[TIL] September In 2022

On 09/05 (Mon)

“생활코딩 JAVA 입문 수업”

Watching video lecture From “자바 설치” to “입력과 출력” in “생활코딩”

After watching video “입력과 출력”, I wonder how to pass arguments in not eclipse but vsCode

launch.json

{
"version": "0.2.0",
"configurations": [
  {
    "type": "java",
    "name": "Launch Current File",
    "request": "launch",
    "mainClass": "${file}"
  },
  {
    "type": "java",
    "name": "Launch App",
    "request": "launch",
    "mainClass": "App",
    "projectName": "test project_6ad5dc36"
  }
]
}

input “args”: “one two” in the first curly brackets, it makes error.
I input “args”: “one two” in the second curly brackets, it dose work.
Reference: Stack overflow

On 09/06 (Tue)

what I learned today

Opentutorials.org
- From “JAVA1 - 12.1. 직접 컴파일하고 실행하기 : 소개” to “JAVA1 - 13.5. 자바 문서 보는 법 - 상속”
- “문자의 비교 : ==과 equals의 차이점”
- “JAVA 제어문 - 7.2. 배열”
- From “JAVA method - 1. 수업소개” to “JAVA method - 9. 부록 - static”
- From “JAVA 객체 지향 프로그래밍 - 1. 수업소개” to “JAVA 객체 지향 프로그래밍 - 9 수업을 마치며”

the awkward or difficult parts

Opentutorials.org
- “JAVA 객체 지향 프로그래밍 - 6. static”
- “JAVA 객체 지향 프로그래밍 - 7. 생성자와 this”

the interesting parts

Opentutorials.org
- “JAVA1 - 13.2. 자바 문서 보는 법 - 패키지,클래스,변수,메소드”
  - pakage: container of classes
  - class: container of related variables and methods
  - ex) java.lang: pakage, String: class
- “JAVA1 - 13.4. 자바 문서 보는 법 - 인스턴스”
  - when I should use methods in a certain class many times, it is good to make instances.
  - basic python grammer videe come to mind that said “클래스는 틀이다. 그 틀로 찍어낸 것이 인스턴스다”.
  - “constructor, 즉 생성자가 없는 애들은 일회용이다”
  - constructor: make instance (ex- PrintWriter p1 = new PrintWriter(“result1.txt”))
- “JAVA1 - 13.5. 자바 문서 보는 법 - 상속”
  - in VSCode, right click at PrintfWriter -> “Show Type Hierarchy” -> the class “PrintWriter” inherit the class “Writer”, and “Writer” inherit the class “Object”
  - inheritance: a child class can use parent classes method
  - override: the situation when I make a method “example()” in a child class, even if parent has a “example()” method.
- “문자의 비교 : ==과 equals의 차이점”
  - all parts of this video is interesting.
- “JAVA 제어문 - 7.2. 배열”
  - two ways:
    1. String[] users = new String[3];
    2. int[] scores = {10, 100, 100};
- “JAVA method - 8. 부록 - access level modifiers”
  - all parts of this video is interesting.
- “JAVA method - 9. 부록 - static”
  - all parts of this video is interesting.

On 09/07 (Wed)

Opentutorials.org

From “Java 상속 - 1. 수업소개” to “Java 상속 - 6 수업을 마치며”

“Do it! 알고리즘 코딩테스트 자바 편”

p.17~p.44
- “숫자의 합 구하기 (문제 001/35p)”
  - Scanner sNum=new Scanner(System.in); //create Scnner instance
  - int N = sc.nextInt(); // returns the Int scanned from the input.
  - String sNum = sc.next(); // scan input data like “12345”
  - char[] cNum = sNum.toCharArray(); // ex) “1234”.toCharArray() -> return character array {1,2,3,4}
  - how to convert ‘1’ to 1
    - In ASCII, the difference between the code values of letters and numbers of the same meaning is 48.
    - ‘1’ - 48 or ‘1’ - ‘0’
- “평균 구하기 (문제 002, 38p)(Baekjoon Online Judge: 1546번) “
  - This problem wasn’t difficult for me, but I failed to solve it.
  - My solving process is as follow:
```
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] grade = new int[n];

    for (int i = 0; i < n; i++) {
        grade[i] = sc.nextInt();
    }

    int max = grade[0];
    for (int i = 0; i < n; i++) {
        if (grade[i] > max) {
            max = grade[i];
        }
    }

    for (int i = 0; i < n; i++) {
        grade[i] = grade[i] * 100 / max;
    }

    int sum = 0;
    for (int i = 0; i < n; i++) {
        sum += grade[i];
    }
    System.out.println((double) sum / n);

    sc.close();
  }
}
```
- 00:00 ~ 12:00 in “코딩 테스트 핵심이론 강의 - 시간복잡도 (Java)”
  - I didn’t understand why “1,000,000log(1,000,000)” is about 20,000,000. Because I thought the base of the log is 10.
  - Therefore, I searched several sites and found out the following facts.
    - First, in the “Big-O” notation, the coefficient is discarded and only the degree is considered.
    - Second, The logarithm has the following properties. Source: https://nacture.kr/73
      
      for example, “log a N” is “log c N / log c a”. this coefficient is “log c a”, so it is discarded. Therefore, O(log a N) is O(log c N). After all, the base dosen’t really matter.

On 09/13 (Tue)

“Do it! 알고리즘 코딩테스트 자바 편” (Video Lectures)

“코딩 테스트 핵심이론 강의 - 시간복잡도 (Java)”
“코딩 테스트 핵심이론 강의 - 디버깅 (Java)”
- In VSCode, “Run and Debug” -> “watch” -> “add expression” -> while debuging, I can check certain expressions such as “A[0]+A[5]”
“코딩 테스트 핵심이론 강의 - 배열과 리스트 (Java)”
“코딩 테스트 문제풀이 강의 - 1 숫자의 합 구하기 (백준 11720)”

“코딩 테스트 문제풀이 강의 - 2 평균 구하기 (백준 1546)”

I changed my code of this problem to the following:

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    double[] grade = new double[n];  //changed!

    for (int i = 0; i < n; i++) {
        grade[i] = sc.nextInt();
    }

    double max = grade[0];  //changed!
    for (int i = 0; i < n; i++) {
        if (grade[i] > max) {
            max = grade[i];
        }
    }

    for (int i = 0; i < n; i++) {
        grade[i] = grade[i] * 100 / max;
    }

    double sum = 0;  //changed!
    for (int i = 0; i < n; i++) {
        sum += grade[i];
    }
    System.out.println(sum / n);

    sc.close();
  }
}

On 09/14 (Wed)

“Do it! 알고리즘 코딩테스트 자바 편” (Video Lectures)

“코딩 테스트 핵심이론 강의 - 구간 합”
“코딩 테스트 문제풀이 강의 - 3 구간 합 구하기1 (백준11659)”
- “받는 게 많을 때는, scanner보다 bufferdReader 사용하는 게 낫다.”
- “2번째 줄에 입력하는 수가 10만개면 매우 길게 옆으로 데이터가 들어옴 -> int형으로 받기 힘듦 -> StringTokenizer를 이용해서 값을 토큰으로 분리해가지고 사용”

On 09/15 (Thu)

“Do it! 알고리즘 코딩테스트 자바 편”

“구간 합 구하기 2(문제 004,38p)(Baekjoon Online Judge: 11660번)”

Two dimensional array: int[][] twoD_arr = new int[10][20]; Three dimensional array: int[][][] threeD_arr = new int[10][20][30];
Today I only solved this problem, and I failed to solve it. Also, I didn’t see the answer because of time constraint. The code I wrote is as follows.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
    public static void main(String[] args) throws IOException {

        // get n,m -> 2d array = new int[n][n]
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());
        int n = Integer.parseInt(st.nextToken());
        int m = Integer.parseInt(st.nextToken());
        int[][] a = new int[n + 1][n + 1];

        /*
        * for(repeat for n times){
        * get line in form of string -> convert string to int -> push these to 2d array
        * }
        */
        for (int i = 1; i <= n; ++i) {
            st = new StringTokenizer(br.readLine());
            for (int j = 1; j <= n; ++j) {
                int tmp = Integer.parseInt(st.nextToken());
                a[i][j] = tmp;
            }
        }

        // by row, get a sum array
        int[][] rSum = new int[n + 1][n + 1];
        for (int i = 1; i <= n; ++i) {
            for (int j = 1; j <= n; ++j) {
                rSum[i][j] = rSum[i][j - 1] + a[i][j];
            }
        }

        // get a line -> get x1,y1,x2,y2 -> get a sum in row from x1 to x2 at y1
        // -> repeat the previous step by y2
        // below code has a problem. time limit is 1 sec, so computations must be less
        // than 100 million.
        // however, in the worst case, the number of operations exceeds 100 million...
        int x1, y1, x2, y2;
        for (int i = 1; i <= m; ++i) {
            st = new StringTokenizer(br.readLine());
            x1 = Integer.parseInt(st.nextToken());
            y1 = Integer.parseInt(st.nextToken());
            x2 = Integer.parseInt(st.nextToken());
            y2 = Integer.parseInt(st.nextToken());

            int sum = 0;
            for (int j = y1; j <= y2; ++j) {
                sum += rSum[j][x2] - rSum[j][x1 - 1];
            }
            System.out.println(sum);
        }

    }
}

On 09/19 (Mon)

“Do it! 알고리즘 코딩테스트 자바 편”

“구간 합 구하기 2(문제 004,38p)(Baekjoon Online Judge: 11660번)”
- I was confused between rows and columns of 2D array, so fixing the code, and changed code is as follows:
```
// get a line -> get x1,y1,x2,y2 -> get a sum in row from y1 to y2 at x1
// -> repeat the previous step by x2

for (int j = x1; j <= x2; ++j) {
    sum += rSum[j][y2] - rSum[j][y1 - 1];
}
```
- however, the fixed codes exceed a time limit.
나머지 합 구하기(문제 005,52p)(Baekjoon Online Judge: 10986번)
- I devoted my time to understanding the solution and explanation of this problem.

On 09/19 (Mon)

“Do it! 알고리즘 코딩테스트 자바 편”

“구간 합 구하기 2(문제 004,38p)(Baekjoon Online Judge: 11660번)”

my codes got an error of ArrayIndexOutOfBounds or incorrect codes
I dont’t know why my codes is wrong and the codes of the book is correct, although my code and the code of the book seem similar.
The last code I tried is as follows:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();

        int a[] = new int[n];
        for (int i = 0; i < n; ++i) {
            a[i] = sc.nextInt();
        }

        int s[] = new int[n];
        int c[] = new int[m];
        int ans = 0;
        s[0] = a[0];
        for (int i = 1; i < n; ++i) {
            s[i] = s[i - 1] + a[i];
        }
        for (int i = 0; i < n; ++i) {
            int remainder = (int) (s[i] % m);
            if (remainder == 0)
                ++ans;
            c[remainder]++;
        }

        for (int i = 0; i < m; ++i) {
            if (c[i] > 1) {
                ans += c[i] * (c[i] - 1) / 2;
            }
        }
        System.out.println(ans);
        sc.close();
    }
}

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person3113

[TIL] September In 2022

On 09/05 (Mon)

“생활코딩 JAVA 입문 수업”

On 09/06 (Tue)

what I learned today

the awkward or difficult parts

the interesting parts

On 09/07 (Wed)

Opentutorials.org

“Do it! 알고리즘 코딩테스트 자바 편”

On 09/13 (Tue)

“Do it! 알고리즘 코딩테스트 자바 편” (Video Lectures)

On 09/14 (Wed)

“Do it! 알고리즘 코딩테스트 자바 편” (Video Lectures)

On 09/15 (Thu)

“Do it! 알고리즘 코딩테스트 자바 편”

On 09/19 (Mon)

“Do it! 알고리즘 코딩테스트 자바 편”

On 09/19 (Mon)

“Do it! 알고리즘 코딩테스트 자바 편”

공유하기

참고

[스프링 핵심 원리(김영한)] ‘이야기 - 자바 진영의 추운 겨울과 스프링의 탄생’ ~ ‘스코프와 프록시’

[제이쌤의 포텐영문법] 품사의 이해 ~ 가정법 (예정)

[Do It! 코딩테스트 자바편(김종관)] 문제 075(백준 11438번)

[Do It! 코딩테스트 자바편(김종관)] 문제 074(백준 11437번)