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Going further · 25

Lists of records and enums

Collect records in a list, and give names to a small fixed set of values.

Put records (bundles of several values) into a list one after another and you have a table.
And a value that is one of a few fixed choices, like low, middle or high, gets a name (an enum).
The example below reads students' names and scores, collects records with a level in a list, prints them line by line, and then prints only the names of the students whose level is HIGH.

// enum이 없어서, 이름 붙인 상수로 정해요(바꾸지 않기로 약속한 변수)LOW = 0;MID = 1;HIGH = 2;LEVEL_NAMES = ["LOW", "MID", "HIGH"]; levelOf(score){    if (score >= 80)    {        return HIGH;    }    if (score >= 60)    {        return MID;    }    return LOW;} n = Console.ReadLine().ToNumber();students = []; // 묶음 자료(이름, 점수, 등급)의 목록for (i = 0; i < n; i++){    parts = Console.ReadLine().Split();    s = Dict(); // 바퀴마다 새 묶음 자료를 만들어요    s["name"] = parts[0];    s["score"] = parts[1].ToNumber();    s["level"] = levelOf(s["score"]);    students.Add(s);} foreach (s in students){    Console.WriteLine(s["name"] + " " + s["score"] + " " + LEVEL_NAMES[s["level"]]);} high = [];foreach (s in students){    if (s["level"] == HIGH)    {        high.Add(s["name"]);    }}Console.WriteLine("HIGH: " + high.Join(" "));
stdin
4
mina 92
jun 58
sora 75
theo 81
stdout
mina 92 HIGH
jun 58 LOW
sora 75 MID
theo 81 HIGH
HIGH: mina theo

Key points

  • One record is the bundle of values about one person (or thing).
    Put records in a list and you get a table where each slot is one record.
    Take one s out of students and its name, score and level come together.
  • Create a new record on every pass inside the loop and add it to the list.
    Compared with keeping a list of names and a separate list of scores, one person's values do not get scattered.
  • An enum is a type that names a small fixed set of values.
    Instead of writing the string "HIGH" or the number 2 directly, you write Level.HIGH.
    In the languages that have enums, a name that does not exist, such as Level.HIHG, is reported as an error (while running in Python, at compile time in the others).
  • What is inside an enum differs by language.
    In C, C++, TypeScript and C# it is a number such as 0, 1, 2.
    To print the name, Java and C# just print it, Python uses .name, TypeScript uses Level[value], and C and C++ look it up in an array of names.
  • JavaScript and Craykon have no enum, so they do the same job with named constants.
    Instead of writing 2 all over the code, you define the name HIGH once and use it.
  • Turning a score into a level is kept in one function (levelOf).
    When the rule changes, there is only one place to fix.

How each language writes it

PythonMake a record as a dictionary {"name": name, "score": score}, add it with students.append(...) and read it with s["name"].
With a class marked @dataclass you read it as s.name.
For an enum, write from enum import Enum, then lines like HIGH = 2 inside class Level(Enum):.
The name is level.name, the inner value is level.value, and Level["HIGH"] looks one up by its name string
JavaScriptMake a record as an object { name: name, score: score }, add it with students.push(...) and read it with s.name.
When the variable and the key have the same name, shorten it to { name, score }.
There is no enum, so use a frozen object: const Level = Object.freeze({ LOW: "LOW", MID: "MID", HIGH: "HIGH" });.
A frozen object does not change even after Level.HIGH = "x".
Run as a plain script, as this site's judge does, the assignment is ignored with no error; in strict mode ("use strict" or an ES module) it stops with a TypeError.
A missing name such as Level.HIHG is not an error but undefined
TypeScriptDescribe the record's shape with type Student = { name: string; score: number; level: Level }; and the list is Student[].
The enum is enum Level { LOW, MID, HIGH }, with 0, 1, 2 inside.
Get the name with Level[s.level].
A type that is one of a few strings, type Level = "LOW" | "MID" | "HIGH";, is also widely used
JavaMake a record with record Student(String name, int score, Level level) {}, create it with new Student(...) and read it with s.name().
The list is List<Student>.
The enum is enum Level { LOW, MID, HIGH }, and printing it gives the name as is.
Its position is level.ordinal(), and Level.valueOf("HIGH") looks one up by its name string
C#Make a record with record Student(string Name, int Score, Level Level);, create it with new Student(...) and read it with s.Name.
The list is List<Student>.
The enum is enum Level { LOW, MID, HIGH }, and printing it gives the name.
The inner number is (int)level, and Enum.Parse<Level>("HIGH") looks one up by its name string
C++The record is struct Student { std::string name; int score; Level level; }; and the list is a std::vector<Student> with push_back({name, score, levelOf(score)}).
The enum is enum class Level { LOW, MID, HIGH };, used as Level::HIGH.
An enum class cannot go straight into std::cout and does not turn into an int by itself, so take the name from an array: LEVEL_NAMES[static_cast<int>(s.level)]
CThe record is typedef struct { char name[32]; int score; Level level; } Student; and the list is an array Student students[100];, used like students[i].score.
The enum is typedef enum { LOW, MID, HIGH } Level;, with the integers 0, 1, 2 inside.
Nothing prints the name for you, so take it from an array of names: LEVEL_NAMES[students[i].level]
CraykonThere are no structs, so a record is a dictionary.
On each pass make a new one with s = Dict();, fill it with s["name"] = ... and add it with students.Add(s).
There is no enum either, so put named constants at the top, LOW = 0; MID = 1; HIGH = 2;, and take the name from a list, LEVEL_NAMES[s["level"]].
Nothing stops a constant from being changed, so it is a promise not to change it

Common mistakes

Adding one record again and again while editing itJavaScript · TypeScript · Python · Craykon

const s = {};
const students = [];
for (const name of ["mina", "jun"]) {
  s.name = name;
  students.push(s);
}
console.log(students);

→ [ { name: 'jun' }, { name: 'jun' } ] — both slots of the list point at the same single record, so only the value written last remains

Create the record inside the loop (const s = { name: name };).
Then every pass makes a new record.

Comparing an enum value with a string or a numberPython

from enum import Enum
class Level(Enum):
    LOW = 0
    MID = 1
    HIGH = 2
level = Level.HIGH
print(level == "HIGH", level == 2)

→ False False — Level.HIGH is neither the string "HIGH" nor the number 2; it is a value of its own. It becomes false with no error, which makes it hard to find

Compare enum with enum: level == Level.HIGH.
Turn a string from the input into the enum first with Level["HIGH"].

Printing an enum gave a number, not the nameTypeScript · C · C++

enum Level { LOW, MID, HIGH }
console.log(Level.HIGH);

→ 2 — the number inside comes out as is. printf("%d\n", level); in C and sending an enum Level to std::cout in C++ also give 2. Sending a C++ enum class to std::cout is a compile error

In TypeScript get the name with Level[Level.HIGH].
In C and C++ make an array of names such as {"LOW", "MID", "HIGH"} and take it from there.

How languages differ
Being an enum does not mean only the listed values can get in.
In C, Level x = 7; compiles as is and prints 7.
Printing (Level)7 in C# gives 7 too.
A C++ enum class stops Level x = 7; with a compile error.
When turning a number from outside into an enum, check its range first.

Practice

  1. Weekend farm plot records8P0319
  2. Which way the turtle faces6P0077
  3. Score ranking8P0067
  4. League table6P0080
  5. Orders by city2P0082
  6. Order state machine6P0156
  7. Filter logs by level6P0357
  8. Hands of five dice6P0198