Structures, Unions, Enumerations, and Type Definitions

(構造体 (struct)、共用体 (union)、列挙型 (enum)、typedef)

Computing Practice I

6th lecture, May 25, 2017

http://www.sw.it.aoyama.ac.jp/2017/CP1/lecture6.html

Martin J. Dürst

© 2005-17 Martin J. Dürst 青山学院大学

Today's Schedule

• Minitest
• About last week's exercises
• Structures (struct)
• Type declarations (typedef)
• Unions (union)
• Enumeration types (enum)
• About today's exercises

ミニテスト

• 授業開始までにログイン済み
• 授業開始まで教科書、資料、筆箱、財布などを鞄に入れ、鞄を椅子の下に
• テスト終了後その場で待つ

Applications of C: gcc

• GNU Compiler Collection, formerly the GNU C Compiler
• First published in 1987
• Input: C, C++, Objective-C, Fortran, Java, Ada, Go,...
• Output for Intel x86, ARM, and over 30 other architectures
• Size of actual program: about 1'750'000 lines, tests: about 550'000 lines
• Uses C++ starting with version 4.8

Results of Previous Exercises

前回の演習問題について

• 発展問題はともかく、そうでない問題は必ず提出
  (これからも使う内容の演習)
• 予習が非常に大事
• Q&A フォーラムはもっと活用

前回の演習: 05C1

• 短い関数が多い
• 関数の名前は長くて分かりやすい
• 関数のおかげで「何を」と「どうやって」を分離
• 大学・学生関連の関数と再利用可能な関数は区別
• 学科の判断を switch (プログラム) からテーブル (データ) に変更

前回の演習: 05C2

• main 関数は超簡単
• 再帰関数は
  • 一行入力 (入力が終了の時、そのまま return)
  • 再帰的呼び出し
  • 入力した一行を出力
• バッファは関数内のため、呼び出しごとに別途に用意
• 行の逆順の出力は再帰呼び出しのパターンによって実現

自覚: 分かることと分からないこと

本授業について、一番分かることと一番分からないことそれぞれ三つ列挙しなさい。

Importance of Data

• Lifetime:
  • Hardware: 3-5 years
  • Software: about 10 years
  • Data: more than 30 years (examples: graduation certificate, social security)
• In Information Technology, the design of data structures is more important than the design of functions

Main Data Types in C

Basic Types

• Integers/characters (char, short, int, long)
• Floating point numbers (float, double)

Derived Types

• Arrays, functions
• Structures (struct), unions (union), enumeration types (enum)
• Pointers

Simple Example of a Structure (struct)

typedef struct {     // declaration of type Person
    char name[40];
    double height;   /* m */
    int weight;      /* kg */
} Person;

Person student = { "Taro Aoyama", 1.70, 70 }; // definition and initialization
double bmi = student.weight /                 // use
      (student.height * student.height);

Structure

• Data type combining several kinds of data
• The parts of a structure are called members
• Operations possible on whole structures:
  • Declaration and definition of variables, initialization
  • Assignment
  • Function arguments and return values
• Operations that can only applied to members (not to whole structures):
  • Calculations
  • Input and output
• Functions abstract functionality, structures abstract data types
  [Object orientation combines both abstractions into one abstraction]

Giving a New Name to a Type: typedef

(see textbook pp. 272-273)

• typedef for types other than struct
• Naming a type
• Program becomes (shorter and) clearer
• Good idea to create a convention to distinguish type names from variable/function names(example: variable names: lower case; type names: start with upper case)

Definition and Use of Type Names (typedef)

Example: 4×4 matrices (used for 3D graphics, games,...)

1. Define a variable with the intended type
   Ex.: double a[4][4];
2. Replace variable name with type name
   Ex.: double Matrix[4][4];
3. Add the typedef keyword at the start to complete the typedef declaration
   Ex.: typedef double Matrix[4][4];
4. Use typedef in the same way as preexisting types
   Ex.: Matrix a; Matrix mul(Matrix, Matrix); ...

For Reference: Different Ways to Declare Structures

1. Historic original (using a structure tag name)
   struct Person {...}; (not allowed in this lecture)
2. Definition using structure tag name, followed by separate typedef
struct Person {...};
typedef struct Person Person; (not allowed in this lecture)
   (structure tag name and typedef type name may be the same or different)
3. Combining structure tag name and typedef
typedef struct Person {...} Person; (tolerated in this lecture)
4. Omitting the structure tag name (recommended)
   typedef struct {...} Person; (preferred in this lecture)

Ways of using the above declarations:

• Person p1; (possible with 2., 3., and 4.)
  • Simple and convenient
• struct Person p1; (possible with 1., 2., and 3.)
  • Leftover from early C history
  • Unnecessary except for recursive data structures (e.g. lists, trees)

Structures and Functions

double bmi (Person p)
{
    return p.weight
        / (p.height * p.height);
}

• Structures can be used both as arguments and as return values
• Pass by value is used in both cases (similar to int)

Example of Structures as Arguments and Return Values

Complex add(Complex a, Complex b)
{
    Complex result;
    /* calculate the real/immaginary parts of result
       from the real/immaginary parts of a and b */
    return result;
}

Unions (union)

• Declaration syntax is the same as for struct, but
  • structs reserve separate memory for each member
  • unions superimpose members
    → only one member can be used at one time
• Applications:
  • Memory savings
  • Low-level data conversion
  • Simulation of object-orientation

Enumeration Types (enum)

• Used to distinguish a small number of things
  Examples: Moves in rock, paper, scissors; colleges/departments; types of bank accounts,...
• Making program easier to understand by using names (instead of numbers)
• In C, enums are implemented as integers (int)
• Similar concepts available in many other language
  (Ruby: Symbol, Lisp: Atom,...)

Example of Enumeration Type

Make 03C2 and 03C3 (rock, paper, scissors) easier to read:

typedef enum
    { Rock, Scissors, Paper } Janken;

Janken moveA, moveB;
 ...
if (moveA == Paper)
    pointA += 5;

Additional Explanations about Exercises

• Start by thingking about types (structures)
• Use a structures that match real objects
  Bad example (by chance, there are two points in the exercise):
  typedef struct { double x1, x2, y1, y2; } Point;

  Good example: typedef struct { double x, y; } Point; Point p1, p2;

• Always use typedef
• Read hints carefully

演習問題の概要

• 06A1: 日付のための構造体
• 06A2: 二つの点の間の距離
• 06B1: 複素数の足し算
• 06C1: 成績表
• 06C2: 複素数の関数による計算 (重要: 来週・再来週に再利用)
• 06C3 (発展問題、月曜締切): 趣味の数のグラフ

入出力の詳細に注意

次回までの準備

• 残りの演習問題を宿題として完成、提出
• 分からないことについてしっかり復習
• 来週の総合復習テスト (紙、約 30 分) の準備

Glossary

structure

構造体

union

共用体

enumeration type

列挙型

lifetime

寿命

graduation certificate

卒業証明証

social security

社会保障

basic type

基本型、基本的なデータ型

derived type

派生型

floating point numbers

浮動小数点数

pointer

ポインタ

member (of a structure)

(構造体の) メンバ

convention

決まり、慣例

pass by value

値渡し

complex number

複素数

real part

実部

immaginary part

虚部

superimpose

重ねる

data conversion

データ変換

object-orient(ed/ation)

オブジェクト指向

naming

命名

matrix (複数 matrices)

行列

structure tag name

構造体タグ (名)