EThe exam board's mark scheme
    Answer: A, B, C, D. AQA lists data structures, algorithms, a modular structure and the user interface.

  4. [1 mark]The robot's screen shows errors only as red text. Which change best improves accessibility?

    1. AAdd a symbol and a clear message, so colour is not the only way the error is shown
    2. BMake the red text brighter
    3. CRemove the error messages
    4. DUse a smaller font to fit more on screen
    Answer: A. Colour alone excludes users who cannot distinguish it; a symbol and words carry the meaning for everyone.

  5. [1 mark]What name is given to design that starts with the whole problem and repeatedly breaks it into smaller sub-problems?

    Answer: top-down design. Top-down design, or stepwise refinement, is what a structure chart records.

  6. [1 mark]A structure chart is stored as a dictionary. What does this print?

    chart = {"robot": ["sense", "act"], "act": ["drive", "beep"]}
def show(module, depth):
    print("-" * depth + module)
    for sub in chart.get(module, []):
        show(sub, depth + 1)
show("robot", 0)
    Answer:
    robot
-sense
-act
--drive
--beep

    Each module is printed, then its sub-modules one level deeper, so drive and beep come after act with two dashes.

The task: print the structure chart

chart is a dictionary describing a structure chart: each key is a module name (a string), and its value is the list of that module's sub-modules, in left-to-right order. A module that is not a key has no sub-modules. Write a recursive procedure show(module, depth) that prints the module's name after 2 * depth spaces, then calls itself for each of the module's sub-modules with depth + 1. Call it as show("delivery robot", 0). While it runs, count every module printed and collect the leaves (modules with no sub-modules) in the order they are printed. After the chart, print modules: <n> and then leaves: <names>, with the names separated by , . The robot stays still.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

chart = {
    "delivery robot": ["plan route", "drive route", "report"],
    "plan route": ["read map", "shortest path"],
    "drive route": ["drive leg", "check gap"],
    "drive leg": ["set motors", "read heading"],
    "report": ["format message", "send message"],
}

The hint students can ask for: A module's line is its name after some spaces that depend on its depth. After printing it, do the same for each of its sub-modules one level deeper. A module that is not a key in the dictionary has no sub-modules, so it is a leaf.

A solution

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

chart = {
    "delivery robot": ["plan route", "drive route", "report"],
    "plan route": ["read map", "shortest path"],
    "drive route": ["drive leg", "check gap"],
    "drive leg": ["set motors", "read heading"],
    "report": ["format message", "send message"],
}

count = 0
leaves = []

def show(module, depth):
    global count
    print("  " * depth + module)
    count += 1
    if module not in chart:
        leaves.append(module)
    for sub in chart.get(module, []):
        show(sub, depth + 1)

show("delivery robot", 0)
print("modules:", count)
print("leaves:", ", ".join(leaves))

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.