What I learned in Java (Part 4) Conditional branching and repetition

Looking back on Java

Last time, I wrote about instruction execution statements in Java. Please see that if you like. What I learned in Java (3) Instruction execution statement

This time I would like to write about conditional branching and repetition.

What is conditional branching?

A Java program consists of a control structure of "rank", "branch", and "repetition". Java is processed in order from the top of the source code. This is called "sequential".

public class Main {
	public static void main(String[] args) {
		int a = 1;
		int b = 2;
		int c = a + b;
		System.out.println(c);
	}
}

Execution result In addition to proceeding "sequentially" as described above, I think it will branch in some cases. For example, if you have trouble with rice or bread at breakfast ... This kind of branching is called "branching". I would like to actually write the source code

public class Main {
	public static void main(String[] args) {
		//Ask them to choose between rice and bread
		System.out.println("What do you eat for breakfast?");
		System.out.println("Enter "0" for rice and "1" for bread");
		int morning = new java.util.Scanner(System.in).nextInt();
		
		//If it was rice
		if (morning == 0) {
			System.out.println("Prepare rice, miso soup and side dishes ...");
		
		//If it was bread
		} else {
			System.out.println("Bake in a toaster, butter, ...");
		}
		
		System.out.println("I will!");
	}
}

Execution result

• If it is rice Here is the part that is actually branching

if (morning == 0) {
    System.out.println("Prepare rice, miso soup and side dishes ...");	
} else {
    System.out.println("Bake in a toaster, butter, ...");
}

This part is called an if statement. When translated into human language "If breakfast is rice, prepare rice, miso soup, and side dishes." "Else (otherwise) bake bread and butter" It is the content. You can "branch" by using the if statement like this.

What is repetition

Next, I would like to compare "repetition" with the distance to the destination. Let's write the above contents in source code.

public class Main {
	public static void main(String[] args) {
		//Number of steps to the destination (this time the vending machine)
		int destination = 10;
		System.out.println(destination + "Let's buy juice from the vending machine at the destination!");
		//Number of steps taken
		int steps = 0;
		
		//Repeat until you reach your destination
		while (steps != destination) {
			System.out.println("Take a step forward");
			steps++;
		}
		System.out.println("I arrived at the vending machine, so I bought juice");
	}
}

Execution result I implemented the above code by walking "repeatedly" until I arrived at the vending machine 10 steps away!

• Sorry for the strange analogy ...

The part that is actually repeated is as follows.

//Number of steps to the destination (this time the vending machine)
int destination = 10;
//Number of steps taken
int steps = 0;		
//Repeat until you reach your destination
while (steps != destination) {
    System.out.println("Take a step forward");
    steps++;
}

It is an image of going step by step to the destination (destination)! On the contrary, if the destination is set to 0 and it is corrected so that it does not approach.

//Number of steps to the destination (this time the vending machine)
int destination = 0;
//Number of steps taken
int steps = 10;		
//Repeat until you reach your destination
while (steps != destination) {
    System.out.println("one step closer");
    steps--;
}

It doesn't change that much, but ... In this way, the while statement is iteratively processed as long as the condition is met.

How to write if statement and while statement

Statements that represent control structures, such as if statements and while statements, are called control syntax. I would like to summarize how to write it below.

//if statement
if (Conditional expression) {
Processing performed when the condition is met
} else {
Processing performed when the conditions are different
}

//while statement
while (Conditional expression) {
Processing performed when the condition is met
}

Conditional expressions are expressions used in if statements and while statements to express "conditions for branching processing" and "conditions for continuing repetition".

//If the variable number is 0
if (number == 0) {}
//You were if the variable s is "Hello"
if (s.equals("Hello")) {}

//If the variable number is 20 or more
while (number >= 20) {}

The "==" and "> =" in the conditional expression are called relational operators. Types and meanings of relational operators

//Age 20+ and 65+
if (age >= 20 && age <= 65) {}
//30 years or younger or male
if (age <= 30 || gender == 0 ) {}

"" in the conditional expression&&」「||Is called a logical operator. Types and meanings of logical operators

Other branch syntax and repeat syntax variations

if statement only syntax

if (Conditional expression) {
Processing performed when the condition is met
}

Since there is no else statement, nothing is done if the conditional expression does not apply.

if-else if-else syntax

if (Conditional expression 1) {
Processing performed when the condition is met
} else if (Conditional expression 2) {
Processing performed when the condition is met
} else if (Conditional expression 3) {
Processing performed when the condition is met
} else {
What happens if the condition is not met
}

One if statement can branch to three or more routes.

switch syntax

switch(Condition value)
case value 1:
Process 1
        break;
case value 2:
Process 2
        break;
case value 3:
Process 3
        break;
    default:
Process 4

switch statement Conditions that can be used (1) An expression that compares whether conditional expressions match, such as "variable == value" and "variable == variable". (2) The value to be compared is an integer, a character string, or a character, not a decimal value or a boolean value.

• As the processing of the switch statement, the processing jumps to the "case label" that matches the condition value. Therefore, if you forget the "break statement", the "sequential" processing will proceed.

do-while syntax

do{
block
} while (Conditional expression)

The while statement first determines the conditional expression (pre-judgment), and if the condition is met, the processing inside the block is executed. The do-while statement judges the conditional expression (postfix judgment) after the processing in the block is performed.

for statement

for (int i = 0; i < 10; i++){
Repeated processing
};

The process is repeated the number of times the int type i increases from "0" to "10" (i ++ part).

Control structure nesting

Conditional branches and repetitive statements (control structures) can be placed in different control structures. This is called "nesting" or "nesting". A simple multiplication table example ...

for (int i = 1; i < 10; i++){
	for (int j = 1; j < 10; j++) {
		System.out.print(i * j);
		System.out.print(" ");
	}
	System.out.println("");
}

Execution result I think that you can play various if statements, while statements, for statements, etc. depending on how you use them, so please try them!

References

https://book.impress.co.jp/books/1113101090

Next time, I would like to write about arrays.