Applications of pointers II: References, Indirection

(ポインタの応用 I：参照、間接)

Computing Practice I

11th lecture, June 29, 2017

http://www.sw.it.aoyama.ac.jp/2017/CP1/lecture11.html

Martin J. Dürst

Today's Schedule

Minitest
Summary of last lecture
Call by reference
Indirection
Functions as arguments
Today's exercises

ミニテスト

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テスト終了後その場で待つ

Summary of Last Lecture

Variables are placed on the stack, on the heap,... depending on their kind (global, local,...)
Array index calculations can be converted to pointer calculations
Dynamic memory is allocated with malloc, resized with realloc, and returned with free
Use patterns for dynamic memory handling, and make sure it is clear who is responsibly for freeing the memory

Results of Previous Exercises

	10A1	10B2	10C1	10C2
100 points	97	50	32	4
60 points	1	48	62	32
partial	1	48	62	17
errors	-	-	3	34
not submitted	1	1	2	13

10C2 の注意点

ダメもとで前のプログラムを出すことは「挑戦」とは言えません
もっと Q&A フォーラムを使うべき!
読み込み済み文字の数と用意されたメモリの長さ
行頭の EOF: NULL を返却
行中の改行: 改行を保持、\0 を追加
行中の EOF: 読み込みを終了、\0 を追加

Usages of Pointers

Low-level address manipulation (device allocation,...)
Speedup of array processing
Dynamic memory management
Reference (passing of arguments to a function by reference,...)
Indirection

References in Natural Language

He, she, they
This, that
Here, there
Names or words for things
...

引数の値渡しと参照渡しのイメージ

値渡し (call by value):

付箋に値を書いて、捨ててもいいように渡す
修理するものを直接渡す (例: 靴の修理)

参照渡し (call by reference):

箱の中に値を用意し、箱の場所を渡す
修理する場所 (住所) を指定 (例: 水道管の修理)

Call by Reference

目的:
- 関数に値を渡すのではなく、
- 変数への参照を渡すことで
- 変数そのものが関数から変更可能
技法:
- 引数をポインタとして定義
- 関数呼び出し時にアドレス演算子 (&) を使用
- 関数の中には常に間接演算子 (*) を使用
問題点:
- 自動ではないので注意
  (特に scanf などコンパイル時に型のチェックが不可能な場合)

Pattern for Call by Reference (Except Arrays)

Callee side: Define and use the argument as a pointer
Example: Add 3 to the variable pointed to by base_p if it is less than 10
```
void maybe_plus_three (int *base_p)
{
    if (*base_p < 10)
        *base_p += 3;
}
```
Caller side: pass the address
```
int i = 5;
maybe_plus_three(&i);
```

Pattern for Call by Reference (Arrays)

Because arrays and pointers are (almost) the same, call by reference is automatic
On the callee side, pointers and arrays are equivalent
(for an argument definition, e.g. int *a; and int a[]; are identical)
Caution: How to know the length of the array:
- Special value to signal the end of the array (e.g.: '\0' for strings, exercise 11C1)
- Pass the length as a separate argument (e.g. second argument of qsort)

How to Pass Arrays by Value

Call by reference, do not change in callee
As in (1), but add const qualifiers to declare and assure there are no changes
Copy data before passing it
Wrap the array in a struct, and pass the struct (which is passed by value)

Application of Call by Reference: Multiple Return Values

Problem: Create a function that returns both n² and n³

Solution: Pass uninitialized variables by reference:

void square_cube(int n, int *pn2, int *pn3)
{
    *pn2 = n * n;
    *pn3 = *pn2 * n;
}

Caller side:

int a=5, a2, a3;
square_cube(a, &a2, &a3);
printf("a=%d, a*a=%d, a*a*a=%d\n", a, a2, a3);

Indirection

名言: 「全ての情報テクノロジーの問題は間接のレベルを一つ増やせば解決可能」

間接の例:

IP アドレスの代わりにドメイン名
学生本人の代わりにあだ名、学生番号、メールアドレス
データベースのインデックス
Web ページ本体とそのアドレス (http://... など)
Google, Yahoo みたいなサーチエンジン

C の場合には間接をポインタで実現

Applications of Indirection

順番を入れ替える時に、実際のデータを入れ替えると:

データの移動が大変
データの場所が変更、他のところで困る
同時に一つの順番しか使えない

間接を使えば全ての問題が解決 (例: 11C2)

Functions as Arguments

(pointers to) functions can be passed as arguments to other functions

Typical example: Execute some 'work' according to some pattern, where the 'work' details are not fixed

Application example: Execute some work (e.g. clean blackbords, clean room, switch off lights, check air quality, check fire alarms...) in all lecture halls of Sagamihara campus

Solution:

Create a "lecture hall tracer" function
Pass the function doing the actual work to the "lecture hall tracer" function
The "lecture hall tracer" function calls the "actual work" function in every lecture hall

How to Implement Sorting

There are many criteria for sorting
(student number, name, grades, increasing/decreasing)
There are many different methods for sorting
(bubble sort, insertion sort,...; we will study them in the course Data Structures and Algorithms in the fall term)

To cover everything, we need many complicated functions

	bubble sort	insertion sort	selection sort	quicksort	...
Student number
Name
Math grades
English grades
...

Introducing Comparison Functions

The sort criterion and the sorting method are independent of each other
The order of data items can be expressed by a comparison function
The comparison function compares two items a and b, and returns:
- A negative number if a comes earlier
- A positive number if b comes earlier
- Zero if a and b are the same
If we can pass the comparison function to the sorting function:
- We need only one sorting function (provided by a library, e.g. qsort)
- The comparison function can be quite simple

Functions as Arguments in Other Programming Languages

The C programming language requires some effort to create a function
The comparison function needs to work with all kinds of data, and therefore uses pointers
There are programming languages where creating functions is much easier:
Functional languages (Lisp, Haskell,...)
In Ruby, many methods (member functions) use blocks (similar to functions) as arguments

Example of Difference between Languages

Ruby: students.sort { |a, b| a.number <=> b.number }

Or even easier: students.sort_by { |a| a.number }

Shorthand: students.sort_by &:number

C (from 11C2):

int compareNumbers (const void* a, const void* b)
{
    Student *studentPa = *((Student **) a);
    Student *studentPb = *((Student **) b);

    return strcmp (studentPa->studentNumber,
                   studentPb->studentNumber);
}
...
qsort(byNumber, studentCount, sizeof(Student*), compareNumbers);

今日の演習

11A1: 参照渡しの基本
11C1: 参照渡しを使って値を帰す
11C2: 全置き換えの作成
11C3: 学生データの整列
- 全点必須 (発展問題ではない)
- 順番を追加するごとに提出 (部分点)
- 帰宅前に必ず2点以上取得
- 締切は月曜日

穴埋めや追加が多いので、インデントに注意

次回の準備

残りの演習問題を宿題として完成
今日の復習
参考書の第 11 章 (pp. 281-311) をもう一回よく読む
参考書の第 12 章 (ファイル操作、pp. 313-345) を読む

Glossary

call (pass) by reference: 参照渡し
array index: 配列の指数
callee side: (呼ばれた) 関数側
caller side: 呼び出し側
sort(ing): ソート、整列
(sort) criterion: (整列の) 基準
bubble sort: バブル整列
insertion sort: 挿入整列
selection sort: 選択整列
quicksort: クイックソート
comparison function: 比較関数