Loop Patterns - Java Programming

A countdown timer needs to go 10, 9, 8... 1, 0. A report processes every 5th record for sampling. A game loop runs until the player quits. Each scenario uses a different loop pattern.

Count up (1 to 10)

The most basic loop pattern - counting from start to end.

                            example
                            start = 1, end = 10 (default)start = 0start = 5end = 5end = 20
                        

                            CountUp.java
                        
Replay: real traced execution (multi-file project)

                            public class CountUp {
    public static void main(String[] args) {
        int start = 1;
        int end = 10;

        System.out.println("Counting from " + start + " to " + end + ":");
        for (int i = start; i <= end; i++) {
            System.out.println(i);
        }

        // Practical: sum numbers in range
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum of 1-100: " + sum);
    }
}
public class CountUp {
    public static void main(String[] args) {
        int start = 0;
        int end = 10;

        System.out.println("Counting from " + start + " to " + end + ":");
        for (int i = start; i <= end; i++) {
            System.out.println(i);
        }

        // Practical: sum numbers in range
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum of 1-100: " + sum);
    }
}
public class CountUp {
    public static void main(String[] args) {
        int start = 5;
        int end = 10;

        System.out.println("Counting from " + start + " to " + end + ":");
        for (int i = start; i <= end; i++) {
            System.out.println(i);
        }

        // Practical: sum numbers in range
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum of 1-100: " + sum);
    }
}
public class CountUp {
    public static void main(String[] args) {
        int start = 1;
        int end = 5;

        System.out.println("Counting from " + start + " to " + end + ":");
        for (int i = start; i <= end; i++) {
            System.out.println(i);
        }

        // Practical: sum numbers in range
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum of 1-100: " + sum);
    }
}
public class CountUp {
    public static void main(String[] args) {
        int start = 1;
        int end = 20;

        System.out.println("Counting from " + start + " to " + end + ":");
        for (int i = start; i <= end; i++) {
            System.out.println(i);
        }

        // Practical: sum numbers in range
        int sum = 0;
        for (int i = 1; i <= 100; i++) {
            sum += i;
        }
        System.out.println("Sum of 1-100: " + sum);
    }
}

                        
1start ← 1, end ← 101public class CountUp {2    public static void main(String[] args) {3        int start→ 1 = 1;   //@start=5, 04        int end→ 10 = 10;    //@end=5, 205        6        System.out.println("Counting from " + start1 + " to " + end10 + ":");7        for (int i = start; i <= end; i++) {⋯outputCounting from 1 to 10:
2for (int i = start; i <= end; i++)pass 1 of 10⋯6System.out.println("Counting from " + start + " to " + end + ":");7for (int i1 = start; i <= end10; i++) {8    System.out.println(i1);9}⋯output1All 10 passes — pass 1 is the card abovepassi
11
22
33
44
55
66
77
88
99
1010
3sum ← 0⋯11// Practical: sum numbers in range12int sum→ 0 = 0;13for (int i = 1; i <= 100; i++) {⋯
4sum ← 1pass 1 of 100⋯12int sum = 0;13for (int i1 = 1; i <= 100; i++) {14    sum→ 1 += i1;15}⋯100 passes — pass 1 is the card abovepassisum
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5System.out.println("Sum of 1-100: " + sum);⋯15    }16    System.out.println("Sum of 1-100: " + sum5050);17}⋯outputSum of 1-100: 5050
1start ← 0, end ← 101public class CountUp {2    public static void main(String[] args) {3        int start→ 0 = 0;4        int end→ 10 = 10;5        6        System.out.println("Counting from " + start0 + " to " + end10 + ":");7        for (int i = start; i <= end; i++) {⋯outputCounting from 0 to 10:
2for (int i = start; i <= end; i++)pass 1 of 11⋯6System.out.println("Counting from " + start + " to " + end + ":");7for (int i0 = start; i <= end10; i++) {8    System.out.println(i0);9}⋯output0All 11 passes — pass 1 is the card abovepassi
10
21
32
43
54
65
76
87
98
109
1110
3sum ← 0⋯11// Practical: sum numbers in range12int sum→ 0 = 0;13for (int i = 1; i <= 100; i++) {⋯
4sum ← 1pass 1 of 100⋯12int sum = 0;13for (int i1 = 1; i <= 100; i++) {14    sum→ 1 += i1;15}⋯100 passes — pass 1 is the card abovepassisum
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5System.out.println("Sum of 1-100: " + sum);⋯15    }16    System.out.println("Sum of 1-100: " + sum5050);17}⋯outputSum of 1-100: 5050
1start ← 5, end ← 101public class CountUp {2    public static void main(String[] args) {3        int start→ 5 = 5;4        int end→ 10 = 10;5        6        System.out.println("Counting from " + start5 + " to " + end10 + ":");7        for (int i = start; i <= end; i++) {⋯outputCounting from 5 to 10:
2for (int i = start; i <= end; i++)pass 1 of 6⋯6System.out.println("Counting from " + start + " to " + end + ":");7for (int i5 = start; i <= end10; i++) {8    System.out.println(i5);9}⋯output5All 6 passes — pass 1 is the card abovepassi
15
26
37
48
59
610
3sum ← 0⋯11// Practical: sum numbers in range12int sum→ 0 = 0;13for (int i = 1; i <= 100; i++) {⋯
4sum ← 1pass 1 of 100⋯12int sum = 0;13for (int i1 = 1; i <= 100; i++) {14    sum→ 1 += i1;15}⋯100 passes — pass 1 is the card abovepassisum
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5System.out.println("Sum of 1-100: " + sum);⋯15    }16    System.out.println("Sum of 1-100: " + sum5050);17}⋯outputSum of 1-100: 5050
1start ← 1, end ← 51public class CountUp {2    public static void main(String[] args) {3        int start→ 1 = 1;4        int end→ 5 = 5;5        6        System.out.println("Counting from " + start1 + " to " + end5 + ":");7        for (int i = start; i <= end; i++) {⋯outputCounting from 1 to 5:
2for (int i = start; i <= end; i++)pass 1 of 5⋯6System.out.println("Counting from " + start + " to " + end + ":");7for (int i1 = start; i <= end5; i++) {8    System.out.println(i1);9}⋯output1All 5 passes — pass 1 is the card abovepassi
11
22
33
44
55
3sum ← 0⋯11// Practical: sum numbers in range12int sum→ 0 = 0;13for (int i = 1; i <= 100; i++) {⋯
4sum ← 1pass 1 of 100⋯12int sum = 0;13for (int i1 = 1; i <= 100; i++) {14    sum→ 1 += i1;15}⋯100 passes — pass 1 is the card abovepassisum
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5System.out.println("Sum of 1-100: " + sum);⋯15    }16    System.out.println("Sum of 1-100: " + sum5050);17}⋯outputSum of 1-100: 5050
1start ← 1, end ← 201public class CountUp {2    public static void main(String[] args) {3        int start→ 1 = 1;4        int end→ 20 = 20;5        6        System.out.println("Counting from " + start1 + " to " + end20 + ":");7        for (int i = start; i <= end; i++) {⋯outputCounting from 1 to 20:
2for (int i = start; i <= end; i++)pass 1 of 20⋯6System.out.println("Counting from " + start + " to " + end + ":");7for (int i1 = start; i <= end20; i++) {8    System.out.println(i1);9}⋯output120 passes — pass 1 is the card abovepassi
11
22
33
44
55
66
77
88
99
⋯ 9 more passes ⋯
1919
2020
3sum ← 0⋯11// Practical: sum numbers in range12int sum→ 0 = 0;13for (int i = 1; i <= 100; i++) {⋯
4sum ← 1pass 1 of 100⋯12int sum = 0;13for (int i1 = 1; i <= 100; i++) {14    sum→ 1 += i1;15}⋯100 passes — pass 1 is the card abovepassisum
110 → 1
221 → 3
333 → 6
446 → 10
5510 → 15
6615 → 21
7721 → 28
8828 → 36
9936 → 45
⋯ 89 more passes ⋯
99994851 → 4950
1001004950 → 5050
5System.out.println("Sum of 1-100: " + sum);⋯15    }16    System.out.println("Sum of 1-100: " + sum5050);17}⋯outputSum of 1-100: 5050

All 10 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
10	10

100 passes — pass 1 is the card above
pass	`i`	`sum`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 11 passes — pass 1 is the card above
pass	`i`
1	0
2	1
3	2
4	3
5	4
6	5
7	6
8	7
9	8
10	9
11	10

100 passes — pass 1 is the card above
pass	`i`	`sum`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 6 passes — pass 1 is the card above
pass	`i`
1	5
2	6
3	7
4	8
5	9
6	10

100 passes — pass 1 is the card above
pass	`i`	`sum`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

All 5 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5

100 passes — pass 1 is the card above
pass	`i`	`sum`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

20 passes — pass 1 is the card above
pass	`i`
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
⋯ 9 more passes ⋯
19	19
20	20

100 passes — pass 1 is the card above
pass	`i`	`sum`
1	1	0 → 1
2	2	1 → 3
3	3	3 → 6
4	4	6 → 10
5	5	10 → 15
6	6	15 → 21
7	7	21 → 28
8	8	28 → 36
9	9	36 → 45
⋯ 89 more passes ⋯
99	99	4851 → 4950
100	100	4950 → 5050

The classic counting loop. Change the bounds to count any range.

for Count-controlled loop: known number of iterations.

Count down (countdown timer)

Looping in reverse order - start high, go low.

                            start
                            start = 10 (default)start = 3start = 5
                        

                            CountDown.java
                        
Replay: real traced execution (multi-file project)

                            public class CountDown {
    public static void main(String[] args) {
        int start = 10;

        System.out.println("Countdown from " + start + ":");
        for (int i = start; i >= 1; i--) {
            System.out.println(i);
        }
        System.out.println("Liftoff!");

        // Practical: reverse array traversal
        int[] arr = {10, 20, 30, 40, 50};
        System.out.println("\nArray in reverse:");
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.println("Index " + i + ": " + arr[i]);
        }
    }
}
public class CountDown {
    public static void main(String[] args) {
        int start = 3;

        System.out.println("Countdown from " + start + ":");
        for (int i = start; i >= 1; i--) {
            System.out.println(i);
        }
        System.out.println("Liftoff!");

        // Practical: reverse array traversal
        int[] arr = {10, 20, 30, 40, 50};
        System.out.println("\nArray in reverse:");
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.println("Index " + i + ": " + arr[i]);
        }
    }
}
public class CountDown {
    public static void main(String[] args) {
        int start = 5;

        System.out.println("Countdown from " + start + ":");
        for (int i = start; i >= 1; i--) {
            System.out.println(i);
        }
        System.out.println("Liftoff!");

        // Practical: reverse array traversal
        int[] arr = {10, 20, 30, 40, 50};
        System.out.println("\nArray in reverse:");
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.println("Index " + i + ": " + arr[i]);
        }
    }
}

                        
1start ← 101public class CountDown {2    public static void main(String[] args) {3        int start→ 10 = 10;  //@start=5, 34        5        System.out.println("Countdown from " + start10 + ":");6        for (int i = start; i >= 1; i--) {⋯outputCountdown from 10:
2for (int i = start; i >= 1; i--)pass 1 of 10⋯5System.out.println("Countdown from " + start + ":");6for (int i10 = start; i >= 1; i--) {7    System.out.println(i10);8}⋯output10All 10 passes — pass 1 is the card abovepassi
110
29
38
47
56
65
74
83
92
101
3int[] arr = {10, 20, 30, 40, 50};⋯8}9System.out.println("Liftoff!");1011// Practical: reverse array traversal12int[] arr = {10, 20, 30, 40, 50};13System.out.println("\nArray in reverse:");14for (int i = arr.length - 1; i >= 0; i--) {⋯outputLiftoff!

Array in reverse:
4for (int i = arr.length - 1; i >= 0; i--)pass 1 of 5⋯13System.out.println("\nArray in reverse:");14for (int i4 = arr.length5 - 1; i >= 0; i--) {15    System.out.println("Index " + i4 + ": " + arr[i]50);16}⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010
1start ← 31public class CountDown {2    public static void main(String[] args) {3        int start→ 3 = 3;4        5        System.out.println("Countdown from " + start3 + ":");6        for (int i = start; i >= 1; i--) {⋯outputCountdown from 3:
2for (int i = start; i >= 1; i--)pass 1 of 3⋯5System.out.println("Countdown from " + start + ":");6for (int i3 = start; i >= 1; i--) {7    System.out.println(i3);8}⋯output3All 3 passes — pass 1 is the card abovepassi
13
22
31
3int[] arr = {10, 20, 30, 40, 50};⋯8}9System.out.println("Liftoff!");1011// Practical: reverse array traversal12int[] arr = {10, 20, 30, 40, 50};13System.out.println("\nArray in reverse:");14for (int i = arr.length - 1; i >= 0; i--) {⋯outputLiftoff!

Array in reverse:
4for (int i = arr.length - 1; i >= 0; i--)pass 1 of 5⋯13System.out.println("\nArray in reverse:");14for (int i4 = arr.length5 - 1; i >= 0; i--) {15    System.out.println("Index " + i4 + ": " + arr[i]50);16}⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010
1start ← 51public class CountDown {2    public static void main(String[] args) {3        int start→ 5 = 5;4        5        System.out.println("Countdown from " + start5 + ":");6        for (int i = start; i >= 1; i--) {⋯outputCountdown from 5:
2for (int i = start; i >= 1; i--)pass 1 of 5⋯5System.out.println("Countdown from " + start + ":");6for (int i5 = start; i >= 1; i--) {7    System.out.println(i5);8}⋯output5All 5 passes — pass 1 is the card abovepassi
15
24
33
42
51
3int[] arr = {10, 20, 30, 40, 50};⋯8}9System.out.println("Liftoff!");1011// Practical: reverse array traversal12int[] arr = {10, 20, 30, 40, 50};13System.out.println("\nArray in reverse:");14for (int i = arr.length - 1; i >= 0; i--) {⋯outputLiftoff!

Array in reverse:
4for (int i = arr.length - 1; i >= 0; i--)pass 1 of 5⋯13System.out.println("\nArray in reverse:");14for (int i4 = arr.length5 - 1; i >= 0; i--) {15    System.out.println("Index " + i4 + ": " + arr[i]50);16}⋯outputIndex 4: 50All 5 passes — pass 1 is the card abovepassiarr[i]
1450
2340
3230
4120
5010

All 10 passes — pass 1 is the card above
pass	`i`
1	10
2	9
3	8
4	7
5	6
6	5
7	4
8	3
9	2
10	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

All 3 passes — pass 1 is the card above
pass	`i`
1	3
2	2
3	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

All 5 passes — pass 1 is the card above
pass	`i`
1	5
2	4
3	3
4	2
5	1

All 5 passes — pass 1 is the card above
pass	`i`	`arr[i]`
1	4	50
2	3	40
3	2	30
4	1	20
5	0	10

Decrement with i-- and check i >= end to count backwards.

Step by N (every nth element)

Skip elements by incrementing by more than 1.

                            step
                            step = 5 (default)step = 2step = 10
                        

                            StepByN.java
                        
Replay: real traced execution (multi-file project)

                            public class StepByN {
    public static void main(String[] args) {
        int step = 5;

        // Count by step
        System.out.println("Counting by " + step + ":");
        for (int i = 0; i <= 50; i += step) {
            System.out.println(i);
        }

        // Practical: process every other element
        String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};
        System.out.println("\nEvery other name:");
        for (int i = 0; i < names.length; i += 2) {
            System.out.println(names[i]);
        }

        // Odd numbers only
        System.out.println("\nOdd numbers 1-20:");
        for (int i = 1; i <= 20; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
}
public class StepByN {
    public static void main(String[] args) {
        int step = 2;

        // Count by step
        System.out.println("Counting by " + step + ":");
        for (int i = 0; i <= 50; i += step) {
            System.out.println(i);
        }

        // Practical: process every other element
        String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};
        System.out.println("\nEvery other name:");
        for (int i = 0; i < names.length; i += 2) {
            System.out.println(names[i]);
        }

        // Odd numbers only
        System.out.println("\nOdd numbers 1-20:");
        for (int i = 1; i <= 20; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
}
public class StepByN {
    public static void main(String[] args) {
        int step = 10;

        // Count by step
        System.out.println("Counting by " + step + ":");
        for (int i = 0; i <= 50; i += step) {
            System.out.println(i);
        }

        // Practical: process every other element
        String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};
        System.out.println("\nEvery other name:");
        for (int i = 0; i < names.length; i += 2) {
            System.out.println(names[i]);
        }

        // Odd numbers only
        System.out.println("\nOdd numbers 1-20:");
        for (int i = 1; i <= 20; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

                        
1step ← 51public class StepByN {2    public static void main(String[] args) {3        int step→ 5 = 5;  //@step=2, 104        5        // Count by step6        System.out.println("Counting by " + step5 + ":");7        for (int i = 0; i <= 50; i += step) {⋯outputCounting by 5:
2for (int i = 0; i <= 50; i += step)pass 1 of 11⋯6System.out.println("Counting by " + step + ":");7for (int i0 = 0; i <= 50; i += step5) {8    System.out.println(i0);9}⋯output0All 11 passes — pass 1 is the card abovepassi
10
25
310
415
520
625
730
835
940
1045
1150
3String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};⋯11// Practical: process every other element12String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};13System.out.println("\nEvery other name:");14for (int i = 0; i < names.length; i += 2) {⋯output
Every other name:
4for (int i = 0; i < names.length; i += 2)pass 1 of 3⋯13System.out.println("\nEvery other name:");14for (int i0 = 0; i < names.length6; i += 2) {15    System.out.println(names[i]Alice);16}⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5System.out.println(" Odd numbers 1-20:");⋯18// Odd numbers only19System.out.println("\nOdd numbers 1-20:");20for (int i = 1; i <= 20; i += 2) {⋯output
Odd numbers 1-20:
6for (int i = 1; i <= 20; i += 2)pass 1 of 10⋯19System.out.println("\nOdd numbers 1-20:");20for (int i1 = 1; i <= 20; i += 2) {21    System.out.print(i1 + " ");22}⋯output1 All 10 passes — pass 1 is the card abovepassi
11
23
35
47
59
611
713
815
917
1019
7System.out.println();⋯22    }23    System.out.println();24}⋯
1step ← 21public class StepByN {2    public static void main(String[] args) {3        int step→ 2 = 2;4        5        // Count by step6        System.out.println("Counting by " + step2 + ":");7        for (int i = 0; i <= 50; i += step) {⋯outputCounting by 2:
2for (int i = 0; i <= 50; i += step)pass 1 of 26⋯6System.out.println("Counting by " + step + ":");7for (int i0 = 0; i <= 50; i += step2) {8    System.out.println(i0);9}⋯output026 passes — pass 1 is the card abovepassi
10
22
34
46
58
610
712
814
916
⋯ 15 more passes ⋯
2548
2650
3String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};⋯11// Practical: process every other element12String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};13System.out.println("\nEvery other name:");14for (int i = 0; i < names.length; i += 2) {⋯output
Every other name:
4for (int i = 0; i < names.length; i += 2)pass 1 of 3⋯13System.out.println("\nEvery other name:");14for (int i0 = 0; i < names.length6; i += 2) {15    System.out.println(names[i]Alice);16}⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5System.out.println(" Odd numbers 1-20:");⋯18// Odd numbers only19System.out.println("\nOdd numbers 1-20:");20for (int i = 1; i <= 20; i += 2) {⋯output
Odd numbers 1-20:
6for (int i = 1; i <= 20; i += 2)pass 1 of 10⋯19System.out.println("\nOdd numbers 1-20:");20for (int i1 = 1; i <= 20; i += 2) {21    System.out.print(i1 + " ");22}⋯output1 All 10 passes — pass 1 is the card abovepassi
11
23
35
47
59
611
713
815
917
1019
7System.out.println();⋯22    }23    System.out.println();24}⋯
1step ← 101public class StepByN {2    public static void main(String[] args) {3        int step→ 10 = 10;4        5        // Count by step6        System.out.println("Counting by " + step10 + ":");7        for (int i = 0; i <= 50; i += step) {⋯outputCounting by 10:
2for (int i = 0; i <= 50; i += step)pass 1 of 6⋯6System.out.println("Counting by " + step + ":");7for (int i0 = 0; i <= 50; i += step10) {8    System.out.println(i0);9}⋯output0All 6 passes — pass 1 is the card abovepassi
10
210
320
430
540
650
3String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};⋯11// Practical: process every other element12String[] names = {"Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"};13System.out.println("\nEvery other name:");14for (int i = 0; i < names.length; i += 2) {⋯output
Every other name:
4for (int i = 0; i < names.length; i += 2)pass 1 of 3⋯13System.out.println("\nEvery other name:");14for (int i0 = 0; i < names.length6; i += 2) {15    System.out.println(names[i]Alice);16}⋯outputAliceAll 3 passes — pass 1 is the card abovepassinames[i]
10Alice
22Charlie
34Eve
5System.out.println(" Odd numbers 1-20:");⋯18// Odd numbers only19System.out.println("\nOdd numbers 1-20:");20for (int i = 1; i <= 20; i += 2) {⋯output
Odd numbers 1-20:
6for (int i = 1; i <= 20; i += 2)pass 1 of 10⋯19System.out.println("\nOdd numbers 1-20:");20for (int i1 = 1; i <= 20; i += 2) {21    System.out.print(i1 + " ");22}⋯output1 All 10 passes — pass 1 is the card abovepassi
11
23
35
47
59
611
713
815
917
1019
7System.out.println();⋯22    }23    System.out.println();24}⋯

All 11 passes — pass 1 is the card above
pass	`i`
1	0
2	5
3	10
4	15
5	20
6	25
7	30
8	35
9	40
10	45
11	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 10 passes — pass 1 is the card above
pass	`i`
1	1
2	3
3	5
4	7
5	9
6	11
7	13
8	15
9	17
10	19

26 passes — pass 1 is the card above
pass	`i`
1	0
2	2
3	4
4	6
5	8
6	10
7	12
8	14
9	16
⋯ 15 more passes ⋯
25	48
26	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 10 passes — pass 1 is the card above
pass	`i`
1	1
2	3
3	5
4	7
5	9
6	11
7	13
8	15
9	17
10	19

All 6 passes — pass 1 is the card above
pass	`i`
1	0
2	10
3	20
4	30
5	40
6	50

All 3 passes — pass 1 is the card above
pass	`i`	`names[i]`
1	0	Alice
2	2	Charlie
3	4	Eve

All 10 passes — pass 1 is the card above
pass	`i`
1	1
2	3
3	5
4	7
5	9
6	11
7	13
8	15
9	17
10	19

Change the step size to process every 2nd, 5th, or nth element.

step How much to increment each iteration: `i++`, `i += 2`, `i += 5`

While with condition

Loop until some condition becomes false.

                            secret
                            secret = 7 (default)secret = 3secret = 9
                        

                            WhileCondition.java
                        
Replay: real traced execution (multi-file project)

                            public class WhileCondition {
    public static void main(String[] args) {
        // Guessing game simulation
        int secret = 7;
        int guess = 1;
        int attempts = 0;

        while (guess != secret) {
            System.out.println("Trying " + guess + "... wrong!");
            guess++;
            attempts++;
        }
        System.out.println("Found " + secret + " in " + attempts + " attempts!");

        // Practical: process until sentinel value
        int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel
        int i = 0;
        int sum = 0;

        System.out.println("\nSum until -1:");
        while (i < data.length && data[i] != -1) {
            sum += data[i];
            i++;
        }
        System.out.println("Sum: " + sum);

    }
}
public class WhileCondition {
    public static void main(String[] args) {
        // Guessing game simulation
        int secret = 3;
        int guess = 1;
        int attempts = 0;

        while (guess != secret) {
            System.out.println("Trying " + guess + "... wrong!");
            guess++;
            attempts++;
        }
        System.out.println("Found " + secret + " in " + attempts + " attempts!");

        // Practical: process until sentinel value
        int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel
        int i = 0;
        int sum = 0;

        System.out.println("\nSum until -1:");
        while (i < data.length && data[i] != -1) {
            sum += data[i];
            i++;
        }
        System.out.println("Sum: " + sum);

    }
}
public class WhileCondition {
    public static void main(String[] args) {
        // Guessing game simulation
        int secret = 9;
        int guess = 1;
        int attempts = 0;

        while (guess != secret) {
            System.out.println("Trying " + guess + "... wrong!");
            guess++;
            attempts++;
        }
        System.out.println("Found " + secret + " in " + attempts + " attempts!");

        // Practical: process until sentinel value
        int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel
        int i = 0;
        int sum = 0;

        System.out.println("\nSum until -1:");
        while (i < data.length && data[i] != -1) {
            sum += data[i];
            i++;
        }
        System.out.println("Sum: " + sum);

    }
}

                        
1secret ← 7, guess ← 1, attempts ← 01public class WhileCondition {2    public static void main(String[] args) {3        // Guessing game simulation4        int secret→ 7 = 7;    //@secret=3, 95        int guess→ 1 = 1;6        int attempts→ 0 = 0;⋯
2guess ← 2, attempts ← 1pass 1 of 6⋯8while (guess1 != secret7) {  //#?while_condition9    System.out.println("Trying " + guess1 + "... wrong!");10    guess→ 2++;11    attempts→ 1++;12}⋯outputTrying 1... wrong!All 6 passes — pass 1 is the card abovepassguessattempts
11 → 20 → 1
22 → 31 → 2
33 → 42 → 3
44 → 53 → 4
55 → 64 → 5
66 → 75 → 6
3i ← 0, sum ← 0⋯12}13System.out.println("Found " + secret7 + " in " + attempts6 + " attempts!");1415// Practical: process until sentinel value16int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel17int i→ 0 = 0;18int sum→ 0 = 0;1920System.out.println("\nSum until -1:");21while (i < data.length && data[i] != -1) {⋯outputFound 7 in 6 attempts!

Sum until -1:
4sum ← 10, i ← 1pass 1 of 3⋯20System.out.println("\nSum until -1:");21while (i0 < data.length6 && data[i]10 != -1) {22    sum→ 10 += data[i]10;23    i→ 1++;24}⋯All 3 passes — pass 1 is the card abovepassdata[i]sumi
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
5System.out.println("Sum: " + sum);⋯24}25System.out.println("Sum: " + sum60);⋯outputSum: 60
1secret ← 3, guess ← 1, attempts ← 01public class WhileCondition {2    public static void main(String[] args) {3        // Guessing game simulation4        int secret→ 3 = 3;5        int guess→ 1 = 1;6        int attempts→ 0 = 0;⋯
2guess ← 2, attempts ← 1pass 1 of 2⋯8while (guess1 != secret3) {9    System.out.println("Trying " + guess1 + "... wrong!");10    guess→ 2++;11    attempts→ 1++;12}⋯outputTrying 1... wrong!
3guess ← 3, attempts ← 2pass 2 of 2⋯8while (guess2 != secret3) {9    System.out.println("Trying " + guess2 + "... wrong!");10    guess→ 3++;11    attempts→ 2++;12}⋯outputTrying 2... wrong!
4i ← 0, sum ← 0⋯12}13System.out.println("Found " + secret3 + " in " + attempts2 + " attempts!");1415// Practical: process until sentinel value16int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel17int i→ 0 = 0;18int sum→ 0 = 0;1920System.out.println("\nSum until -1:");21while (i < data.length && data[i] != -1) {⋯outputFound 3 in 2 attempts!

Sum until -1:
5sum ← 10, i ← 1pass 1 of 3⋯20System.out.println("\nSum until -1:");21while (i0 < data.length6 && data[i]10 != -1) {22    sum→ 10 += data[i]10;23    i→ 1++;24}⋯All 3 passes — pass 1 is the card abovepassdata[i]sumi
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
6System.out.println("Sum: " + sum);⋯24}25System.out.println("Sum: " + sum60);⋯outputSum: 60
1secret ← 9, guess ← 1, attempts ← 01public class WhileCondition {2    public static void main(String[] args) {3        // Guessing game simulation4        int secret→ 9 = 9;5        int guess→ 1 = 1;6        int attempts→ 0 = 0;⋯
2guess ← 2, attempts ← 1pass 1 of 8⋯8while (guess1 != secret9) {9    System.out.println("Trying " + guess1 + "... wrong!");10    guess→ 2++;11    attempts→ 1++;12}⋯outputTrying 1... wrong!All 8 passes — pass 1 is the card abovepassguessattempts
11 → 20 → 1
22 → 31 → 2
33 → 42 → 3
44 → 53 → 4
55 → 64 → 5
66 → 75 → 6
77 → 86 → 7
88 → 97 → 8
3i ← 0, sum ← 0⋯12}13System.out.println("Found " + secret9 + " in " + attempts8 + " attempts!");1415// Practical: process until sentinel value16int[] data = {10, 20, 30, -1, 40, 50};  // -1 is sentinel17int i→ 0 = 0;18int sum→ 0 = 0;1920System.out.println("\nSum until -1:");21while (i < data.length && data[i] != -1) {⋯outputFound 9 in 8 attempts!

Sum until -1:
4sum ← 10, i ← 1pass 1 of 3⋯20System.out.println("\nSum until -1:");21while (i0 < data.length6 && data[i]10 != -1) {22    sum→ 10 += data[i]10;23    i→ 1++;24}⋯All 3 passes — pass 1 is the card abovepassdata[i]sumi
1100 → 100 → 1
22010 → 301 → 2
33030 → 602 → 3
5System.out.println("Sum: " + sum);⋯24}25System.out.println("Sum: " + sum60);⋯outputSum: 60

All 6 passes — pass 1 is the card above
pass	`guess`	`attempts`
1	1 → 2	0 → 1
2	2 → 3	1 → 2
3	3 → 4	2 → 3
4	4 → 5	3 → 4
5	5 → 6	4 → 5
6	6 → 7	5 → 6

All 3 passes — pass 1 is the card above
pass	`data[i]`	`sum`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

All 3 passes — pass 1 is the card above
pass	`data[i]`	`sum`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

All 8 passes — pass 1 is the card above
pass	`guess`	`attempts`
1	1 → 2	0 → 1
2	2 → 3	1 → 2
3	3 → 4	2 → 3
4	4 → 5	3 → 4
5	5 → 6	4 → 5
6	6 → 7	5 → 6
7	7 → 8	6 → 7
8	8 → 9	7 → 8

All 3 passes — pass 1 is the card above
pass	`data[i]`	`sum`	`i`
1	10	0 → 10	0 → 1
2	20	10 → 30	1 → 2
3	30	30 → 60	2 → 3

Use while when you don't know how many iterations ahead of time.

while Condition-controlled loop: unknown iterations, check before each.

Do-while (always runs once)

Sometimes you need to run the body at least once before checking.

                            DoWhile.java
                        
Replay: real traced execution (multi-file project)

                            public class DoWhile {
    public static void main(String[] args) {
        // Menu that shows at least once
        int choice = 0;

        do {
            System.out.println("=== Menu ===");
            System.out.println("1. Play");
            System.out.println("2. Settings");
            System.out.println("3. Exit");
            System.out.println("Choice: " + choice);

            // Simulate getting input
            choice = 3;
        } while (choice != 3);

        System.out.println("Goodbye!");

        // Difference: while vs do-while when condition is false
        boolean runLoop = false;

        System.out.println("\n=== while (runLoop=false) ===");
        while (runLoop) {
            System.out.println("This never prints");
        }
        System.out.println("while skipped entirely");

        System.out.println("\n=== do-while (runLoop=false) ===");
        do {
            System.out.println("This prints once!");
        } while (runLoop);
        System.out.println("do-while executed once");

    }
}

                        
1choice ← 01public class DoWhile {2    public static void main(String[] args) {3        // Menu that shows at least once4        int choice→ 0 = 0;⋯
2choice ← 3⋯6do {7    System.out.println("=== Menu ===");8    System.out.println("1. Play");9    System.out.println("2. Settings");10    System.out.println("3. Exit");11    System.out.println("Choice: " + choice0);12    13    // Simulate getting input14    choice→ 3 = 3;  //#?simulate_input15} while (choice != 3);⋯output=== Menu ===
1. Play
2. Settings
3. Exit
Choice: 0
3runLoop ← false⋯17System.out.println("Goodbye!");1819// Difference: while vs do-while when condition is false20boolean runLoop→ false = false;2122System.out.println("\n=== while (runLoop=false) ===");23while (runLoop) {24    System.out.println("This never prints");25}26System.out.println("while skipped entirely");2728System.out.println("\n=== do-while (runLoop=false) ===");29do {⋯outputGoodbye!

=== while (runLoop=false) ===
while skipped entirely

=== do-while (runLoop=false) ===
4do⋯28System.out.println("\n=== do-while (runLoop=false) ===");29do {30    System.out.println("This prints once!");31} while (runLoop);⋯outputThis prints once!
5System.out.println("do-while executed once");⋯31} while (runLoop);32System.out.println("do-while executed once");⋯outputdo-while executed once

do-while runs the body first, then checks the condition.

do-while Like while but checks AFTER running body (runs at least once).

Exercise: InfinitePatterns.java

Explore intentional infinite loops and how to use them safely