2.8 for Loops

1. The three parts of the header

A for loop is a type of iterative statement. Its header has three parts, separated by semicolons: the initialization, the Boolean expression and the update 2.8.A.1.

for (int lap = 1; lap <= 4; lap++) {
    System.out.println("Lap " + lap);
}

Output:

Lap 1
Lap 2
Lap 3
Lap 4

Part	In the example	Job
Initialization	int lap = 1	Create and set the loop control variable
Boolean expression	lap <= 4	Decide whether to run the body again
Update	lap++	Change the loop control variable after each iteration

2. The exact order of execution

The initialization statement runs only once, before the Boolean expression is evaluated for the first time. The variable it initializes is called the loop control variable. The Boolean expression is evaluated right after the initialization and then after each update, until it is false. In each iteration, the update runs after the entire loop body and before the Boolean expression is evaluated again 2.8.A.2.

for (int i = 0; i < 3; i++) {
    System.out.println("body sees i = " + i);
}

Output:

body sees i = 0
body sees i = 1
body sees i = 2

Step	Action	i	Result
1	initialization	0
2	check i < 3	0	true
3	body	0	prints 0
4	update i++	1
5	check i < 3	1	true
6	body	1	prints 1
7	update i++	2
8	check i < 3	2	true
9	body	2	prints 2
10	update i++	3
11	check i < 3	3	false, loop ends

Count from the table: the initialization ran 1 time, the Boolean expression 4 times, the body 3 times and the update 3 times.

A loop control variable declared in the header exists only inside the loop:

for (int i = 0; i < 3; i++) {
    System.out.println(i);
}
System.out.println(i);

error: cannot find symbol

3. Counting iterations and other updates

You can count iterations without tracing every step:

Header	Values of the loop control variable	Iterations
for (int i = 0; i < 10; i++)	0 to 9	10
for (int i = 1; i <= 10; i++)	1 to 10	10
for (int i = 1; i < 10; i++)	1 to 9	9
for (int i = 0; i <= 10; i += 2)	0, 2, 4, 6, 8, 10	6
for (int i = 10; i > 0; i--)	10 down to 1	10
for (int i = 1; i < 100; i *= 3)	1, 3, 9, 27, 81	5

The update does not have to be i++. This loop counts down by 5:

for (int seconds = 20; seconds >= 0; seconds -= 5) {
    System.out.print(seconds + " ");
}

Output:

20 15 10 5 0 

A for loop is a good fit for an accumulator, such as the sum of the even numbers from 2 to 10:

int sum = 0;
for (int n = 2; n <= 10; n += 2) {
    sum += n;
}
System.out.println("Sum: " + sum);

Output:

Sum: 30

4. for and while are interchangeable

A for loop can be rewritten as an equivalent while loop, and the other way around 2.8.A.3. Move the initialization above the loop, keep the Boolean expression, and make the update the last statement of the body.

for (int day = 1; day <= 3; day++) {
    System.out.println("Day " + day);
}

int day = 1;
while (day <= 3) {
    System.out.println("Day " + day);
    day++;
}

Both print the same three lines:

Output:

Day 1
Day 2
Day 3

One difference: in the while version, day is declared before the loop, so it still exists after the loop, with the value 4.

Going the other way, a while loop that halves a value becomes a for loop:

int steps = 0;
for (int size = 64; size > 1; size /= 2) {
    steps++;
}
System.out.println(steps);

Output:

6

5. Common for loop errors

A semicolon after the header gives the loop an empty body. The empty body runs 5 times, then the block runs once:

int total = 0;
for (int i = 0; i < 5; i++); {
    total++;
}
System.out.println(total);

Output:

1

Changing the loop control variable in the body as well as in the header skips values:

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

Output:

0 2 4 6 8 

Each iteration adds 2 to i: once in the body and once in the update. Keep all changes to the loop control variable in the header.