Инструменты проектирования программ
| English | Русский |
|---|---|
| structure chart/ˈstrʌktʃə tʃɑːt/ | структурная диаграмма |
| state-transition diagram/steɪt trænˈsɪʃn ˈdaɪəɡræm/ | диаграмма переходов состояний |
| pseudocode/ˈsuːdəʊkəʊd/ | псевдокод |
| decomposition/ˌdiːkɒmpəˈzɪʃn/ | разложение |
| subroutines/ˈsʌbruːtiːnz/ | подпрограммы |
| parameters/pəˈræmɪtəz/ | параметрами |
| top-down design/tɒp daʊn dɪˈzaɪn/ | проектирование сверху вниз |
| states/steɪts/ | утверждает |
The year software became engineering
- In October 1968, fifty of the world's leading programmers met in Garmisch, Germany, to discuss why large programs were failing: late, over budget, unreliable. They coined a phrase for what was missing: software engineering.
- The complaint was simple. Builders draw before they build. Engineers calculate before they cut. Programmers were typing code before anyone had drawn what the program was.
- The drawings that came out of that decade are the ones you will use in the design stage: the structure chart 结构图, which shows how a program is broken into parts, and the state-transition diagram 状态转换图, which shows how it behaves.
- This lesson is how to read them, how to construct them, and how to turn a structure chart into pseudocode.
What design decides
- Analysis said what the program must do. Design decides how: the data structures, the algorithms, the modules and the interface.
- The design stage produces drawings a programmer can code from: a flowchart for the logic of one algorithm, pseudocode 伪代码 for the same thing in text, a structure chart for the modules, and a state-transition diagram for the behaviour.
- Each tool answers a different question, and the exam asks which one fits.
One algorithm's logic, drawn before it is coded
Лаборатория программного процесса
Классифицируйте примеры разработки по этапе или инструменту, к которым они относятся.
The structure chart
- A structure chart shows the hierarchical decomposition 分解 of a program into modules, its subroutines 子程序, and the parameters 参数 passed between them. This top-down design 自顶向下设计 breaks one large problem into smaller sub-problems, each of which becomes a module.
- Each module is a rectangle. A line joins a caller, above, to the module it calls, below. Modules on the same level are called from left to right.
- Small arrows beside the lines carry data: a parameter passed down into the module, a result returned up to the caller. A diamond marks a selection, a curved arrow a loop.
Hierarchy on the lines, data on the arrows
Структурная диаграмма показывает:
Структурная диаграмма — это иерархическое разделение на модули, где параметры идут вниз, а результаты вверх.
Разбиение задачи на модули сверху вниз называется:
Проектирование сверху вниз порождает модульное решение.
Worked example: read the signatures off the chart
CalculatePay
/ | \
GetEmployee CalculateBonus CalculateTax
returns: takes: sales takes: gross
employeeID returns: bonus returns: tax
GetEmployeereceives nothing and returns an employee ID:FUNCTION GetEmployee() RETURNS INTEGER.CalculateBonustakes the sales figure down and sends the bonus back:FUNCTION CalculateBonus(Sales : REAL) RETURNS REAL.CalculateTaxtakes the gross pay and returns the tax. Every arrow on the chart is a parameter or a return value in the header; a header with a parameter the chart does not show is wrong.
На структурной диаграмме маленькая стрелка, указывающая вниз от вызывающего модуля в модуль, обозначает ____, передаваемый ему.
Стрелки вниз — это входные параметры; стрелки вверх — возвращаемые результаты. Вместе они образуют заголовок модуля.
Worked example: construct a structure chart
- A program reads a student's marks, calculates the average, and outputs a grade. Draw a structure chart.
- Top module:
ProcessStudent. Below it, left to right:ReadMarks, which returns the array of marks;CalculateAverage, which takes the marks array down and returns the average;OutputGrade, which takes the average down and returns nothing. - Three things score: the hierarchy with the main task at the top, the sub-tasks in the order they run, and named parameters on the arrows in the right direction. An unlabelled arrow is half a mark at best.
From structure chart to pseudocode
- The top module becomes the main program; each rectangle below it becomes a procedure or function whose header is read off the arrows; the main program calls them in left-to-right order.
PROCEDURE ProcessStudent()
DECLARE Marks : ARRAY[1:10] OF INTEGER
DECLARE Average : REAL
Marks ← ReadMarks()
Average ← CalculateAverage(Marks)
CALL OutputGrade(Average)
ENDPROCEDURE
- A returned value means a
FUNCTION … RETURNS; a module that returns nothing is aPROCEDURE. The parameter list is exactly the downward arrows.
Расставьте шаги проектирования программы со структурной диаграммой по порядку.
Сверху вниз: задача, подзадачи, потоки данных, заголовки, вызовы. Диаграмма завершается до написания первой строки кода.
Модуль, у которого стрелки структурной диаграммы показывают возврат значения вверх, следует писать как PROCEDURE (процедуру).
Возвращаемое значение делает его FUNCTION (функцией) … RETURNS. PROCEDURE ничего не возвращает.
The state-transition diagram
- A state-transition diagram documents the behaviour of a system: the states 状态 it can be in and the events that move it from one state to another.
- Each state is a circle or rounded box; each transition is an arrow labelled with the event that causes it, sometimes with the action taken. A marker shows the start state.
- It suits systems that wait for events and react: a vending machine, a traffic light, a door lock, a user interface.
Every state, every event, every arrow
Диаграмма переходов состояний показывает:
Состояния — это круги; переходы — стрелки с подписями событий. Идеально подходит для автоматов, замков, светофоров.
Что показывает диаграмма переходов состояний? Выберите все подходящие варианты.
Состояния, подписанные переходы и начальный маркер. Время не входит в состав диаграммы.
Worked example: read the door-lock diagram
- The lock opens on the code 2, 5, 9. Start in Locked. Pressing 2 moves to One digit correct; pressing 5 from there moves to Two digits correct; pressing 9 from there moves to Unlocked.
- Any other key from any of the waiting states returns to Locked: the diagram shows those arrows too, and a diagram that leaves them out has a gap. What happens if 2 is pressed while Unlocked? If no arrow says, the design has not decided.
- That is what the diagram is for: every state must say what happens on every event, so the missing transitions are found on paper and not by a customer.
Диаграмма переходов состояний позволяет легко заметить отсутствующие или необработанные переходы, так как все состояния и события между ними отображены.
Отображение всех состояний и событий выявляет случаи, которые вы не учли, например, неожиданную монету во втором броске в автомате.
Choosing the tool
- To show how a program is broken into modules and what passes between them: a structure chart.
- To show how a system behaves over time in response to events, especially a machine or an interface with modes: a state-transition diagram.
- To show the step-by-step logic of one algorithm: a flowchart or pseudocode. Say which and why.
Соотнесите каждый инструмент проектирования с тем, что он показывает.
Каждый инструмент рассматривает проект по-разному: структуру (модули), поведение (состояния), поток (блок-схема) или шаги (псевдокод).
Управление светофором должно реагировать на таймер и кнопку пешехода. Какой инструмент проектирования лучше всего документирует его поведение?
Красный, красно-желтый, зеленый, желтый — это состояния; таймер и кнопка — события. Структурная диаграмма показала бы модули, а не поведение.
Marks that slip away
- A structure chart is not a flowchart. It shows hierarchy and parameters, not the sequence of decisions inside a module.
- Label every arrow with the name of the parameter or result and point it the right way. A bare arrow says nothing.
- A state is a condition the system is in, waiting; an event is what happens to it. "Press 5" is an event, not a state.
- The pseudocode headers must match the chart: same parameters, same return values, same order of calls.
You've got it
- a structure chart shows top-down decomposition into modules, with the parameters passed down and the results returned up on labelled arrows
- read the pseudocode headers off it: downward arrows are the parameter list, an upward arrow makes it a
FUNCTION … RETURNS, and the main module calls left to right - a state-transition diagram shows the states and the events that move between them, and exposes the transitions nobody has decided
- decomposition → structure chart; behaviour → state-transition diagram; one algorithm's logic → flowchart or pseudocode