データ型とレコード

The most expensive data-type decision in history

• In the 1960s memory cost more than programmers. To save two characters per date, years were stored as two digits: 99 meant 1999.
• Nobody expected the code to outlive the century. It did, and on 1 January 2000 every one of those fields would read as 1900. Governments and companies spent an estimated $300 billion checking and fixing it.
• The bug was not a typo. It was a data type 数据类型 chosen for the memory of 1965 and kept for the dates of 2000.
• This lesson is how to pick the type that fits the data, and how to bundle values that belong together into a record 记录.

The six basic types

Pseudocode also names two structured types: ARRAY and FILE

Choose by how the value is used

• A type is decided by what the program will do with the value, not by how it looks.
• The number of students is INTEGER; the class's average mark is REAL, because it has a fractional part; whether a student has paid is BOOLEAN.
• An array index is always INTEGER. A single grade letter is CHAR; an email address is STRING.
• A phone number is a STRING: it starts with 0, and nobody ever adds two phone numbers.

Worked example: give the appropriate data type

Records

• A record (a record structure 记录结构) holds a set of data items of different data types under one identifier. Each item is a field 字段.
• Use one when several values always describe one thing: a stock item, a customer, a student.

TYPE TStockItem
    DECLARE ItemID : INTEGER
    DECLARE Category : STRING
    DECLARE ItemCost : REAL
    DECLARE InStock : BOOLEAN
ENDTYPE

One identifier, four fields, four types

Declaring and using a record

• TYPE … ENDTYPE defines the type. Then declare variables of it, singly or as an array:

DECLARE Item1 : TStockItem
DECLARE Items : ARRAY[1:100] OF TStockItem

• Reach a field with dot notation 点号表示法, to write and to read:

Item1.Category ← "Fruit"
OUTPUT Item1.Category, " costs ", Item1.ItemCost
Items[7].InStock ← FALSE

Worked example: the club membership file

TYPE Student
    DECLARE StudentID : STRING
    DECLARE Name : STRING
    DECLARE DateOfBirth : DATE
    DECLARE Club : ARRAY[1:3] OF INTEGER
ENDTYPE
DECLARE Membership : ARRAY[1:3000] OF Student
Membership[1].Name ← "Li Wei"

Worked example: state the error in the declaration

TYPE TBooking
    DECLARE Seats : INTEGER
    DECLARE Price
    DECLARE Total : STRING

Record, array, or plain variables

• Values that always belong together and have different types: a record.
• Many values of the same type under one name: an array (next lesson).
• A table of rows where each row has mixed fields: an array of records, Members[i].Name.
• Unrelated values: separate variables. Bundling them gains nothing.

Marks that slip away

You've got it