Looping statements

Intro

Problem

  • Что если нужно вывести сообщение 100 раз?

  • Повторим код по выводу 100 раз!

Solution

Вместо того чтобы писать один и тот же код 100 раз, можно использовать цикл.

Control Flow Statements

Looping statements

Control Flow Statements

while statement

Block schema

Block schema

Syntax

while (booleanExpression) {
    // statement(s) (body of loop)
}

Syntax

while (booleanExpression)
    // ONE statement

Do not recommend

Example

// declare variables
int i = 1;
int n = 5;

// while loop from 1 to 5
while (i <= n) {
    System.out.println(i);
    i++;
}
1
2
3
4
5

How this program works?

IterationVariablei <= nAction

1st

i = 1, n = 5

true

1 is printed. i is increased to 2.

2nd

i = 2, n = 5

true

2 is printed. i is increased to 3.

3rd

i = 3, n = 5

true

3 is printed. i is increased to 4.

4th

i = 4, n = 5

true

4 is printed. i is increased to 5.

5th

i = 5, n = 5

true

5 is printed. i is increased to 6.

6th

i = 6, n = 5

false

The loop is terminated.

Example

// declare variables
int i = 1;

// infinity while loop
while (true) {
    System.out.println(i);
    i++;
}
1
2
...

do-while statement

Block schema

Block schema

Syntax

do {
    // statement(s) (body of loop)
} while (booleanExpression);

Syntax

do
    // ONE statement
while (booleanExpression);

Do not recommend

Example

int i = 1;
int n = 5;

// do-while loop from 1 to 5
do {
    System.out.println(i);
    i++;
} while (i <= n);
1
2
3
4
5

Example

int i = 1;

// infinity do-while loop
do {
    System.out.println(i);
    i++;
} while (true);
1
2
...

for statement

Block schema

Block schema

Syntax

for (initialization; termination; increment) {
    // statement(s) (body of loop)
}

Syntax

for (initialization; termination; increment)
    // ONE statement

Do not recommend

Concepts

for (initialization; termination; increment) {
    // statement(s) (body of loop)
}

  • The initialization expression initializes the loop; it’s executed ONCE, as the loop begins.

  • When the termination expression evaluates to false, the loop terminates.

  • The increment expression is invoked after EACH iteration through the loop

Example

int n = 5;

// for loop
for (int i = 1; i <= n; i++) {
    System.out.println(i);
}
1
2
3
4
5

How this program works?

IterationVariablei <= nAction

1st

i = 1, n = 5

true

1 is printed. i is increased to 2.

2nd

i = 2, n = 5

true

2 is printed. i is increased to 3.

3rd

i = 3, n = 5

true

3 is printed. i is increased to 4.

4th

i = 4, n = 5

true

4 is printed. i is increased to 5.

5th

i = 5, n = 5

true

5 is printed. i is increased to 6.

6th

i = 6, n = 5

false

The loop is terminated.

Example

int sum = 0;
int n = 1000;

// for loop
for (int i = 1; i <= n; i++) {
    // body inside for loop
    sum += i; // sum = sum + i
}

System.out.println("Sum = " + sum);
Sum = 500500

Example

int sum = 0;
int n = 1000;

// for loop
for (int i = n; i >= 1; i--) {
    // body inside for loop
    sum += i; // sum = sum + i
}

System.out.println("Sum = " + sum);
Sum = 500500

Example

// infinity for loop
for ( ; ; ) {
    ...
}

Example

for (int i = 1; i <= 10; i--) {
    System.out.println("Hello");
}
Hello
...

for-each loop

Syntax

T[] array = initArray();

// for-each loop
for (T element : array) {
    // statement(s)
}

T - data type for elements from Array/Collection

Syntax

// for-each loop
for (T element : array)
    // ONE statement

T - data type for elements from Array/Collection

Example

// create an array
int[] numbers = {3, 9, 5, -5};

// for-each loop
for (int number: numbers) {
    System.out.println(number);
}
3
9
5
-5

How to this program works?

  • In the first iteration, item will be 3.

  • In the second iteration, item will be 9.

  • In the third iteration, item will be 5.

  • In the fourth iteration, item will be -5.

for vs for-each

Example

char[] vowels = {'a', 'e', 'i', 'o', 'u'};
// iterating through an array using a for loop
for (int i = 0; i < vowels.length; ++ i) {
    System.out.println(vowels[i]);
}
// iterating through an array using the for-each loop
for (char item: vowels) {
    System.out.println(item);
}
a
e
i
o
u

Total

Total

  • while statement

    • когда количество итераций зависит от результата вычисления в теле цикла

  • do-while statement

    • когда количество итераций зависит от результата вычисления в теле цикла, НО нужно выполнить как минимум одну итерацию

Total

  • for statement

    • когда количество итераций заранее известно или легко вычисляется

  • for-each statement

    • при работе с каждым элементом Collection или Array