[JAVA] I will explain the nesting of for statements that kill beginners

Demonstration

for (int i = 0; i < 10; i++) {
  System.out.println("Hello!");
}

• int
• Declaring type ʻint`
• i = 0
• Substitute 0 for the variable ʻi`
• i < 10
• If ʻi is less than 10, it is true. In other words, the execution content is repeated until ʻi is 9.
• i++
• ʻI` is increased by one
• System.out.println("Hello!")
• Outputs the characters Hello!

Movement

Let's see it based on the demonstration. Initialization is the first thing that works when the for statement is started. The value of the variable ʻi is 0. The for statement confirms that ʻi is 0 and then moves on to the condition. The condition is ʻi <10. At this point it is the same as 0 <10. When translated into Japanese, it can be said that "0 is less than 10", so the condition is true. Since it was confirmed that the condition is true`, the part of the execution content moves.

When System.out.println ("Hello! ") Is executed, the next step is change. The change is ʻi ++ . At this point, it's 0 ++, so it's now 1. This 1moves to the condition to assign to the variable of execution. At this point it is1 <10. It's true`. That's why I will move on to the execution contents again.

If this flow continues many times, the condition will be 10 <10. Is this true? Is it false? It's false. This concludes the for statement. Thank you for your hard work.

The flow is listed below.

1. Start the for statement
2. Initialization
3. Conditions
4. Execution content
5. Change
6. Conditions (abbreviated below)
7. End of for statement if condition is false

Nesting for statements that was actually easy

Now that we have the basics of the for statement, the next step is nesting. I thought I somehow understood the nesting of the for statement, but when I actually wrote it, I didn't understand how the upper for statement and the lower for were entwined. did.

Let's see the movement of nesting while unraveling the multiplication table.

Program to display multiplication tables

public class MultiplicationTable {
	public void multiply() {
		for (int x = 1; x < 10; x++) {
			for (int y = 1; y < 10; y++) {
				System.out.print(x * y);
				System.out.print(" ");
			}
				System.out.println();
		}
	}
}

From this point onward, the expression is "outer for statement "instead of" upper for statement", and "inner for statement" instead of "lower for statement". Then, from the third line. Executed from ʻint x = 1; . Then x <10;. This time it is true`, so let's move on to the execution content. If you are a beginner, it will be blank to see where the execution is, but please see the figure below.

The for statement has been enlarged and color coded. As you can see from this, the execution content of the outer for statement is the inner blue for statement. The initial value of the inner for statement (blue), ʻint y = 1, is executed to move to the condition. Since y <10;, substitute it and it is 1 <10. Since it is true, it moves to the execution content. It is System.out.print (x * y). x was a variable in the outer forstatement. At this point,x = 1. So here 1 * 1 is executed and the result is 1. 1is displayed on the screen and moves to the next line.System.out.print ("") `displays a half-width space.

Once this is done, the next step is to change. Since it is y ++, y increases by one and changes to 2, and then the condition moves on. Since it is y <10, it becomes 2 <10 and it is true, so it moves to the execution contents again. It is System.out.print (x * y). Since it is 1 * 2, the result will be 2 and will be displayed on the screen, and a half-width space on the next line will also be displayed. When the value of y reaches 9 by such a movement, 1 2 3 4 5 6 7 8 9 is displayed on the screen. Let's see what happens after this.

With y ++, y becomes 10 and the condition moves. Since 10 <10 is false, System.out.println () at the bottom of the inner for statement (blue) is executed to start a new line. This ends the inner for statement (blue) and returns to the outer for statement (red). From here on, it's repeated. If you start with 2 * 1 and go to 2 * 9, 3 * 1 will be executed with a line break. Let's see the actual execution result.

1 2 3 4 5 6 7 8 9 
2 4 6 8 10 12 14 16 18 
3 6 9 12 15 18 21 24 27 
4 8 12 16 20 24 28 32 36 
5 10 15 20 25 30 35 40 45 
6 12 18 24 30 36 42 48 54 
7 14 21 28 35 42 49 56 63 
8 16 24 32 40 48 56 64 72 
9 18 27 36 45 54 63 72 81 

This is the behavior of the multiplication table program using the nesting of for statements. It's easy to unravel. Applying this, I made a question that I asked at the beginning. The result is here.

■
■■
■■■
■■■■
■■■■■
■■■■■■
■■■■■■■
■■■■■■■■
■■■■■■■■■
■■■■■■■■■■
■■■■■■■■■■
■■■■■■■■■
■■■■■■■■
■■■■■■■
■■■■■■
■■■■■
■■■■
■■■
■■
■

What I'm doing is using the for statement to increase the number of■by line to express a triangle. I made a shape that increases by one and a shape that decreases by one, and displayed the top and bottom combined. Below is the program.

public class TriangleTop {
	public void traiangle() {
		for (int x = 0; x < 10; x++) {
			for (int y = 0; y < x+1; y++) {
				System.out.print("■");
			}
				System.out.println();
		}
	}
}

public class TriangleBottom {
	public void traiangleReverse() {
		for (int x = 0; x < 10; x++) {
			for (int y = 10; y > x; y--) {
				System.out.print("■");
			}
				System.out.println();
		}
	}
}

If you've read this far, you don't have to explain it anymore.

Afterword

It is difficult for programming beginners to immediately understand the behavior of nesting for statements. It is recommended that you do not just look at it, but first try and error with your own hands. I wanted to make a triangle that was flipped to the left, so I tried a quadrangle, but I gave up because I couldn't display half-width spaces continuously. Someday I would like to draw a simple picture with the for statement.

Also, is it okay for Qiita to post something like this?