Imperative programming V.S. declarative programming

Programming: Imperative & Declarative

• Imperative Programming: “How to” – a language written for computers
  • Operation statements
  • Conditional statements
  • Looping statements
• Declarative programming: “What to do” — a language written for people
  • Functional programming
  • DSL: Domain specific language

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Consider the following questions:

Check the average score of the students in students and output the first place in the average score.

Given some definition code:

struct Student {
    let name: String
    let scores: [Subject: Int]}enum Subject: String.CaseIterable {
    case Chinese.Math.English
}

let s1 = Student(
    name: "Foo",
    scores: [.Chinese: 86, .Math: 92, .English: 73])let s2 = Student(
    name: "Bar",
    scores: [.Chinese: 99, .Math: 52, .English: 97])let s3 = Student(
    name: "Joe",
    scores: [.Chinese: 91, .Math: 92, .English: 100])let students = [s1, s2, s3]
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Here’s an attempt to solve this problem using imperative and functional programming, respectively:

Imperative programming

Tell the computer what to do step by step.

// Command.swift
var best: (Student.Double)?

for s in students {
    var totalScore = 0
    for key in Subject.allCases {
        totalScore + = s.scores[key] ?? 0
    }

    let averageScore = Double(totalScore) /
    	Double(Subject.allCases.count)

    if let temp = best {
        if averageScore > temp.1 {
            best = (s, averageScore)
        }
    } else {
        best = (s, averageScore)
    }
}

print(best?.0.name ?? "no students")
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This is probably the way most of you are familiar with programming.

Declarative programming

Tell the computer what to do, and the computer decides what to do.

functional

// The function.swift

func average(_ scores: [Subject: Int]) -> Double {
    return Double(scores.values.reduce(0.+)) / Double(Subject.allCases.count)
}

let bestStudent = students
    .map { ($0, average($0.scores)) }
    .sorted { $0.1 > The $1.1 }
    .first
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(Written in January 2021: I kind of like functions, but SICP hasn’t finished yet.) Update(2021.07.18): Now we’re done!!

I always like function, recommend everyone to see SICP (must put the inside of the code to write it again, or to the basic behind do not understand) 👀.

DSL

Such as SQL:

select name, avg(score) as avs_score 
from scores group by name order by avg_score;
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(I don’t like SQL. I hate group by keywords that are more than one word, so I use ORM every day for my projects.)

reference

Wang Wei (@onevcat) SwiftUI and Combine Programming Chapter 1