Java Tutorial/Statement Control/For Loop

A loop allows you to execute a statement or block of statements repeatedly

public class MainClass {
  public static void main(String[] args) {
    for (int i = 0; i < 8; i++) {
      System.out.println("Hi.");
    }
  }
}

Hi.
Hi.
Hi.
Hi.
Hi.
Hi.
Hi.
Hi.

Declare multiple variables in for loop

public class Main {
  public static void main(String[] args) {
    for (int i = 0, j = 1, k = 2; i < 5; i++){
      System.out.println("I : " + i + ",j : " + j + ", k : " + k);
    }
  }
}
/*
I : 0,j : 1, k : 2
I : 1,j : 1, k : 2
I : 2,j : 1, k : 2
I : 3,j : 1, k : 2
I : 4,j : 1, k : 2
*/

For statement in detail

It is common to declare a variable and assign a value to it in the initialization part. The variable declared will be visible to the expression and update parts as well as to the statement block.

For example, the following for statement loops five times and each time prints the value of i. Note that the variable i is not visible anywhere else since it is declared within the for loop.

public class MainClass {
  public static void main(String[] args) {
    for (int i = 0; i < 5; i++) {
        System.out.println(i + " ");
    }
  }
}

Infinite For loop Example

public class Main {
  public static void main(String[] args) {
    for (;;) {
      System.out.println("Hello");
      break;
    }
  }
}
//Hello

initialization_expression: define two variables in for loop

public class MainClass {
  public static void main(String[] arg) {
    int limit = 10;
    int sum = 0;
    for (int i = 1, j = 0; i <= limit; i++, j++) {
      sum += i * j;
    }
    System.out.println(sum);
  }
}

330

Java"s "labeled for" loop

public class MainClass {
  public static void main(String[] args) {
    int i = 0;
    outer: for (; true;) {
      inner: for (; i < 10; i++) {
        System.out.println("i = " + i);
        if (i == 2) {
          System.out.println("continue");
          continue;
        }
        if (i == 3) {
          System.out.println("break");
          i++;
          break;
        }
        if (i == 7) {
          System.out.println("continue outer");
          i++;
          continue outer;
        }
        if (i == 8) {
          System.out.println("break outer");
          break outer;
        }
        for (int k = 0; k < 5; k++) {
          if (k == 3) {
            System.out.println("continue inner");
            continue inner;
          }
        }
      }
    }
  }
}

i = 0
continue inner
i = 1
continue inner
i = 2
continue
i = 3
break
i = 4
continue inner
i = 5
continue inner
i = 6
continue inner
i = 7
continue outer
i = 8
break outer

Keeping the middle element only in for loop

public class MainClass {
  public static void main(String[] arg) {
    int limit = 10;
    int sum = 0;
    int i = 1;
    for (; i <= limit;) {
      sum += i++;
    }
    System.out.println(sum);
  }
}

55

Multiple expressions in for loops

public class Main {
  public static void main(String[] args) {
    for (int i = 0, j = 0; i < 5; i++, j--)
      System.out.println("i = " + i + " j= " + j);
  }
}
/*
i = 0 j= 0
i = 1 j= -1
i = 2 j= -2
i = 3 j= -3
i = 4 j= -4
*/

Nested for Loop

public class MainClass {
  public static void main(String[] args) {
    long limit = 20L;
    long factorial = 1L;
    for (long i = 1L; i <= limit; i++) {
      factorial = 1L;
      for (long factor = 2; factor <= i; factor++) {
        factorial *= factor;
      }
      System.out.println(i + "! is " + factorial);
    }
  }
}

1! is 1
2! is 2
3! is 6
4! is 24
5! is 120
6! is 720
7! is 5040
8! is 40320
9! is 362880
10! is 3628800
11! is 39916800
12! is 479001600
13! is 6227020800
14! is 87178291200
15! is 1307674368000
16! is 20922789888000
17! is 355687428096000
18! is 6402373705728000
19! is 121645100408832000
20! is 2432902008176640000

Print out a Diamond

class Diamond {
  public static void main(String[] args) {
    for (int i = 1; i < 10; i += 2) {
      for (int j = 0; j < 9 - i / 2; j++)
        System.out.print(" ");
      for (int j = 0; j < i; j++)
        System.out.print("*");
      System.out.print("\n");
    }
    for (int i = 7; i > 0; i -= 2) {
      for (int j = 0; j < 9 - i / 2; j++)
        System.out.print(" ");
      for (int j = 0; j < i; j++)
        System.out.print("*");
      System.out.print("\n");
    }
  }
}
/*
         *
        ***
       *****
      *******
     *********
      *******
       *****
        ***
         *
*/

The for Statement

The for statement is like the while statement, i.e. you use it to create loop. The for statement has following syntax:

for ( init ; booleanExpression ; update ) {
    statement (s)
}

1. init is an initialization that will be performed before the first iteration.
2. booleanExpression is a boolean expression which will cause the execution of statement(s) if it evaluates to true.
3. update is a statement that will be executed after the execution of the statement block.
4. init, expression, and update are optional.

The for statement will stop only if one of the following conditions is met:

1. booleanExpression evaluates to false
2. A break or continue statement is executed
3. A runtime error occurs.

The numerical for loop

for (initialization_expression ; loop_condition ; increment_expression) {
  // statements
}

sum = 210

To omit any or all of the elements in "for" loop: but you must include the semicolons

public class MainClass {
  public static void main(String[] arg) {
    int limit = 10;
    int sum = 0;
    for (int i = 1; i <= limit;) {
      sum += i++;
    }
    System.out.println(sum);
  }
}

55

Using the Floating-Point Values as the control value in a for loop

public class MainClass {
  public static void main(String[] arg) {
    for (double radius = 1.0; radius <= 2.0; radius += 0.2) {
      System.out.println("radius = " + radius + "area = " + Math.PI * radius * radius);
    }
  }
}

radius = 1.0area = 3.141592653589793
radius = 1.2area = 4.523893421169302
radius = 1.4area = 6.157521601035994
radius = 1.5999999999999999area = 8.04247719318987
radius = 1.7999999999999998area = 10.178760197630927
radius = 1.9999999999999998area = 12.566370614359169