[JAVA] 14. 이중 for문으로 구구단 만들기

구구단

• 구구단을 2단부터 9단까지 출력하시오.

문제 분석

• 구구단은 특정 수를 1부터 9까지 곱해야 함
• 반복문 사용 : for

• 2단부터 9단까지 한 번에 표현해야 함
• 반복문을 반복 : 이중 반복문

Step 1. Hard Coding

package ex03;

public class Gugudan01 {
    public static void main(String[] args) {
		    // 1. 2단
        System.out.println("2*1=2");
        System.out.println("2*2=4");
        System.out.println("2*3=6");
        System.out.println("2*4=8");
        System.out.println("2*5=10");
        System.out.println("2*6=12");
        System.out.println("2*7=14");
        System.out.println("2*8=16");
        System.out.println("2*9=18");
        System.out.println();
				
				// 2. 3단
        System.out.println("3*1=3");
        System.out.println("3*2=6");
        System.out.println("3*3=9");
        System.out.println("3*4=12");
        System.out.println("3*5=15");
        System.out.println("3*6=18");
        System.out.println("3*7=21");
        System.out.println("3*8=24");
        System.out.println("3*9=27");
        System.out.println();
        
				// 3. 4단
        System.out.println("4*1=4");
        System.out.println("4*2=8");
        System.out.println("4*3=12");
        System.out.println("4*4=16");
        System.out.println("4*5=20");
        System.out.println("4*6=24");
        System.out.println("4*7=28");
        System.out.println("4*8=32");
        System.out.println("4*9=36");
        System.out.println();
        
        // 5단, 6단, 7단, ..., 9단
    }
}

• Hard Coding을 통한 반복되는 구간, 변수 찾기

Step 2. Moduling

package ex03;

public class Gugudan01 {
    public static void main(String[] args) {

        int a = 1;

        // 1. 2단
        a++;
        System.out.println(a + "*" + 1 + "=" + (a * 1));
        System.out.println(a + "*" + 2 + "=" + (a * 2));
        System.out.println(a + "*" + 3 + "=" + (a * 3));
        System.out.println(a + "*" + 4 + "=" + (a * 4));
        System.out.println(a + "*" + 5 + "=" + (a * 5));
        System.out.println(a + "*" + 6 + "=" + (a * 6));
        System.out.println(a + "*" + 7 + "=" + (a * 7));
        System.out.println(a + "*" + 8 + "=" + (a * 8));
        System.out.println(a + "*" + 9 + "=" + (a * 9));
        System.out.println();

        // 2. 3단
        a++;
        System.out.println(a + "*" + 1 + "=" + (a * 1));
        System.out.println(a + "*" + 2 + "=" + (a * 2));
        System.out.println(a + "*" + 3 + "=" + (a * 3));
        System.out.println(a + "*" + 4 + "=" + (a * 4));
        System.out.println(a + "*" + 5 + "=" + (a * 5));
        System.out.println(a + "*" + 6 + "=" + (a * 6));
        System.out.println(a + "*" + 7 + "=" + (a * 7));
        System.out.println(a + "*" + 8 + "=" + (a * 8));
        System.out.println(a + "*" + 9 + "=" + (a * 9));
        System.out.println();

        // 3. 4단
        a++;
        System.out.println(a + "*" + 1 + "=" + (a * 1));
        System.out.println(a + "*" + 2 + "=" + (a * 2));
        System.out.println(a + "*" + 3 + "=" + (a * 3));
        System.out.println(a + "*" + 4 + "=" + (a * 4));
        System.out.println(a + "*" + 5 + "=" + (a * 5));
        System.out.println(a + "*" + 6 + "=" + (a * 6));
        System.out.println(a + "*" + 7 + "=" + (a * 7));
        System.out.println(a + "*" + 8 + "=" + (a * 8));
        System.out.println(a + "*" + 9 + "=" + (a * 9));
        System.out.println();
        
        // 5단, 6단, 7단, ..., 9단
    }
}

• 공통되는 부분을 변수 추가를 통한 모듈화

Step 3. 반복 (9바퀴)

package ex03;

public class Gugudan01 {
    public static void main(String[] args) {

        int a = 1;
        
        for (int i = 0; i < 8; i++) {
            a++;
            System.out.println(a + "*" + 1 + "=" + (a * 1));
            System.out.println(a + "*" + 2 + "=" + (a * 2));
            System.out.println(a + "*" + 3 + "=" + (a * 3));
            System.out.println(a + "*" + 4 + "=" + (a * 4));
            System.out.println(a + "*" + 5 + "=" + (a * 5));
            System.out.println(a + "*" + 6 + "=" + (a * 6));
            System.out.println(a + "*" + 7 + "=" + (a * 7));
            System.out.println(a + "*" + 8 + "=" + (a * 8));
            System.out.println(a + "*" + 9 + "=" + (a * 9));
            System.out.println();
        }
    }
}

• 모듈화를 통해 압축시킨 부분을 반복문을 통해 추가 압축 및 반복

최종 코드 작성

package ex03;

public class Gugudan01 {
    public static void main(String[] args) {

        for (int a = 2; a < 10; a++) {
            for (int b = 1; b < 10; b++) {
                System.out.println(a + " * " + b + " = " + (a * b));
            }
            System.out.println();
        }
    }
}

• for문을 사용하여 1부터 9까지 곱하여 출력
• 이중 반복문을 활용하여 2단부터 9단까지 출력

결과